Vsb Engineering College, Karur Department Of

VSB ENGINEERING COLLEGE, KARUR

DEPARTMENT OF ELECTRICAL AND ELECTRONICS ENGINEERING

ACADEMIC YEAR 2019-2020 (EVEN SEMESTER)

III YEAR/VI SEMESTER 2 MARK QUESTION BANK

S.No SUBJECT CODE SUBJECT NAME PAGE NO.

1 EE8601 Solid State Drives 2-21

Protection and 2 EE8602 22-53 Switchgear

3 EE8691 Embedded Systems 54-68

Design of Electrical 4 EE8002 69-100 Apparatus

Special Electrical 5 EE8005 101–128 Machines

EE8601 SOLID STATE DRIVES

TWO MARKS QUESTIONS AND ANSWERS

UNIT – I TWO MARKS

1. What is meant by electrical drives? (Nov/Dec 2014)

Systems employed for motion control are called drives and they employ any of the prime movers such as diesel or petrol engines, gas or steam turbines, hydraulic motors and electric motors for supplying mathematical energy for motion control. Drives employing electric motion are called electric drives.

2. What are the requirements of an electric drive?

Stable operation should be assured. The drive should have good transient response

3. Specify the functions of power modulator. Power modulator performs one or more of the following four functions.

a. Modulates flow of power from the source to the motor in such a manner that motor is imparted speed-torque characteristics required by the load.

b. During transient operations, such as starting, braking and speed reversal, it restricts source and motor currents within permissible values; excessive current drawn from source may overload it or may cause a voltage dip.

4. Mention the different types of drives. 1) Group drive 2) Individual drive 3) Multi motor drive

5. List the different types of electrical drives. 1) dc drives 2) ac drives

6. What are the advantages of electric drives?

They have flexible control characteristics. The steady state and dynamic characteristics of electrical drives can be shaped to satisfy load requirements.

1) Drives can be provided with automatic fault detection systems, programmable logic controllers and computers can be employed to automatically ctrl the drive operations in a desired sequence. 2) They are available in which range of torque, speed and power.

3) It can operate in all the four quadrants of speed-torque plane. Electric braking gives smooth deceleration and increases life of the equipment compared to other forms of braking.

4) Control gear required for speed control, starting and braking is usually simple and easy to operate.

7. What are the functions performed by electric drives?

Various functions performed by electric drives include the following. a. Driving fans, ventilators, compressors and pumps etc. b. Lifting goods by hoists and cranes c. Imparting motion to conveyors in factories, mines and warehouses and

d. Running excavators and escalators, electric locomotives, trains, cars, trolley buses, lifts and drums winders etc.

8. What are the disadvantages of electric drives?

The disadvantages of electric drives are a. Electric drives system is tied only up to the electrified area.

b. The condition arising under the short circuits, leakage from conductors and breakdown of overhead conductor may lead to fatal accidents.

c. Failure in supply for a few minutes may paralyses the whole system.

9. What are the advantages of group drive over individual drive? The advantages of group drive over individual drive are

a. Initial cost: Initial cost of group drive is less as compared to that of the individual drive.

b. Sequence of operation: Group drive system is useful because all the operations are stopped simultaneously.

c. Space requirement: Less space is required in group drive as compared to individual drive.

d. Low maintenance cost: It requires little maintenance as compared to individual drive.

10. What the group drive is not used extensively.

Although the initial cost of group drive is less but yet this system is not used extensively because of following disadvantages.

a. Power factor: Group drive has low power factor

b. Efficiency: Group drive system when used and if all the machines are not working together the main motor shall work at very much reduced load.

c. Reliability: In group drive if the main motor fails whole industry will come to stand still.

d. Flexibility: Such arrangement is not possible in group drive i.e., this arrangement is not suitable for the place where flexibility is the prime factor. e. Speed: Group drive does not provide constant speed.

f. Types of machines: Group drive is not suitable fro driving heavy machines such as cranes, lifts and hoists etc.

11. Write short notes on individual electric drives.

In individual drive, each individual machine is driven by a separate motor. This motor also imparts motion to various other parts of the machine. Examples of such machines are single spindle drilling machines (Universal motor is used) and lathes. In a lathe, the motor rotates the spindle, moves the feed and also with the help of gears, transmits motion to lubricating and cooling pumps. A three phase squirrel cage induction motor is used as the drive. In many such applications the electric motor forms an integral part of the machine.

12. Mention the different factors for the selection of electric drives?

1) Steady state operation requirements. 2) Transient operation requirements. 3) Requirements related to the source. 4) Capital and running cost, maintenance needs life. 5) Space and weight restriction. 6) Environment and location. 7) Reliability.

13.Mention the parts of electrical drives. 1) Electrical motors and load. 2) Power modulator 3) Sources 4) Control unit 5) Sensing unit

14. Mention the applications of electrical drives(Nov/Dec 2016)

• Paper mills • Electric traction Cement mills • Steel mills

15. Mention the types of enclosures Screen projected type Drip proof type Totally enclosed type

16. Mention the different types of classes of duty Continuous duty, Discontinuous duty, Short time duty, intermittent duty.

17. What is meant by regenerative braking?

Regenerative braking occurs when the motor speed exceeds the synchronous speed. In this case the IM runs as the induction m\c is converting the mechanical power into electrical power which is delivered back to the electrical system. This method of braking is known as regenerative braking.

18. What is meant by dynamic braking?

Dynamic braking of electric motors occurs when the energy stored in the rotating mass is dissipated in an electrical resistance. This requires a motor to operate as a gen. to convert the stored energy into electrical.

19. What is meant by plugging?

It is one method of braking of IM. When phase sequence of supply of the motor running at the speed is reversed by interchanging connections of any two phases of stator with respect to supply terminals, operation shifts from motoring to plugging region.

20. What is critical speed?

It is the speed that separates continuous conduction from discontinuous conduction mode.

21. Which braking is suitable for reversing the motor?

Plugging is suitable for reversing the motor.

22. Define equivalent current method

The motor selected should have a current rating more than or equal to the current. It is also necessary to check the overload of the motor. This method of determining the power rating of the motor is known as equivalent current method.

23. Define cooling time constant

It is defined as the ratio between C and A. Cooling time constant is denoted as Tau. Tau = C/A Where C=amount of heat required to raise the temp of the motor body by 1 degree Celsius A=amount of heat dissipated by the motor per unit time per degree Celsius. 24. What are the methods of operation of electric drives? Steady state Acceleration including starting Deceleration including starting

25. Define four quadrant operations.

The motor operates in two mode: motoring and braking. In motoring, it converts electrical energy into mechanical energy which supports its motion. In braking, it works as a generator, converting mathematical energy into electrical energy and thus opposes the motion. Motor can provide motoring and braking operations for both forward and reverse directions.

26. What is meant by mechanical characteristics?

The curve is drawn between speed and torque. This characteristic is called mechanical characteristics.

27. Mention the types of braking Regenerative braking Dynamic braking Plugging

28. What are the advantage and disadvantages of D.C. drives? The advantages of D.C. drives are, a. Adjustable speed b. Good speed regulation c. Frequent starting, braking and reversing.

The disadvantage of D.C. drives is the presence of a mechanical commutator which limits the maximum power rating and the speed.

29. Give some applications of D.C. drives. The applications of D.C. drives are, a. Rolling mills b. Paper mills c. Mine winders d. Hoists e. Machine tools f. Traction g. Printing presses h. Excavators i. Textile mils j. Cranes

30. Why the variable speed applications are dominated by D.C. drives?

The variable speed applications are dominated by D.C. drives because of lower cost, reliability and simple control.

31. List out the examples of active load torque in drive system(Apr/May 2017) Load torque which have the potential to drive the motor under equilibrium conditions are called active load torques. Ex. Torque due to force of gravity, torque due tension torque, due to comparession and torsion, Torque due to friction, cutting etc.

32.State the condition of steady state stability of motor load system? (May/Jun 2016) i) A decrease in speed, the motor torque is greater that the load torque. ii) An increase in speed, the load torque is greater that the motor torque.

33. What are active and passive load torques? Give examples.(Apr/May 2015) A passive load is a load consisting of only a resistor, capacitor or inductor, or a combination of them.Ex. Torque due to friction, cutting An active load is a load which includes something which is current or voltage controlled, particularly a semiconductor device. Ex.Torque due to force of gravity, torque due tension torque.

UNIT – II TWO MARKS

1. What is the use of flywheel? Where it is used? It is used for load equalization. It is mounted on the motor shaft in compound motor.

2. What are the advantages of series motor? The advantages of series motors are, a. High starting torque 3.Define and mention different types of braking in a dc motor?

In breaking the motor works as a generator developing a negative torque which opposes the motion. Types are regenerative braking, dynamic or rheostat braking and plugging or reverse voltage braking.

4. How the D.C. motor is affected at the time of starting?

A D.C. motor is started with full supply voltage across its terminals, a very high current will flow, which may damage the motor due to heavy sparking at commuter and heating of the winding. Therefore, it is necessary top limit the current to a safe value during starting.

5. List the drawbacks of armature resistance control?

In armature resistance control speed is varied by wasting power in external resistors that are connected in series with the armature. since it is an inefficient method of speed control it was used in intermittent load applications where the duration of low speed operations forms only a small proportion of total running time.

6. What is static Ward-Leonard drive?

Controlled rectifiers are used to get variable d.c. voltage from an a.c. source of fixed voltage controlled rectifier fed dc drives are also known as static Ward-Leonard drive.

7. What is aline commutated inverter?

Full converter with firing angle delay greater than 90 deg. is called line commutated inverter. such an operation is used in regenerative braking mode of a dc motor in which case a back emf is greater than applied voltage.

8. Mention the methods of armature voltage controlled dc motor? When the supplied voltage is ac, Ward-Leonard schemes

Transformer with taps and un controlled rectifier bridge Static Ward-Leonard scheme or controlled rectifiers when the supply is dc: Chopper control

9. How is the stator winding changed during constant torque and constant horsepower operations?

For constant torque operation, the change of stator winding is made form series - star to parallel - star, while for constant horsepower operation the change is made from series-delta to parallel-star. Regenerative braking takes place during changeover from higher to lower speeds.

10. Define positive and negative motor torque.

Positive motor torque is defined as the torque which produces acceleration or the positive rate of change of speed in forward direction. Positive load torque is negative if it produces deceleration.

11. Write the expression for average o/p voltage of full converter fed dc drives? Vm=(2Vm/pi)cospi ...... continuous conduction Vm=[Vm(cos alpha-cos beta)+(pi+alpha+beta)]/pi] discontinuous conduction

12. What are the disadvantages of conventional Ward-Leonard schemes?

Higher initial cost due to use of two additional m\cs. Heavy weight and size.

Needs more floor space and proper foundation. Required frequent maintenance

13. Mention the drawbacks of rectifier fed dc drives?

Distortion of supply. Low power factor. Ripple in motor current

14. What are the advantages in operating choppers at high frequency?

The operation at a high frequency improves motor performance by reducing current ripple and eliminating discontinuous conduction.

15. Why self commutated devices are preferred over thyristors for chopper circuits?

Self commutated devices such as power MOSFETs power transistors, IGBTs, GTOs and IGCTs are preferred over thyristors for building choppers because they can be commutated by a low power control signal and don't need commutation circuit.

16. State the advantages of dc chopper drives?

DC chopper device has the advantages of high efficiency, flexibility in control, light weight, small size, quick response and regeneration down to very low speed.

17. What are the advantages of closed loop c of dc drives?

Closed loop control system has the adv. of improved accuracy, fast dynamic response and reduced effects of disturbance and system non-linearities.

18. What are the types of control strategies in dc chopper? • Time ratio control. • Current limit control.

19. What are the adv. of using PI controller in closed loop ctrl. of dc drive? Stabilize the drive • Adjust the damping ratio at the desired value

• Makes the steady state speed error close to zero by integral action and filters out noise again due to the integral action. 20. What are the different methods of braking applied to the induction motor? Regenerative braking Plugging, Dynamic braking.

21. What are the different methods of speed control of IM?

Stator voltage control, Supply frequency control, Rotor resistance control, Slip power recovery control.

21. What is meant by stator voltage control.? The speed of the IM can be changed by changing the stator voltage. Because the torque is proportional to the square of the voltage.

23. Mention the application of stator voltage control.

This method is suitable for applications where torque demand reduced with speed, which points towards its suitability for fan and pump drives.

24. Mention the applications of ac drives.

AC drives are used in a no. of applications such as fans, blowers, mill run-out tables, cranes, conveyors, traction etc.

25. What are the three regions in the speed-torque characteristics in the IM?

Motoring region (0<=s<=1) Generating region(s<0) Plugging region (1<=s<=2) where s is the slip.

26. What are the advantages of stator voltage control method?

• The control circuitry is simple • Compact size • Quick response time

• There is considerable savings in energy and thus it is economical method as compared to other methods of speed ctrl.

27. What is meant by soft start?

The ac voltage controllers show a step less control of supply voltage from zero to rated voltage they are used for soft start for motors.

28. List the adv of squirrel cage IM? • Cheaper • light in weight • Rugged in construction • More efficient • Require less maintenance • It can be operated in dirty and explosive environment

29. Define slip

The difference between the synchronous speed (Ns)and actual speed(N)of the rotor is known as slip speed. the % of slip is gn by,

%slip s=[(Ns-N)/Ns]x 100

30. Define base speed. The synchronous speed corresponding to the rated freq is called the base speed.

UNIT – III

TWO MARKS

1. What is meant by frequency control of IM?

The speed of IM can be controlled by changing the supply freq because the speed is directly proportional to supply frequency. This method of speed ctrl is called freq control.

2. What is meant by V/F control l?

When the freq is reduced the i/p voltage must be reduced proportionally so as to maintain constant flux otherwise the core will get saturated resulting in excessive iron loss and magnetizing current. This type of IM behavior is similar to the working of dc series motor.

3. What are the advantages of V/F control?

• Smooth speed ctrl • Small i/p current and improved power factor at low freq. start

• Higher starting torque for low case resistance

3. What is meant by stator current control?

The 3 phase IM speed can be controlled by stator current control. The stator current can be varied by using current source inverter.

5. What are the 3 modes of region in the adjustable-freq IM drives characteristics? • Constant torque region • Constant power region • High speed series motoring region

6. What are the two modes of operation in the motor?

The two modes of operation in the motor are, motoring and braking. In motoring, it converts electrical energy to mechanical energy, which supports its motion. In braking, it works as a generator converting mechanical energy to electrical energy and thus opposes the motion.

7. How will you select the motor rating for a specific application?

When operating for a specific application motor rating should be carefully chosen that the insulation temperature never exceed the prescribed limit. Otherwise either it will lead to its immediate thermal breakdown causing short circuit and damage to winding, or it will lead to deterioration of its quality resulting into thermal breakdown in near future.

8. What is braking? Mention its types.

The motor works as a generator developing a negative torque which opposes the motion is called barking. It is of three types. They are, a. Regenerative braking. b. Dynamic or rheostat braking. c. Plugging or reverse voltage braking.

9. What are the three types of speed control? The three types of speed control as, a. Armature voltage control b. Field flux control c. Armature resistance control. 10. What are the advantages of armature voltage control?

The advantages of armature voltage control are, a. High efficiency b. Good transient response c. Good speed regulation.

11. What are the methods involved in armature voltage control? When the supply in A.C. a. Ward-Leonard schemes

10 b. Transformer with taps and an uncontrolled rectifier bridge. c. Static ward Leonard scheme or controlled rectifiers when the supply in D.C. d. Chopper control.

12. Give some drawbacks and uses of Ward-Leonard drive. The drawbacks of Ward . Leonard drive are.

a. High initial cost b. Low efficiency

The Ward-Leonard drive is used in rolling mills, mine winders, paper mills, elevators, machine tools etc.

13. Give some advantages of Ward-Leonard drive. The advantages of Ward-Leonard drive are, a. Inherent regenerative barking capability b. Power factor improvement.

14. What is the use of controlled rectifiers?

Controlled rectifiers are used to get variable D.C. Voltage form an A.C. Source of fixed voltage.

15. What is known as half-controlled rectifier and fully controlled rectifier?

The rectifiers provide control of D.C. voltage in either direction and therefore, allow motor control in quadrants I and IV. They are known as fully-controlled rectifiers.

The rectifiers allow D.C. Voltage control only in one direction and motor control in quadrant I only. They are known as half-controlled rectifiers.

16. What is called continuous and discontinuous conduction?

A D.C. motor is fed from a phase controlled converter the current in the armature may flow in discrete pulses in called continuous conduction.

A D.C. motor is fed from a phase controlled converter the current in the armature may flow continuously with an average value superimposed on by a ripple is called discontinuous conduction.

17. What are the three intervals present in discontinuous conduction mode of single phase half and fully controlled rectifier? The three intervals present in half controlled rectifier are, a. Duty interval b. Free, wheeling interval c. Zero current intervals. The two intervals present in fully controlled rectifier are a. Duty interval b. Zero current intervals.

18. What is called inversion?

Rectifier takes power from D.C. terminals and transfers it to A.C. mains is called inversion.

19. What are the limitations of series motor? Why series motor is not used in traction applications now a days?

1. The field of series cannot be easily controlled. If field control is not employed, the series motor must be designed with its base speed equal to the highest desired speed of the drive.

2. Further, there are a number of problems with regenerative braking of a series motor. Because of the limitations of series motors, separately excited motors are now preferred even for traction applications.

20. What are the advantages of induction motors over D.C. motors?

The main drawback of D.C. motors is the presence of commutate and brushes, which require frequent maintenance and make them unsuitable for explosive and dirty environments. On the other hand, induction motors, particularly squirrel-cage are rugged, cheaper, lighter, smaller, more efficient, require lower maintenance and can operate in dirty and explosive environments.

21. Give the applications of induction motors drives.

Although variable speed induction motor drives are generally expensive than D.C. drives, they are used in a number of applications such as fans, blowers, mill run-out tables, cranes, conveyors, traction etc., because of the advantages of induction motors. Other applications involved are underground and underwater installations, and explosive and dirty environments.

22. How is the speed controlled in induction motor?

The induction motor speed can be controlled by supplying the stator a variable voltage, variable frequency supply using static frequency converters. Speed control is also possible by feeding the slip power to the supply system using converters in the rotor circuit, basically one distinguishes two different methods of speed control.

a. Speed control by varying the slip frequency when the stator is fed from a constant voltage, constant frequency mains.

b. Speed control of the motor using a variable frequency variable voltage motor operating a constant rotor frequency.

23. How is the speed control by variation of slip frequency obtained? Speed control by variation of slip frequency is obtained by the following ways. a. Stator voltage control using a three-phase voltage controller. b. Rotor resistance control using a chopper controlled resistance in the rotor circuit. c. Using a converter cascade in the rotor circuit to recover slip energy. d. Using a cyclconverter in the rotor circuit.

24. Mention the effects of variable voltage supply in a cage induction motor.

When a cage induction motor is fed from a variable voltage for speed control the following observations may be made. a. The torque curve beyond the maximum torque point has a negative shape. A stable operating point in this region is not possible for constant torque load.

12 b. The voltage controlled must be capable of withstanding high starting currents. The range of speed control is rather limited. c. The motor power factor is poor.

25. Classify the type of loads driven by the motor.

The type of load driven by the motor influences the current drawn and losses of the motor as the slip various. The normally occurring loads are 2. Constant torque loads. 3. Torque varying proportional to speed. 4. Torque varying preoperational to the square of the speed.

26. What are the disadvantages of constant torque loads?

The constant torque loads are not favored due to increase in the losses linearly with slip and becoming maximum at s= 1.0. This is obvious form the variation of flux as the voltage is varied for speed control. To maintain constant torque the motor draws heavy current resulting in poor torque/ampere, poor efficiency ad poor power factor at low speeds.

27. In which cases, torque versus speed method is suitable. Torque versus speed method is suitable only for the following cases. a. For short time operations where the duration of speed controls ids defined.

b. For speed control of blowers or pumps having parabolic or cubic variations of torque with speed. This is not suitable for constant torque loads due to increases and heating.

28. How is the speed of a squirrel cage induction motor controlled?

The speed of a squirrel cage induction motor can be controlled very effectively by varying the stator frequency. Further the operation of the motor is economical and efficient, if it operates at very small slips. The speed of the motor is therefore, varied by varying the supply frequency and maintaining the rotor frequency at the rated value or a value corresponding to the required torque on the linear portion of the torque-speed curve.

29. Why the control of a three-phase indication motor is more difficult than D.C. motors.

The control of a three-phase induction motor, particularly when the dynamic performance involved is more difficult than D.C. motors. This is due to a. Relatively large internal resistance of the converter causes voltage fluctuations following load fluctuations because the capacitor cannot be ideally large.

b. In a D.C. motor there is a decoupling between the flux producing magnetizing current and torque producing armature current. They can be independently controlled. This is not the case with induction motors.

c. An induction motor is very poorly damped compared to a D.C. motor.

30. Where is the V/f control used?

The V/f control would be sufficient in some applications requiring variable torque, such as centrifugal pumps, compressors and fans. In these, the torque varies as the square of the speed.

Therefore at small speeds the required torque is also small and V/f control would be sufficient to drive these leads with no compensation required for resistance drop. This is true also for the case of the liquid being pumped with minimal solids.

UNIT – IV

TWO MARKS

1. What are the components of the applied voltage to the induction motor?

The applied voltage to the induction motor has two components at low frequencies. They are a. Proportional to stator frequency. b.To compensate for the resistance drop in the stator. The second component deepens on the load on the motor and hence on rotor frequency. 2. What is indirect flux control?

The method of maintaining the flux constant by providing a voltage boost proportional to slip frequency is a kind of indirect flux control. This method of flux control is not desirable if very good dynamic behaviour is required.

3. What is voltage source inverter?

Voltage source inverter is a kind of D.C. link converter, which is a two stage conversion device.

4. What is the purpose of inductance and capacitance in the D.C. link circuit?

The inductance in the D.C. link circuit provides smoothing whereas the capacitance maintains the constancy of link voltage. The link voltage is a controlled quality.

5. What are the disadvantages of square wave inverter in induction motor drive?

Square wave inverters have commutation problems at very low frequencies, as the D.C. link voltage available at these frequencies cannot charge the commutating capacitors sufficiently enough to commutate the thrusters. Those puts a limit on the lower frequency of operation. To extend the frequency towards zero, special charging circuits must be used.

6. What is slip controlled drive?

When the slip is used as a controlled quantity to maintain the flux constant in the motor the drive is called slip enrolled drive. By making the slip negative (i.e., decreasing the output frequency of the inverter) The machine may be made to operate as a generator and the energy of the rotating parts fed back to the mains by an additional line side converter or dissipated in a resistance for dynamic barking. By keeping the slip frequency constant, braking at constant torque and current can be achieved. Thus braking is also fast.

7. What are the effects of harmonics in VSI fed induction motor drive?

The motor receives square wave voltages. These voltage has harmonic components. The harmonics of the stator current cause additional losses and heating. These harmonics are also responsible for torque pulsations. The reaction of the fifth and seventh harmonics with the fundamental gives rise to the seventh harmonic pulsations in the torque developed. For a given induction motor fed from a square wave inverter the harmonic content in the current tends to

14 remain constant independent of input frequency, with the rang of operating frequencies of the inverter.

8. What is a current source inverter?

In a D.C. link converter, if the D.C. link current is controlled, the inverter is called a current source inverter, The current in the D.C. link is kept constant by a high inductance and he capacitance of the filter is dispensed with . A current source inverter is suitable for loads which present a low impedance to harmonic currents and have unity p.f.

9. Explain about the commutation of the current source inverter.

The commutation of the inverter is load dependent. The load parameters form a part of the commutation circuit. A matching is therefore required between the inverter and the motor. Multimotor operation is not possible. The inverter must necessarily be a force commutated one as the induction motor cannot provide the reactive power for the inverter. The motor voltage is almost sinusoidal with superimposed spikes.

10. Give the features from which a slip controlled drive is developed.

The stator current of an induction motor operating on a variable frequency, variable voltage supply is independent of stator frequency if the air gap flux is maintained constant. However, it is a function of the rotor frequency. The torque developed is also a function of rotor frequency. The torque developed is also a function of rotor frequency only. Using these features a slip controlled drive can be developed employing a current source inverter to feed an induction motor.

11. How is the braking action produced in plugging?

In plugging, the barking torque is produced by interchange any two supply terminals, so that the direction of rotation of the rotating magnetic field is reversed with respect to the rotation of the motor. The electromagnetic torque developed provides the braking action and brings the rotor to a quick stop.

12. Where is rotor resistance control used?

Where the motors drive loads with intermittent type duty, such as cranes, ore or coal unloaders, skip hoists, mine hoists, lifts, etc. slip-ring induction motors with speed control by variation of resistance in the rotor circuit are frequently used. This method of speed control is employed for a motor generator set with a flywheel (Ilgner set) used as an automatic slip regulator under shock loading conditions.

13. What are the advantages and disadvantages of rotor resistance control?

Advantage of rotor resistance control is that motor torque capability remains unaltered even at low speeds. Only other method which has this advantage is variable frequency control. However, cost of rotor resistance control is very low compared to variable frequency control.

Major disadvantage is low efficiency due to additional losses in resistors connected in the rotor circuit.

14. Where is rotor resistance control used?

15. What are the advantages and disadvantages of rotor resistance control?

Major disadvantage is low efficiency due to additional losses in resistors connected in the rotor circuit.

16. How is the resistance in the output terminals of a chopper varied?

The resistance connected across the output terminals of a chopper can be varied form O to R by varying the time ratio of the chopper. When the chopper is always OFF, the supply is always connected to the resistance R. The time ratio in this case is zero and the effective resistance connected in R. Similarly when the chopper is always ON, the resistance is short circuited. The time ratio in the case is unity and the effective resistance connected is 0. Hence by varying the time ratio from 0 to 1, the value of resistance can be varied from R to O.

17. What is the function of inductance L and resistance R in the chopper resistance circuit?

A smoothing inductance L is used in the circuit to maintain the current at a constant value. Any short circuit in the chopper does not become effective due to L.

The value of R connected across the chopper is effective for all phases and its value can be related to the resistance to be connected in each phase if the conventional method has been used. The speed control range is limited by the resistance.

18. What are the disadvantages and advantages of chopper controlled resistance in the rotor circuit method?

The method is very inefficient because of losses in the resistance. It is suitable for intermittent loads such as elevators. At low speeds, in particular the motor has very poor efficiency. The rotor current is non-sinusoidal. They harmonics of the rotor current produce torque pulsations. These have a frequency which is six times the slip frequency. Because of the increased rotor resistance, the power factor is better.

19. How is the range of speed control increased?

The range of speed control can be increased if a combination of stator voltage control and rotor resistance control is employed. Instead of using a high resistance rotor, a slip ring rotor with external rotor resistance can be used when stator voltage control is used for controlling the speed.

20. Why the static scherbius drive has a poor power factor?

Drive input power is difference between motor input power and the power fed back. Reactive input power is the sum of motor and inverter reactive power. Therefore, drive has a poor power factor throughout the range of its options.

21. How is super synchronous speed achieved?

Super synchronous speed can be achieved if the power is fed to the rotor from A.C. mains. This can be made possible by replacing the converter cascade by a cycloconverter. A cycloconverter allows power flow in either direction making the static sherbets drive operate at both sub and supper synchronous speeds.

22. Give the features of static scherbius drive

The torque pulsations and other reactions are minimal. The performance of the drive improves with respect to additional losses and torque pulsations. A smooth transition is possible from sub to super synchronous speeds without any commutation problems. Speed reversal is not possible. A step up transformer may be interposed between the lines and the converter, to reduce the voltage rating of the converter.

23. Where is Kramer electrical drive system used?

Some continuous rolling mills, large air blowers, mine ventilators, centrifugal pumps and any other mechanisms including pumps drives of hydraulic dredgers require speed adjustment in the range from 15 to 30% below or above normal. If the induction motor is of comparatively big size (100 to 200 KW) it becomes uneconomical to adjust speed by mean's pf external resistances due to copper losses as slip power is wasted as heat in the retort circuit resistance. In these case , the Kramer electrical drive system is used , where slip power recovery takes places.

24. What is the use of sub synchronous converter cascades?

Sub synchronous converter cascades have been used, till now, in applications requiring one quadrant operation. These can be employed for drives where at least one electrical barking is required. A four quadrant operation can also be made possible in these cascades, using suitable switching.

25. How is the speed control obtained in static Kramer drive?

For speed control below synchronous speed, the slip power is pumped back to the supply, where as for the case of speed above synchronous speed, additional slip power is injected into the rotor circuit.

26. What is static Kramer drive?

Instead of wasting the slip power in the rotor circuit resistance, it can be converted to 60 Hz A.C. and pumped back to the line. The slip power controlled drive that permits only a sub synchronous range of speed control through a converter cascade is know as static Kramer drive.

27. What is the use and functions of step down transformer is static Kramer drive?

For a restricted speed range closer to synchronous speed, the system power factor can be further improved by using a step -down transformer.

The step-down transformer has essentially two functions: besides improving the line power factor, it also helps to reduce the converter power ratings.

28. What are the advantages of static Kramer drive?

The static Kramer drive has been very popular in large power pump and fan-type drives, where the range of speed control is limited near, but below the synchronous speed. The drive system is very efficient and the converted power rating is low because t has to handle only the slip power, In fact, the power rating becomes lower with a more restricted range of speed control. The additional advantages are that the drive system has D.C. machine like characteristics and the control is very simple.

29. What are the causes of harmonic currents in static Kramer drive? The rectification of slip power causes harmonic currents in the rotor, and these harmonics are reflected to the stator by the transformer action of the machine. The harmonic currents are also injected into the A.C. line by the inverter. As a result, the machine losses are increased and some amount of harmonic torque is produced. Each harmonic current in the rotor will create a reading magnetic filed and its direction of rotation will depend on the order pf the harmonic.

UNIT – V TWO MARKS

1. Give the four modes of operation of a Scherbius drive The four modes of operation of static Scherbius drive are, Sub synchronous motoring. Sub synchronous regeneration Super synchronous motoring Super synchronous regeneration 2. Give the use of synchronous motors. Synchronous motors were mainly used in constant speed applications. The development of semiconductor variable frequency sources, such as inverters and cycloconverters, has allowed their use in draft fane, main line traction, servo drives, etc. 3. How are the stator and rotor of the synchronous motor supplied? The stator of the synchronous motor is supplied from a thyristor power converter capable of providing a variable frequency supply. The rotor, depending upon the situation, may be constructed with slip rings, where it conforms to a conventional rotor. It is supplied with D.C. through slip rings. Sometimes rotor may also be free from sliding contacts (slip rings), in which case the rotor is fed from a rectifier rotating with rotor.

4. What is the difference between an induction motor and synchronous motor? An induction motor operates at lagging power factor and hence the converter supplying the same must invariable is a force commutated one. A synchronous motor, on the other hand, can be operated at any power factor by controlling the field current.

5. List out the commonly used synchronous motors. Commonly used synchronous motors are, a. Wound field synchronous motors. b. Permanent magnet synchronous motors c. Synchronous reluctance synchronous motors. d. Hysterias motors.

6. Mention the main difference between the wound field and permanent magnet motors.

When a wound filed motor is started as an induction motor, D.C. field is kept off. In case of a permanent magnet motor, the field cannot be 'turned off’.

7. Give the advantages and applications of PMSM. a. High efficiency b. High power factor c. Low sensitivity to supply voltage variations. The application of PMSM is that it is preferred of industrial applications with large duty cycle such as pumps, fans and compressors.

8. Give the uses of a hysteresis synchronous motor.

Small hysteresis motors are extensively used in tape recorders, office equipment and fans. Because of the low starting current, it finds application in high inertia application such as gyrocompasses and small centrifuges.

9. Mention the two modes employed in variable frequency control

Variable frequency control may employ and of the two modes. a. True synchronous mode b. Self- controlled mode

10. Define load commutation Commutation of thyristors by induced voltages pf load is known as load commutation.

11. List out the advantages of load commutation over forced commutation.

Load commutation has a number of advantages over forced commutation It does not require commutation circuits Frequency of operation can be higher It can operate at power levels beyond the capability of forced commutation.

12. Give some application of load commutated inverter fed synchronous motor drive.

Some prominent applications of load commutated inverter fed synchronous motor drive are high speed and high power drives for compressors, blowers, conveyers, steel rolling mills, main-line traction and aircraft test facilities.

13. How the machine operation is performed in self-controlled mode?

For machine operation in the self-controlled mode, rotating filed speed should be the same as rotor speed. This condition is relaised by making frequency of voltage induced in the armature. Firning pulses are therefore generated either by comparison of motor terminal voltages or by rotor position sensors.

14. What is meant by margin angle of commutation?

The difference between the lead angle of firing and the overlap angle is called the margin angle of commutation. If this angle of the thyristor, commutation failure occurs. Safe commutation is assured if this angle has a minimum value equal to the turn off angle f the thyristor.

15. What are the disadvantages of VSI fed synchronous motor drive?

VSI synchronous motor drives might impose fewer problems both on machine as well as on the system design. A normal VSI with 180° conduction of thyristors required forced commutation and load commutation is not possible.

16. How is PNM inverter supplied in VSI fed synchronous motor?

When a PWM inverter is used, two cases may arise the inverter may be fed from a constant D.C. source in which case regeneration is straight forward. The D.C. supply to the inverter may be obtained form a diode rectifier. In this case an additional phase controlled converter is required on the line side.

17. What is D.C. link converter and cycloconverter?

D.C. link converter is a two stage conversion device which provides a variable voltage, variable frequency supply. Cycloconverter is a single stage conversion device which provides a Variable voltage, variable frequency supply.

18. What are the disadvantages of cycloconverter?

A cycloconverter requires large number of thyristors and ts control circuitry is complex. Converter grade thyristors are sufficient but the cost of the converter is high.

19. What are the applications of cycloconverter?

A cycloconverter drive is attractive for law speed operation and is frequently employed in large, low speed reversing mils requiring rapid acceleration and deceleration. Typical applications are large gearless drives, e.g. drives for reversing mills, mine heists, etc.

20. Give the application of CSI fed synchronous motor.

Application of this type of drive is in gas turbine starting pumped hydroturbine starting, pump and blower drives, etc.

21. What are the disadvantages of machine commutation? The disadvantages of machine commutation are, a. Limitation on the speed range. b. The machine size is large c. Due to overexciting it is underutilized.

22. What is the use of an auxiliary motor?

Sometimes when the power is small an auxiliary motor can be used to run up the synchronous motor to the desired speed.

23. What are the advantages of brushless D.C. motor?

The brushless D.C. motor is in fact an inverter-fed self controlled permanent synchronous motor drive. The advantages of brushless D.C. motor are low cost, simplicity reliability and good performance.

24. When can the synchronous motor be load commutated?

When the synchronous motor operates at a leading power factor thyristors of load side converter can be commutated by the motor induced voltages same way as the thyristors of a line commutated converter are commutated by line voltages. 25. What are the characteristics of self controlled mode operated synchronous motor?

a) It operates at like dc motor also commutator less motor. b) These machines have better stability behavior. c) Do not have oscillatory behavior.

26. What are the characteristics of true synchronus mode operated synchronous motor? The motor behaves like conventional synchronous motor i.e) hunting oscillations exists. The change in frequency is slow enough for rotor to truck the changes. Multi motor operation is possible here.

27. What is meant by sub synchronous speed operation? The sub synchronous speed operation means the SRIM speed can be controlled below the synchronous speed. i.e) the slip power is fed back to the supply.

28. What is meant by super synchronous speed operation? The super synchronous speed operation means the SRIM speed can be controlled above the synchronous speed. i.e) the supply is fed back to the rotor side.

EE8602 - PROTECTION AND SWITCHGEAR

UNIT I PROTECTION SCHEMES PART A 1. How does the over voltage surge affect the power system? The over voltage of the power system leads to insulation breakdown of the equipments. It causes the line insulation to flash over and may also damage the nearby transformer, generators and the other equipment connected to the line. 2. What are symmetrical components? It is a mathematical tool to resolve unbalanced components into balanced components. The symmetrical components of three phase system are, i) Positive sequence components. ii) i)Negative sequence components iii) i)Zero sequence components. 3. Define negative sequence component. It has three vectors equal in magnitude and displaced from each other by an angle 120 degrees and has the phase sequence in opposite to its original phasors. 4. State the essential qualities of protection. i) Reliability ii) Selectivity iii)Fastness of operation and iv) Discrimination. 5. Give the consequences of short circuit or What are the effects of short circuit faults in power system if uncleared? (Nov/Dec 2018) When a short-circuit occurs, the current in the system increases to an abnormally high value while the system voltage decreases to a low value. The heavy current due to short-circuit causes excessive heating which may result in fire or explosion. Sometimes short-circuit takes the form of an arc and causes considerable damage to the system. If the voltage remains low for even a few seconds, the consumer’s motors may shut down and generators on the power system may become unstable. 6. What is the need of relay coordination? The operation of a relay should be fast and selective, i.e., it should isolate the fault in the shortest possible time causing minimum disturbance to the system. Also, if a relay fails to operate, there should be sufficiently quick backup protection so that the rest of the system is protected. By coordinating relays, faults can always be isolated quickly without serious disturbance to the rest of the system. 7. Define: energizing quantity. The electrical quantity i.e., current or voltage either alone or in combination with other 22 electrical quantities required for the functioning of the relay. The quantity either current or voltage which is the input to the relay energizes the trip coil of the relay which in turn trips the circuit in case of faults. 8. What is protected zone? (Apr/May 2015) Protected zones are those which are directly protected by a protective system such as relays, fuses or switchgears. When a fault occurs in a zone, it can be immediately detected and isolated by a protection scheme which is dedicated to that particular zone. To limit the extent of the fault, power system protection is arranged in zones. Ideally, the zones of protection should overlap, so that no part of the power system is left unprotected. 9. What are the various faults that would affect an alternator? i) Phase to phase faults ii) Phase to earth faults iii) Inter turn faults iv) Earth faults v)Fault between turns vi) Loss of excitation due to fuel failure vii) Over speed viii) Loss of drive ix) Vacuum failure resulting in condenser pressure rise, resulting in shattering of the turbine low pressure casing. 10. State the significance of double line fault. Double line to ground fault occurs when two lines are short circuited and is in contact with the ground. This type of fault occurrence ranges from 15 to 25%. It has no zero sequence component and the positive and negative sequence networks are connected in parallel. Since zero sequence components are absent there is no circulating current. 11. What is primary protection? (Nov/Dec 2017) Primary protection is the protection in which the fault occurring in a line will be cleared by its own relay and circuit breaker. It serves as the first line of defense. 12. What are the different types of earthing ? (Apr/May 2015) i) Resistive earthing ii) Reactance earthing iii) Resonant earthing 13. State the significance of single line to ground fault. In single line to ground fault all the sequence networks are connected in series. All the sequence currents are equal and the fault current magnitude is three times its sequence currents. 14. Differentiate between a fuse and a circuit breaker. Fuse is a low current interrupting device. It is a copper or an aluminum wire. Circuit breaker is a high current interrupting device and it act as a switch under normal operating conditions.

15. What is surge absorber? How do they differ from surge diverter? (Nov/Dec 2011) Surge absorber is a device designed to protect electrical equipment from transient high voltage to limit the duration and amplitude of the following current. Surge diverter discharges the impulse surge to the earth and dissipates energy in the form of heat. 16. Define the term “insulation coordination” (Nov/Dec 2011) The selection of the insulation strength of equipment in relation to the voltages, which can appear on the system for which the equipment is intended and taking into account the service environment and the characteristics of the available protective device. 17.What are the various types of faults occurring in a power system? (May/June 2012)(May/June 2014 ) (May/June 2017 ) (Nov/Dec 2017)

Series Fault: a) One open conductor fault b) Two open conductor fault Shunt Fault: (a) Symmetrical or balanced fault (i) Three phase Fault (LLLG) (b) Unsymmetrical or unbalanced fault ( i) Line to line fault(LL)(ii) Line to ground fault (LG)(iii) Double line to ground fault.(LLG). 18. How are arcing grounds avoided? (May/June 2012) The presence of inductive and capacitive currents in the isolated neutral system leads to formation of arcs called as arcing grounds. The surge voltage due to arcing ground can be removed by using the arc suppression coil or Peterson coil. The arc suppression coil has an iron cored tapped reactor connected in neutral to ground connection. The reactor of the arc suppression coil extinguishes the arcing ground by neutralising the capacitive current. 19. What are the effects of power system faults? (Nov/Dec 2012) Increase in current above rated value, Insulation failure, Equipment damage. 20. What is back up protection? (Nov/Dec 2012) Back up protection is the second line of defence, which operates if the primary protection fails to activate within a definite time delay. 21. What is meant by pick-up current? (May/June 2013)(Nov/Dec 2014) The minimum current at which the relay armature is attracted to close the trip circuit is called pick-up current. In most of the relays, the pick up current is also indicated with the relay. 22. Write the sources of fault power. (Nov/Dec 2013)

The fault power can be originated from the generation (faults in alternator) or transmission (short circuit) or from the distribution side(loads). Also the fault power can be from external sources like lightning. 23. List out the duties of fault limiting reactors. (Nov/Dec 2013) The duties of fault limiting reactors are to limit the fault current and to eliminate the arcing ground. 24. What are the functions of protective relays? (May/June 2013) (Apr/May 2015) To detect the fault and initiate the operation of the circuit breaker and to isolate the defective element from the rest of the system, thereby protecting the system from damages occurring due to fault. 25. What is the necessity for earthing? (Nov/Dec 2014) (May/June 2014 ) (Nov/Dec 2015) When earthing is provided it ensures the safety of personnel against electrical shocks and avoids accidents. The potential of earthed body does not reach to dangerously high value above earth since it is connected to earth. Also the earth fault current flows through the earthing and may cause operation of fuse or an earth relay. 26. What is the difference between short circuit and an overload?(Nov/Dec2015)(May/June2016) On the occurrence of short circuit, the voltage at the point of fault falls to zero and the current in the network increases abnormally to a higher value. But in the case of overload reduction in the terminal voltage of the equipment occurs but the voltage will never fall to zero. Similarly the current also increases to a higher value but not as high as in the case of short circuit. 28. What is the difference between primary and back up protection? (May/June 2016) Primary protection is the protection in which the fault occurring in a line will be cleared by its own relay and circuit breaker. It serves as the first line of defense. Instantaneous relays are used. Back up protection is the second line of defense, which operates if the primary protection fails to activate within a definite time delay. Relays with definite time lag is used. 29. Why earth wire is provided in overhead transmission lines? (May/June 2016)

Earthing wire usually consists of a Low Resistance wire connected to earth or buried into Earth. It's nothing but a Low Resistance path. Whenever there is a fault or abnormal operation or any external activities, the current flows through the earth wire and charges are discharged into the ground. If a fault occurs, current follows through earth wire first and the electrical equipment is protected. 30. What do you mean by dead spot in zones of protection? In practice, various protective zones are overlapped. The overlapping of protective zones is done to ensure complete safety of each and every element of the system. The zone which is unprotected is called dead spot. The zones are overlapped and hence there is no chance of existence of a dead spot in a system. If there are no overlaps, then dead spot may exist which means the circuit breakers lying within the zone may not trip even though the fault occurs. This may cause damage to the healthy system. 30. State the difference between circuit breaker and switch. (May/June 2017)

Circuit breaker Switch A mechanical switching device capable of A mechanical switching device capable of making , carrying and breaking currents making , carrying and breaking currents under normal conditions and abnormal under normal conditions but not breaking conditions like short circuit. under abnormal conditions such as short It is an automatic device. circuit. A switch is operated manually. 31. Why protection scheme is required for power system? (April/May 2018) An electrical power system consists of generators, transformers, transmission lines and distribution stations etc., Short circuits and other abnormalities often occur in power systems which cause heavy short circuit currents. The heavy current associated with short circuits will cause damage to the equipment if suitable protective relays and circuit breakers are not provided. 32. Write down the importance of symmetrical components for fault current calculation. (April/May 2018) The method of symmetrical components is used to simplify fault analysis by converting a three-phase unbalanced system into two sets of balanced phasors and a set of single-phase phasors, or symmetrical components. These sets of phasors are called the positive-, negative-, and zero-sequence components. These components allow for the simple analysis of power systems under faulted or other unbalanced

conditions. Once the system is solved in the symmetrical component domain, the results can be transformed back to the phase domain. 33. How protective relays are classified based on functions? (Nov/Dec 2018) The protective relays are classified in the following few categories. • Directional Over current Relay • Distance Relay • Over voltage Relay • Differential Relay • Reverse Power Relay UNIT II ELECTROMAGNETIC RELAYS Part A 1. Name the different kinds of over current relays. Induction type non-directional over current relay, Induction type directional over current relay & current differential relay. 2. Define operating time of a relay. It is the time which elapses from the instant at which actuating quantity exceeds the relay pick up value to the instant at which the relay closes its contacts. 3. Define resetting time of a relay. It is the time which elapses from the moment the actuating quantity falls below its reset value to the instant when the relay comes back to its normal (initial) position. 4. What is ‘Time grading’ of relays. (Nov/Dec 2018) It is the setting of time of operation of various relays protecting the different sections of a line. It is set so that the relay which is nearest to the fault location alone will operate first and clear the fault. 5. What are Over and Under current relays? Over current relays are those that operate when the current in a line exceeds a predetermined value. (e.g.: Induction type non-directional/directional over current relay, differential over current relay) whereas Undercurrent relays are those which operate whenever the current in a circuit/line drops below a predetermined

value.(e.g.: differential over-voltage relay) 6. What is biased differential beam relay? The biased beam relay is designed to respond to the differential current in terms of its fractional relation to the current flowing through the protected zone. It is essentially an over-current balanced beam relay type with an additional restraining coil. The restraining coil produces a bias force in the opposite direction to the operating force. 7. Give the limitations of Merz Price protection. Since neutral earthing resistances are often used to protect circuit from earth-fault currents, it becomes impossible to protect the whole of a star-connected alternator. If an earth-fault occurs near the neutral point, the voltage may be insufficient to operate the relay. Also it is extremely difficult to find two identical CT’s. In addition to this, there always an inherent phase difference between the primary and the secondary quantities and a possibility of current through the relay even when there is no fault. 8. Why is an under frequency relay required in a power system?(May/June 2012) (Nov/Dec 2013) (Nov/Dec2014) An under frequency relay is one which operates when the frequency of the system (usually an alternator or transformer) falls below a certain value. Under frequency relays are used to shed automatically certain portion of load whenever the system frequency falls to such a low level which threatens the stability of the power system. 9. What are the features of directional relay? High speed operation; high sensitivity; ability to operate at low voltages; adequate short-time thermal ratio; burden must not be excessive. 10. What is static relay? It is a relay in which measurement or comparison of electrical quantities is made in a static network which is designed to give an output signal when a threshold condition is passed which operates a tripping device. 11. What is a programmable relay? A static relay which has one or more programmable units such as microprocessors or microcomputers embedded in its circuit is called a programmable relay.

12. What are the advantages of static relay over electromagnetic relay? (Nov/Dec 2011) (May/June 2014) (Nov/Dec 2014) i) Low power consumption as low as 1mW ii) No moving contacts; hence associated problems of arcing, contact bounce, erosion, replacement of contacts iii) No gravity effect on operation of static relays. Hence can be used in vessels ie, ships, aircrafts etc. iv) A single relay can perform several functions like over current, under voltage, single phasing protection by incorporating respective functional blocks. This is not possible in electromagnetic relays v) Static relay is compact 13. What are the different types of over current relays i) Definite time ii)Inverse definite minimum time(IDMT) iii)Very Inverse iv)Extremely Inverse. 14. What is earth fault protection? A ground fault (earth fault) is any failure that allows unintended connection of power circuit conductors with the earth. Such faults can cause objectionable circulating currents, or may energize the housings of equipment at a dangerous voltage. Under such condition residual current flowing to the ground is calculated. Such a protective scheme used for the protection of an element of a power system against earth faults is called as earth fault protection 15. List out the applications of static relays. (Nov/Dec 2012) ) (May/June 2016) i) Protection of generators ii) Protection of transformers iii) Protection of transmission lines, and iv) Protection of motors. 16. What is meant by directional relay? (May/June 2012) A directional relay detects the whether the point of fault lies in the forward or reverse direction with respect to relay location. The relay which is able to sense the direction of power flow and act for a particular direction of power flow is called directional relay. 17. What is meant by differential relay? (May/June 2013) (Apr/May 2015) A differential relay is one that operates when the phasor difference of two or more

similar electrical quantities exceeds a predetermined value. It has two coils viz., operating coil which produces operating torque and restraining coil which produces restraining torque. 18. What are the types of fuses? (Nov/Dec 2013) a) Low voltage fuses i) Semi-enclosed rewireable fuse ii)HRC fuse b) High voltage fuses i) cartridge type ii)liquid type iii)metal clad type. 19. List out the different types of distance relay.(May/June 2014) Dependent on the ratio of V and I there are three types of distance relays which are i) Impedance relay which is based on measurement of impedance Z ii) Reactance relay which is based on measurement of reactance X iii)Admittance or Mho relay which is based on measurement of component of admittance Y.

20. In what way distance relay is superior to over current protection? (Nov/Dec 2015) Distance relays are preferred to overcurrent relays because they are not nearly so much affected by changes in short-circuit-current magnitude as overcurrent relays are, and, hence, are much less affected by changes in generating capacity and in system configuration. This is because distance relays achieve selectivity on the basis of impedance rather than current. 21. Where are negative sequence relays employed? Negative sequence relays are employed for negative sequence protection of generators against the unbalanced load condition. The negative phase sequence filter along with the over current relay provides the necessary protection against the unbalanced loads. 22. Write the effects of arc resistance. The effect of arc resistance is most significant on short lines where the reach of the relay setting is small. It can be a problem if the fault occurs near the end of the reach.

High fault-arc resistances tend to occur during midspan flashovers to ground on transmission lines carried on wood poles without earth wires. These problems can usually be overcome by using relays having different shaped characteristics. 23. What is the significance of PSM and TSM? (Nov/Dec 2016) Time setting multiplier TSM: TSM determines the operating time of the relay. Lower the value of TSM, lower will be the operating time. Plug setting multiplier PSM: The plug position ensures the current setting value of the relay. Plug setting multiplier (PSM) indicates the severity of the fault. 24. A relay is connected to 400/5 ratio current transformer with current setting of 150%. Calculate the plug setting multiplier when circuit carries a fault current of 4000A. (Nov/Dec 2016) Fault Current = 4000A C.T. ratio = 400/5 Fault current in the relay coil = 4000 * (5/400) = 50A Plug Setting Multiplier (PSM) = Fault Current in the relay coil / (Rated secondary C.T. Current * Current setting) Plug Setting Multiplier (PSM) = 50 / (5*1.5) = 6.667 25. Why shaded ring is provided in induction disc relay? (May/June 2017) In the induction disc relay, a metal disc is allowed to rotate between two electromagnets. The shaded pole structure is generally actuated by current flowing in a single coil on a magnetic structure containing an air gap.The air gap flux produced by this current is split into two out-of-phase components by a so called “shading ring” generally of copper, that encircles part of the pole face of each pole at the air gap. 26. Give the principle of negative sequence relay. (Nov/Dec 2017) A relay which protects the electrical system from negative sequence component is called a negative sequence relay or unbalance phase relay. The actuating quantity is negative sequence current. When the negative sequence current exceeds a certain value, the relay operates. This is used to protect electrical machines against

overheating due to unbalanced currents. 27. Write the torque equation of the universal relay. (Nov/Dec 2017)

2 2 T=K1I +K2V +K3VI Cos (θ - ) + K4 where K1, K2, K3 are the tap setting or constant of Voltage V and current I. The K4 is the mechanical restraint due to spring or gravity. 28. Mention the principle of operation of distance relay. (April/may 2018) There is one voltage element from potential transformer and a current element fed from current transformer of the system. The deflecting torque is produced by secondary current of CT and restoring torque is produced by voltage of potential transformer. In normal operating condition, restoring torque is more than deflecting torque. Hence relay will not operate. But in faulty condition, the current becomes quite large whereas voltage becomes less. Consequently, deflecting torque becomes more than restoring torque and dynamic parts of the relay starts moving which ultimately close the No contact of relay. Hence clearly operation or working principle of distance relay depends upon the ratio of system voltage and current. 29. Determine the plug setting multiplier of a 5 ampere, 3 second over current relay having a current setting of 125% and a time setting multiplier of 0.6 connected to supply circuit through a 400/5 current transformer when the circuit carries a fault current of 4000A. (April/may 2018) Plug Setting Multiplier = Fault current in relay coil/(Rated CT secondary current * Current Setting) Fault current in relay coil = 4000*(5/400) = 50A. Therefore, PSM= 50/(5*1.25)=8 30. What are the factors affecting the performance of differential relays? (Nov/Dec 2018) • Phasor sum of currents • CT ratio • Polarity of transformers

UNIT III APPARATUS PROTECTION PART A 1. What are the causes of over speed and how alternators are protected from it? (April/may 2018) Sudden loss of all or major part of the load causes over-speeding in alternators. Modern alternators are provided with mechanical centrifugal devices mounted on their driving shafts to trip the main valve of the prime mover when a dangerous over- speed occurs. 2. What are the uses of Buchholz’s relay? Bucholz relay is used to give an alarm in case of incipient (slow-developing) faults in the transformer and to disconnect the transformer from the supply in the event of severe internal faults. It is usually used in oil immersion transformers with a rating over 750KVA. 3. What are the various faults that would affect an alternator? (May/June 2013) (Apr/May 2015) )(May/June 2016) (a) Stator faults i) Phase to phase faults ii) Phase to earth faults iii) Stator inter turn faults (b) Rotor faults i)Rotor earth faults ii)Field over loading iii) Heating of rotor c) Abnormal Running Conditions i) Over speeding ii) Over loading iii) Unbalanced Loading iv)Over voltage v)Failure of Prime mover. 3. What are faults associated with a transformer? a) Overheating b) Winding Faults i)phase to phase fault ii) Earth fault iii)Interturn faults c)Open circuits d)Through faults e)Over fluxing. 4. What are the main safety devices available with transformer? (May/June 2012) Oil level gauge, sudden pressure delay, oil temperature indicator, winding temperature indicator. 5. What are the limitations of Buchholz relay? (May/June 2017) (a) Only fault below the oil level are detected.(b) Mercury switch setting should be

very accurate, otherwise even for vibration, there can be a false operation.(c) The relay is of slow operating type, which is unsatisfactory. 6. What are the problems arising in differential protection in power transformer and how are they overcome? (May/June 2012) (Nov/Dec 2015) i) Difference in lengths of pilot wires on either sides of the relay. This is overcome by connecting adjustable resistors to pilot wires to get equipotential points on the pilot wires. ii) Difference in CT ratio error difference at high values of short circuit currents that makes the relay to operate even for external or through faults. This is overcome by introducing bias coil.iii) Tap changing alters the ratio of voltage and currents between HV and LV sides and the relay will sense this and act. Bias coil will solve this. iv) Magnetizing inrush current will be identified as short circuit current. A harmonic restraining unit is added to the relay which will block it when the transformer is energized. 7. What is REF relay? It is Restricted Earth Fault relay. When the fault occurs very near to the neutral point of the transformer, the voltage available to drive the earth circuit is very small, which may not be sufficient to activate the relay, unless the relay is set for a very low current. Hence the zone of protection in the winding of the transformer is restricted to cover only around 85%. Hence the relay is called REF relay. 8. What is over fluxing protection in transformer? (Nov/Dec 2016) If the turn’s ratio of the transformer is more than 1:1, there will be higher core loss and the capability of the transformer to withstand this is limited to a few minutes only. This phenomenon is called over fluxing. 9. Why bus-bar protection is needed? (May/June 2013) (i) Fault level at bus-bar is high (ii) The stability of the system is affected by the faults in the bus zone.(iii) A fault in the bus bar causes interruption of supply to a large portion of the system network. 10. What are the causes of bus zone faults? i)Failure of support insulator resulting in earth fault ii) Flashover across support

insulator during over voltage iii )Heavily polluted insulator causing flashover iv) Earthquake, mechanical damage etc. 11. What are the problems in bus zone differential protection? i)Large number of circuits, different current levels for different circuits for external faults ii) Saturation of CT cores due to dc component and ac component in short circuit currents. The saturation introduces ratio error.iii) Sectionalizing of the bus makes circuit complicated. iv) Setting of relays need a change with large load changes. 12. What are the disadvantages of time graded protection? i) Time lag is not desirable on short circuits ii) Not suitable for ring main distribution iii) Difficult to coordinate & needs changes with new connection iv) Not suitable for long distance relaying. 13. How does the over voltage surge affect the power system? The over voltage of the power system leads to insulation breakdown of the equipments. It causes the line insulation to flash over and may also damage the nearby transformer, generators and the other equipment connected to the line. 14. What is the general connection rule for Current transformers in differential protection? If the windings of the power transformer are delta connected then the current transformers are star connected and if the windings of the power transformer are star connected then the current transformers are delta connected. 15. Write the coordination equation for inverse over-current relay?

TA=TB+CBB+OA+F

Where TA operating time of relay at station A,TB operating time of relay at station

B,CBB operating time of circuit breaker at station B,OA over travel time of relay at station A,F factor of safety 16. Explain why secondary of current transformer should not be open. (Nov/Dec 2011)(Dec 2014) (Apr/May 2015) (May/June 2016)

Current transformers generally work at a low flux density. Core is then made of very good metal to give small magnetizing current. On open-circuit, secondary impedance now becomes infinite and the core saturates. This induces a very high voltage in the primary upto approximately system volts and the corresponding volts in the secondary will depend on the number of turns. Since secondary of CT has more turns compared to the primary, the voltage generated on the open-circuited CT will be high, leading to flashovers. Hence as a safety precaution, CT secondary should not be open-circuited. 17. What is meant by time graded system protection? (Nov/Dec 2018) In a time graded system, the operating time of the relay is increased from the far end of protected circuit towards the generating source. Definite time overcurrent relays are used which after a preset time will trip the circuit. The difference in time setting of the two adjacent relays are kept at 0.5s. This difference is to cover the operating time of CB and errors in CT and relay. 18. Write the function of earth fault relay. (Nov/Dec 2012) Earth fault relay is used for the protection of an element of a power system against earth faults. Earth relay calculates the residual current. If the residual current is zero the relay will not operate. Restricted earth fault relay is used in differential protection which will not operate for external faults. 19. What is meant by relay operating time? (Nov/Dec 2012) It is the time which elapses from the instant at which actuating quantity exceeds the relay pick up value to the instant at which the relay closes its contacts. 20. What are the different types of zones of protection? (Nov/Dec 2013) i) Primary protection and ii) backup protection and Unit and Non-Unit protection. 21. State the methods of protection of busbars. (Nov/Dec 2014) (Nov/Dec 2016) i)Frame leakage protection of busbar ii) Circulating current protection of busbar iii) High impedance differential protection of busbar 22. List the applications of current transformer. (May/June 2014) i) To the supply the stepped down current to the relay coil in the event of any

overloading or short-circuiting of the equipment lines. ii) To measure power of a load in conjunction with a wattmeter. The secondary of the CT is connected to the current coil of the wattmeter.iii)To measure large currents in conjunction with medium/Small range meters. 23. Give examples of Unit and Non – Unit Protection Schemes (Nov/Dec 2015) The concept of 'Unit Protection', whereby sections of the power system are protected individually as a complete unit without reference to other sections. eg. Differential Protection, Overcurrent Protection. eg. of Non – Unit Protection: Distance Protection. 24. What are the difficulties encountered through differential protection? (May/June 2017) Though the saturation in Current transformer is avoided, there exist difference in the C.T. characteristics due to ratio error at high values of short circuit currents. This causes an appreciable difference in the secondary currents which can operate the relay. So the relay operates for external faults. Due to the difference in lengths of the pilot wires on both sides, the unbalance condition may result. Due to the magnetizing current inrush current in transformers which may be as great as 10 times the full load current of the transformer, the differential relay may operate falsely. 25. What is the need of instrumentation transformer? (May/June 2017) Instrument transformers are high accuracy electrical devices used to isolate or transform voltage or current levels. The most common usage of instrument transformers is to measure high voltage or high current (with common meters) by safely isolating secondary control circuitry from the high voltages or currents. 26. Why secondary of a transformer should not be opened? (Nov/Dec 2017) The secondary side of a current transformer should never be kept in open condition because, when kept open, there is a very high voltage found across the secondary side. This high voltage causes a high magnetizing current to build up on the secondary side which in turn causes high flux and makes the core to saturate.

27. List the types of bus bar protection. (Nov/Dec 2017) Frame-Earth Protection Differential Protection for Sectionalized Bus bars High-Impedance Differential Protection Low-Impedance Differential Protection Digital Bus bar Protection 28. What are the protection methods used for transmission lines?(April/may 2018) Over current protection; Simple Impedance Relay; Mho relay; Reactance relay 29. In the event of fault in generator windings, the field excitation should be suppressed as early as possible. Why? (Nov/Dec 2018) Failure of excitation that is failure of field system in the generator makes the generator run at a speed above the synchronous speed. In that situation the generator or alternator becomes an induction generator which draws magnetizing current from the system. Although this situation does not create any problem in the system immediately but over loading of the stator and overheating of the rotor due to continuous operation of the machine in this mode may create problems in the system in long-run. Therefore special care should be taken for rectifying the field or excitation system of the generator immediately after failure of that system. The generator should be isolated from rest of the system till the field system is properly restored. 30. Which type of protection is used for EHV and UHV lines? (Nov/Dec 2018) Carrier current protective scheme Pilot wire protective scheme UNIT IV STATIC RELAYS AND NUMERICAL PROTECTION

1. Define static relay? (Nov/Dec 2017) It is a relay in which measurement or comparison of electrical quantities is made in a

static network which is designed to give an output signal when a threshold condition is passed which operates a tripping device. 2. What is CPMC? It is combined protection, monitoring and control system incorporated in the static system. For example static relays employ microprocessor units which incorporate protection principles such as overcurrent, inverse time etc., in their operation. Also these units sense the fault current each and every time. The fault current can also be controlled by changing the code embedded into the processor. 4. What are the advantages of static relay over electromagnetic relay? (Nov/Dec 2018) ➢ Low power consumption as low as 1mW ➢ No moving contacts; hence associated problems of arcing, contact bounce, erosion, replacement of contacts ➢ No gravity effect on operation of static relays. Hence can be used in vessels ie, ships, aircrafts etc. ➢ A single relay can perform several functions like over current, under voltage, single phasing protection by incorporating respective functional blocks. This is not possible in electromagnetic relays ➢ Static relay is compact. Superior operating characteristics and accuracy ➢ Static relay can think , programmable operation is possible with static relay ➢ Effect of vibration is nil, hence can be used in earthquake-prone areas ➢ Simplified testing and servicing. Can convert even non-electrical quantities to electrical in conjunction with transducers. 5. What is pick up value? It is the minimum current in the relay coil at which the relay starts to operate. The relay should not operate when the current does not exceed the pick up value. 6. Define target. It is the indicator used for showing the operation of the relay. This helps the operator to know the cause of tripping of the circuit breaker.

7. Define blocking. Blocking is preventing the relay from tripping due to its own characteristics or due to additional relays. False operation of relay may lead to unnecessary opening of circuit. 8. What are the advantages of numerical relays over conventional relays? (May/June 2014) (May/June 2015) (Nov/Dec 2016) No moving parts and therefore no friction Easy to replace and service. Numeric relays are not affected by gravity Are compact and has modular arrangement Various characteristics can be obtained. 9. What are the drawbacks of analogue and active analogue filters? (May/June 2014) They are bulky, especially inductors require large space; High precision components are needed making them expensive; Their characteristics drift with respect to time and temperature; Filters for very low frequencies need impracticably high component values; They are not programmable and adaptable 10. Draw the block diagram of FIR and IIR filter

11. Compare FIR and IIR filters

S.No FIR Filter IIR Filter 1. Difficult to control and have no Always make a linear phase. particular phase 2. FIR is always stable IIR can be unstable

3. FIR has no limited cycles Can have limited cycles 4. FIR has no analog history IIR is derived from analog. 5. FIR can always be made casual IIR filters make polyphase implementation possible 6. FIR filters are Finite IR filters IIR is infinite and used for applications which are required for linear- where linear characteristics are not of phase characteristics. concern. 12. What is Fourier analysis? The analysis of a complex waveform expressed as a series of sinusoidal functions, the frequencies of which form a harmonic series. If a function is periodic, then it can be written as a discrete sum of trigonometric or exponential functions with specific frequencies. 13. What is discrete Fourier transform? As the name implies, the Discrete Fourier Transform (DFT) is purely discrete: discrete-time data sets are converted into a discrete-frequency representation. This is in contrast to the DTFT that uses discrete time, but converts to continuous frequency. Since the resulting frequency information is discrete in nature, it is very common for computers to use DFT calculations when frequency information is needed. 14. What is Aliasing? Aliasing is a phenomenon where the high frequency components of the sampled signal interfere with each other because of inadequate sampling. It results in loss of signal and its place will be taken by a different lower frequency wave. 15. What is sampling? Sampling is the process of converting a signal (for example, a function of continuous time or space) into a numeric sequence (a function of discrete time or space). 16. What is sample and hold circuit? A sample and hold circuit is an analog device that samples (captures, grabs) the voltage of a continuously varying analog signal and holds (locks, freezes) its value at a constant level for a specified minimum period of time. Sample and hold circuits

and related peak detectors are the elementary analog memory devices. They are typically used in analog-to-digital converters to eliminate variations in input signal that can corrupt the conversion process. 17. What is digital filter? In signal processing, a digital filter is a system that performs mathematical operations on a sampled, discrete-time signal to reduce or enhance certain aspects of that signal. 18. Draw the block diagram of static differential relay

19. Draw the block diagram of numerical relay

20. Draw the block diagram of static directional relay

21. Draw the block diagram of static over current relay

22. Draw the block diagram of static relay

23. What are the building blocks of static relay? or What are the basic circuits in static relay? (April/may 2018) (Nov/Dec 2018) Rectifier, Comparator, Amplifier, Transducer are some of the building blocks of static relay. 24. What is least error squared technique? (May/June 2015) The least error squared technique is directly related to the Fourier technique. If a given function were to be synthesized by using a dc component, a sine wave of fundamental frequency and harmonics of this fundamental, then the amplitudes of

various components given by the Fourier analysis are the ones which give the least squared error. We can directly find out the amplitudes of the components by using the LES technique. 25. List out the applications of static relays. (Nov/Dec 2016) (May/June 2016) • Ultra high speed protection of EHV AC transmission lines utilizing distance protection. • In over current and earth fault protection schemes • As main element in differential relay 26. State Nyquist–Shannon sampling theorem (May/June 2017) If a function x(t) contains no frequencies higher than B hertz, it is completely determined by giving its ordinates at a series of points spaced 1/(2B) seconds apart. A band limited signal can be reconstructed exactly if it is sampled at a rate atleast twice the maximum frequency component in it. fs≥2fm where fs = sampling frequency; fm = frequency of the signal which is reconstructed. 27. Write about numerical transformer differential protection. (May/June 2017) It provides fast and selective tripping for two winding transformer. It quickly discriminates between faults that occur in the protected zone and those occurring outside this zone and thus provides selective and fast tripping. The faults within protected zone are short circuit between turns, windings and cables and earth faults inside transformer housing and protected zone. It discriminates between above internal faults and the operational conditions like inrush, over-fluxing and faults external to protected zone using numerical algorithms. 28. What is phase comparator? (Nov/Dec 2017) A phase detector or phase comparator is a frequency mixer, analog multiplier or logic circuit that generates a voltage signal which represents the difference in phase between two signal inputs. It is an essential element of the phase-locked loop (PLL). 29. List out the general characteristics of numerical protection. (April/may 2018) • The numerical relay relies on one system for all approach and use indication

on LCD for relay activation, ensuring less space. • Since the numerical relay system relies on software, customized modifications can be made for getting the desired protection features. This saves the cost of replacing hardware. Fewer interconnections ensure reliability. • The range of operation of traditional models is narrow while numerical relays are diverse and evolution adaptable. • It also has the feature of auto resetting and self-diagnosis. • The benefit of using microprocessor based relays in the numerical system is that it gives minimum burden on the instrument transformers. The sensitivity of the system is pretty nifty and boasts a high pickup ratio.

UNIT V CIRCUIT BREAKERS Part A

1. What is dielectric test of a circuit breaker? It consists of over voltage withstand test of power frequency lightning and impulse voltages. Tests are done for both internal and external insulation with switch in both open and closed conditions. 2. Define composite testing of a circuit breaker. In this method the breaker is first tested for its rated breaking capacity at a reduced voltage and afterwards for rated voltage at a low current. It is the combination of both field type testing station and laboratory type testing station. This method does not give a proper estimate of the breaker performance. 3. What is making capacity? (Nov/Dec 2015) It is the capacity of the circuit breaker to be closed onto a short circuit. It is expressed as 1.414 X 1.8 X symmetrical breaking capacity = 2.55 X symmetrical breaking capacity. 4. What are the advantages of synthetic testing methods? i) The breaker can be tested for desired transient recovery voltage and RRRV. ii)

Both test current and test voltage can be independently varied. This gives flexibility to the test iii) The method is simple iv) With this method a breaker capacity (MVA) of five times of that of the capacity of the test plant can be tested. 5. Write are the types of test conducted on circuit breakers. (May/June 2012) (Apr/May 2015) i) Type test ii) Routine test iii) Reliability test iv) Commissioning test Type test can be classified into mechanical performance test, thermal test, dielectric test and short circuit tests.

6. What are the characteristic of SF6 gas? It has good dielectric strength and excellent arc quenching property. It is inert, non- toxic, non inflammable and heavy. At atmospheric pressure, its dielectric strength is 2.5 times that of air. At three times atmospheric pressure, its dielectric strength is equal to that of the transformer oil.

7. Give the advantage of SF6 circuit breaker over air blast circuit breaker (May/June 2013) (Apr/May 2015) (May/June 2016) High electro negativity, compactness, reduced switching over voltages, reduced insulation time, superior arc interruption and increased safety.

8. What is meant by electro negativity of SF6 gas?

SF6 has high affinity for electrons. When a free electron comes and collides with a neutral gas molecule, the electron is absorbed by the neutral gas molecule and negative ion is formed. This is called as electro negativity of SF6 gas. 9. What are the demerits of using oil as an arc quenching medium? i)The air has relatively inferior arc quenching properties ii)The air blast circuit breakers are very sensitive to variations in the rate of rise of restriking voltage iii)Maintenance is required for the compression plant which supplies the air blast 10. What are the advantages of air blast circuit breaker over oil circuit breaker? i)The risk of fire is diminished ii)The arcing time is very small due to rapid buildup of dielectric strength between contacts iii)The arcing products are completely removed by the blast whereas oil deteriorates with successive operations

11. What are the types of air blast circuit breaker? i)Arial-blast type ii) Cross blast iii) Radial-blast 12. What are the disadvantages of MOCB over a bulk oil circuit breaker? i)The degree of carbonization is increased due to smaller quantity of oil ii) There is difficulty of removing the gases from the contact space in time iii)The dielectric strength of the oil deteriorates rapidly due to high degree of carbonization. 13. What are the advantages of MOCB over a bulk oil circuit breaker? i) It requires lesser quantity of oil ii) It requires smaller space iii) There is a reduced risk of fire iv) Maintenance problem are reduced. 14. What are the advantages of oil as arc quenching medium? i) It absorbs the arc energy to decompose the oil into gases, which have excellent cooling properties ii) It acts as an insulator and permits smaller clearance between line conductors and earthed components 15. What are demerits of MOCB? i)Short contact life ii)Frequent maintenance iii)Possibility of explosion iv)Larger arcing time for small currents v)Prone to restricts 16. Mention different types of circuit breakers? (May/June 2012) i) Air break circuit breaker ii) Oil circuit breaker iii) Minimum oil circuit breaker iv) Air blast circuit breaker v) SF6 circuit breaker vi)Vacuum circuit breaker 17. What are the different types of oil circuit breakers? i) Plain break oil circuit breakers ii) Arc control circuit breakers iii) Minimum oil circuit breakers 18. What are the advantages of using vacuum as an arc interrupting medium? Vacuum offers the utmost insulating strength. Interruption occurs in the first current zero. So it has superior arc quenching properties than any other medium. Also the dielectric strength of vacuum is superior to those of porcelain, oil, air and SF6. 19. Write any two properties of contact material used in vacuum circuit breaker? i) Good electrical conductivity to pass normal load currents without over heating.

ii) Good thermal conductivity to dissipate rapidly the large heat generated during arcing 20. What are the basic requirements of circuit breaker? (Nov/Dec 2011) i)To make or break a circuit either manually or by remote control under normal conditions ii) Break a circuit automatically under fault condition iii) Make a circuit automatically either manually or by remote control after the fault is cleared. 21. Write the difference between fuse and circuit breaker. (Nov/Dec 2012)(May/June 2014). Fuse is a low current interrupting device. It is a copper or an aluminum wire. Circuit breaker is a high current interrupting device and it act as a switch under normal operating conditions. 22. Enumerate breaking capacity of circuit breaker. (Nov/Dec 2012) (Nov/Dec 2014) The capacity of the circuit breaker which can break under specified conditions of recovery voltage. The breaking capacity of a circuit breaker is expressed in MVA and given as 1.732 X (rated voltage in kV) X (rated current in kA). 23. Write the ratings of the circuit breaker. (Nov/Dec 2013) Circuit breaker has three ratings. i) Breaking capacity ii) Making capacity and iii) Short time capacity. 24. Define the opening time of circuit breaker.(May/June 2014) The time interval which is passed in between the energization of the trip coil to the instant of contact separation is caused the opening times. It is dependent on fault current level. 25. What is meant by current chopping? (Nov/Dec 2014) At the time of interruption of a large fault current, the arc energy is high enough to keep the arc column ionized until the arc is interrupted at natural current zero. On keep the arc column ionized until the arc is interrupted at natural current zero. On the other hand, while interrupting small inductive currents such as unloaded currents of transformers and currents of shunt reactor, there is a possibility of overvoltage

depending on the value of the chopping current. This small inductive current is interrupted just before natural current zero and thus induces high transient voltages, which is known as current chopping. 26. What is resistance switching? (Nov/Dec 2013) It is the method of connecting a resistance in parallel with the contact space (arc). The inserted resistance reduces the re striking voltage frequency and it diverts part of the arc current. It assists the circuit breaker in interrupting the magnetizing current and capacity current. 27. What is an arc? Arc is a phenomenon occurring when the two contacts of a circuit breaker separate under heavy load or fault or short circuit condition. 28. Give the two methods of arc interruption? (May/June 2012) (Apr/May 2015) i) High resistance interruption:-the arc resistance is increased by elongating, and splitting the arc so that the arc is fully extinguished. ii)Current zero method:-The arc is interrupted at current zero position that occurs100 times a second in case of 50Hz power system frequency in AC. 29. What is restriking voltage? (Nov/Dec 2011) (May/June 2017) It is the transient voltage appearing across the breaker contacts at the instant of arc being extinguished. 30. What is meant by recovery voltage? (Nov/Dec 2011) (May/June 2013) The power frequency RMS voltage appearing across the breaker contacts after the arc is extinguished and transient oscillations die out is called recovery voltage. 31. Define the term RRRV? (May/June 2012) (Apr/May 2015) The transient voltage which appears across the circuit breaker contacts at the instant of arc extinction is called restriking voltage. RRRV is the Rate of Rise of Restriking Voltage, expressed in volts per microsecond. It is the rate at which the restriking voltage changes per microsecond. It is closely associated with natural frequency of oscillation. 32. What is the main problem of the circuit breaker?

When the contacts of the breaker are separated, an arc is struck between them. This arc delays the current interruption process and also generates enormous heat which may cause damage to the system or to the breaker itself. This is the main problem. 33. What are the factors the arc resistance depends upon? i) Degree of ionization ii)Length of the arc iii) Cross section area of the arc 34. Mention the details circuit breaker rating i)Rated voltage & rated current ii)Rated Frequency iii)Rated breaking capacity, symmetrical & asymmetrical iv)Rated making capacity v)Rated short time current vi)Rated operating duty 35. What are the factors the ARC phenomenon depends upon? (May/June 2013) i) The nature and pressure of the medium ii) The external ionizing and de-ionizing agent present iii) Voltage across the electrodes and its variation with time iv) The nature shape & separation of electrodes v) The nature and shape of vessel and its position in relation to the electrodes 36. Define symmetrical breaking capacity. (Nov/Dec 2017) The symmetrical value of breaking capacity is the value of the symmetrical breaking current which the circuit breaker is capable of breaking at the stated recovery voltage and restriking voltage under prescribed condition 37. What are the two theories explaining current zero interruption? i) Recovery rate theory or voltage race theory or slepain’s theory. ii) Energy balance theory or Cassie’s theory. 38. What are the factors the recovery voltage depends upon? (Nov/Dec 2011) i) Power factor, ii) Armature reaction &iii) Circuit condition 39. What is the basic requirement of DC circuit breaking? Lengthening of the arc is basic requirements of D.C circuit breaker. Loss of energy increases with increasing length of arc and more power will be required to maintain the arc. 40. What are the problems associated with DC circuit breakers? i) Natural current zero does not occur as in the case of A.C circuit breakers. ii) The

amount of energy to be dissipated during the short interval of breaking is very high as compared to conventional A.C circuit breakers. 41. What is the purpose of protective spark gap? A protective spark gap can be used across the circuit breaker to reduce the size of commutation capacitor. The spark gap acts as an energy dissipating device for high frequency currents. 42. List out the various methods of arc interruptions. (Nov/Dec 2012) (Apr/May 2015) i) High resistance interruption ii) Current zero method. 43. How do you classify the circuit breakers? (Nov/Dec 2012) (Nov/Dec 2014)

i) Air break circuit breaker, ii)Oil circuit breaker, iii)Air blast circuit

breaker iv)SF6 circuit breaker, and v)Vacuum circuit breaker 44. What is meant by auto reclosing? (Nov/Dec 2013) (May/June 2016) In electric power distribution, an auto recloser is a circuit breaker equipped with a mechanism that can automatically close the breaker after it has been opened due to a fault. 45. Write the function of isolating switch. (Nov/Dec 2013) (May/June 2016) A disconnector, disconnect switch or isolator switch is used to ensure that an electrical circuit is completely de-energised for service or maintenance. The disconnector is usually not intended for normal control of the circuit, but only for safety isolation. Disconnector can be operated either manually or automatically (motorized disconnector). 46. Mention any two advantages of vacuum circuit breakers.(Nov/Dec 2014) i) They are compact in size and have longer life. ii) There are no fire hazards.iii) No generation of gas during and after operation. iv)They require less maintenance and quiet in operation. v) They can successfully withstand lightning surges. 47. Why current chopping is not required in MOCB? MOCB has superior arc quenching properties when compared to air blast circuit

breakers due to the cooling oil and hence there is no special mechanism required for current chopping. 48. How does a circuit breaker differ from a switch? (Nov/Dec 2015) (May/June 2016) Switches are not automatic as they need to be manually turned on or off while circuit breakers just trips off on certain conditions. Switches allow users to cut off power supply to a certain area or equipment while circuit breakers are more preventive in nature. Circuit breakers are essentially automatic off switches designed for a very specific purpose, which is to prevent unnecessary electrical circuit damage. 49. Name the materials used for contacts of vacuum circuit breakers. (Nov/Dec 2015) Compounds of copper and Chromium are used most widely for making the contacts of circuit breakers. 50. What is the difference between re-striking voltage and recovery voltage? (Nov/Dec 2016) Re-striking voltage: It is the transient voltage appearing across the breaker contacts at the instant of arc being extinguished. Recovery voltage: The power frequency RMS voltage appearing across the breaker contacts after the arc is extinguished and transient oscillations die out is called recovery voltage. 51. State the difference between D.C. and A.C. circuit breaking. (Nov/Dec 2016) DC circuit breaker, like their name suggests, is used for the protection of electrical devices that operate with direct current. The main difference between direct current and alternating current is that in DC the voltage output is constant, while in AC it cycles several times per second. 52. What is rupturing Capacity? (May/June 2017) Rupturing capacity is the current that a fuse, circuit breaker, or other electrical apparatus is able to interrupt without being destroyed or causing an electric arc with unacceptable duration. The prospective short-circuit current which can occur under short circuit conditions should not exceed the rated breaking capacity of the

apparatus. This theory states that the rate at which positive ions and electrons recombine to form neutral molecules is compared with rate of rise of restriking voltage and if the restriking voltage rises more rapidly than the dielectric strength, gap space breaks down and arc strikes again persists. 53. What are the factors responsible for increase in arc resistance? (April/may 2018) The arc resistance increases • When ionized particle between contact decreases. • As the separation between contact increases and length of the arc also increases • With decrease in cross section area of the arc 54. A circuit breaker is rated as 1500A, 1000MVA, 3second, 3 phase oil circuit breaker . Find the rated making current. (April/may 2018) Given: Breaking capacity= 1000MVA; Breaking current = 1500A; Soln: Making current = 2.55*breaking current=2.55*1500=3825A. 55. Why rate of rise of restriking voltage plays an important role in circuit breaker operation? (Nov/Dec 2018)

The rate of rise of restriking voltage denotes the rate at which transient voltage increases or decreases. This factor plays an important role in circuit breaker operation since it decides the interruption of current by the circuit breaker. Transient recovery voltage depends upon natural frequency and power factor. 56. Why oil circuit breakers are not suitable for heavy current interruption at low voltages? (Nov/Dec 2018) Oil circuit breakers cannot be used for heavy current interruption because high current causes arc which produces flammability of oil. Thus it requires high maintenance.

EE8691 EMBEDDED SYSTEMS

UNIT – I INTRODUCTION TO EMBEDDED SYSTEMS PART A 1. What is an embedded system? An Embedded system is one that has computer-hardware with software embedded in it as one of its most important component. It is a dedicated computer-based system for an application or product. It may be either an independent system or a part of a larger system. Ex: VCRs, digital watches, elevators, automobile engines, thermostats & instruments that are driven by microprocessors and their software. 2. Define embedded microcontroller. Microcontrollers are particularly suited for use in embedded systems for real-time control applications with on-chip program memory and devices. An embedded microcontroller is particularly suited for embedded applications to perform dedicated task or operation. Example: 68HC11xx, 8051, PIC, 16F877, etc. 3. List out the microprocessors used in embedded systems. The various microprocessors used in embedded systems are:Intel 8051 family, Zilog Z80 family,Intel 80x86 family, Motorola 68000 family, SPARC, IBM PowerPC 601, 604 4. Define DMA. Direct Memory Access (DMA) allows large quantities of information to be transferred to or from memory, without using a core processor. Data is transferred into DMA memory, while the CPU is freed to handle other tasks. 5. Define Bus handshaking. The process of exchanging control information between the processor and other hardware units before exchanging actual data is called bus handshaking.

6. What is the use of DMA? DMA is a circuitry that can read data from an I/O device, such as a serial port or a network, and then write it into memory or read from memory and write to an I/O device, all without the software assistance and the associated overhead. 7. What is Interrupt vector table? To find out the location of interrupt routine, a table somewhere in memory contains interrupt vectors, the addresses of the interrupt routines. When an interrupt occurs, the microprocessor will look up the address of the interrupt routine in the interrupt vector table. This table must be set properly by the users. 8. What is Target Hardware Debugging? During development of embedded systems under some operating systems, a hardware debugger is used which connects physical device to target hardware via a JTAG (Joint Test Action Group) interface. This is necessary for development of drivers, and possibly user applications, because they're linked into the same memory space as the kernel. If a driver or application crashes, the kernel and system may crash as a result. This makes using software debuggers difficult, because they depend on a running system. 9. Define glue logic circuit. The glue logic circuit of an embedded system is a circuit for interconnecting the processor to external memories so that the appropriate chip-select signals, according to the system

memory, map each of the memory chips. The glue logic circuit also includes a circuit to interconnect the parallel ports to the peripherals. This circuit is designed by programming and configuring PAL, GAL, CPLD, and PLD. 10. What is ROM image? The final stage of embedded software is called ROM image. Image is a unique sequence and arrangement of pixels, embedded software is also a unique placement and arrangement of bytes for instructions and data. 11. What is embedded processor? Give example. A microcontroller or microprocessor which is specially designed with the following capabilities is called embedded processor. Fast context switching, Atomic ALU operation, RISC core for fast, more precise and intensive calculations by the embedded software.Example: ARM 7, AMD family 29050 12. Explain Stand alone and real time embedded system? Standalone system doesn’t require any additional hardware to implement the targetedapplication, but in real time system the task in hand must be completed withina bounded time. 13. Define ASIP. ASIP is Application Specific Instruction Processor. A processor designed for specific application on a VLSI chip. 14. Define virtual memory Virtual Memory is a concept by which the application is made to believe that it has much more memory than is physically available. 15. Define In-circuit Emulator. In-circuit emulation (ICE) is the use of a hardware device or in-circuit emulator is used to debug the software of an embedded system. It operates by using a processor with the additional ability to support debugging operations, as well as to carry out the main function of the system. 16. What are the different types of memory used in embedded system? (DEC 2014) RAM, ROM, Flash memory, Static RAM, Dynamic RAM. 17.How embeddedsystems are different from conventional PC?(DEC 2014) An embedded system is a computer system with a dedicated function within a larger mechanical or electrical system, often with real-time computing constraints. It is embedded as part of a complete device often including hardware and mechanical parts. Embedded systems control many devices in commonly used today.

18. Why embedded systems are suitable for real time systems? (MAY 2015) Real-time systems are computer systems that monitor, respond to, or control an external environment. This environment is connected to the computer system through sensors, actuators, and other input-output interfaces. It may consist of physical or biological objects of any form and structure 19. List the major embedded computer applications. (MAY 2015) Space research, Defence, satellite communication, mobile, Automotive, Smart card system. 20.What is the need of Watchdog timer?(MAY 2016) (DEC 2017)

A watchdog timer resets the microprocessor and starts the software over from the beginning if the software does not restart it periodically. It is used to rescue the system if a fault develops and the program gets stuck. 21. List out the challenges in building an embedded system.(MAY 2016) (DEC 2016) We have a great deal of control over the amount of computing power we apply to our problem. We cannot only select the type of microprocessor used, but also select the amount of memory, the peripheral devices, and more. Since we often must meet both performance deadlines and manufacturing cost constraints ,the choice of hardware is important—too little hardware and the system fails to meet its deadlines, too much hardware and it becomes too expensive to meet deadlines,minimize power consumption, design for upgradability, Complex testing,Limitedobservability and controllability, Restricted development environments. 22. Write the function of One time device programmer? A programmable read-only memory (PROM) or field programmable read-only memory (FPROM) or one-time programmable non-volatile memory (OTP NVM) is a form of digital memory where the setting of each bit is locked by a fuse or antifuse. They are a type of ROM (read-only memory) meaning the data in them is permanent and cannot be changed. PROMs are used in digital electronic devices to store permanent data, usually low level programs such as firmware (microcode). The key difference from a standard ROM is that the data is written into a ROM during manufacture, while with a PROM the data is programmed into them after manufacture. So ROMs are used only for large production runs, while PROMs are used for smaller production where the program may have to be changed. 23. How embedded systems are different from conventional PC? (Dec 2014) An embedded system is a computer system with a dedicated function within a larger mechanical or electrical system, often with real-time computing constraints. It is embedded as part of a complete device often including hardware and mechanical parts. Embedded systems control many devices in commonly used today. 24. What is the need of a watch dog timer? (Dec 2017) A watchdog timer (WDT) is a hardware timer that automatically generates asystem reset if themain program neglects to periodically service it. It is often used to automatically reset an embedded device that hangs because of a software or hardware fault. 25.Define Cyclic & Acyclic mode of operation in counter. Cyclic Mode: Once the counter reaches the done state, it is automatically reloaded and counting process continues. Acyclic Mode: The counter/Timer waits for an explicit signal from the microprocessor to resume counting. 26.What are the functional requirements of embedded systems? (MAY2018) Functional requirements are those which are related to the technical functionalityof the system. 27.What are the parameters to be considered for selecting memory devices for embedded systems? (MAY 2015) Program information are the instructions i.e. Opcodes that are to be executed by the processor. Generally they are stored in a non-volatile memory that is mapped directly to the address space of the processor. Or they might be stored in external memories (say as files in a partition) and loaded on to a volatile memory just prior to execution of the program.

Data memory can be used to store primarily two kind of information. One is relating to the intermediate data being processed- for e.g. a variable storing a value during course of execution of an algorithm or a Process Control Block in an OS etc. The other is the Stack which is used by the processor to store its return functions and local variables. In either case the memory type is volatile. 28. List the characteristics of an embedded system? (MAY2017), (MAY2018) 1. Real time and multi rate operations 2. Complex Algorithms 3. Complex graphic user interface (GUIs) and other user interfaces 4. Dedicated fuctions. 29. What are the steps involved in build process?(DEC 2016) (MAY2017) Building embedded system can be focused into two technical perspective such as (i) Software Perspective (ii) Hardware Perspective (i) Software Perpective a. Processor begins executing at the reset vector. The initial program loader (IPL) locates the OS image and transfers control to the startup program in the image. b. Startup program configures the system and transfer control to the microkernel and process manager. c. The procnto module loads additional drivers and application Programs. (ii) Hardwar Perpective a. Processor b. Source of initialization and configuration info c. Strage media d. Input devices.

30. Draw the block diagram of embedded system. (DEC 2017)

UNIT – II EMBEDDED NETWORKING PART A 1. Define bus? (MAY2018) Buses: The exchange of information. Information is transferred between units of the microcomputer by collections of conductors called buses. There will be one conductor for each bit of information to be passed, e.g., 16 lines for a 16 bit address bus. There will be address, control, and data buses. 2. What is SerDes? • Serialization and De-serialization subunit in an IO device • It is used to perform serial to parallel and parallel to serial conversion during serial data communication automatically.

3. How error is detected in CAN bus? Receiver node uses the CRC code of the Control Area Network bus frame format to detect error. 4. Define On-board bus? On-board bus will be the bus which connects between the microcontroller unit and units like EEPROM, SD card, LCD display etc. 5. What is data-push programming style? Master writes a slave by sending slaves address & the data byte,Slavescannot initiate transfer, master sends read request & slave respond for that.

6. What is CSMA/AMP? • CSMA/AMP: Carrier Sense Multiple Access with Arbitration on Message Priority • A node stops transmitting on sensing a dominant bit (0), which indicates that another node is transmitting. 7. Differentiate hardware and software interrupt. When a device port is ready, a device or port generates an interrupt or when it completes the assigned action it generates an interrupt. These interrupts are called hardware interrupts. When software runtime exception condition is detected, either processor hardware or a software instruction SWI generates an interrupt for exception. An SWI instruction INT n in 80X86. 8. Explain the sequence of steps to be followed when an interrupt event occurred. CPU on interrupt event may initiate a further action by vectoring to a vector address and calling an ISR or else it continues with current process if the interrupt is masked or disabled. 9. Give the limitations of Polling Technique. (MAY2018) When using polling there is a greater chance of missing the events , and also the chances of two or more events may be triggered at the same time for execution and there is a possibility of the same event being executed again and again. 10. Define a device decoder. A circuit to take the system address bus signals as the as the input and generate a device select signals, CS for the port address selection during the device read or write instruction of the system processor. 11. Name the device drivers classification. 1.Block device drivers 2. Character device drivers 3. Network device drivers. 12. What is a Block device driver? This is a driver that is well suited for block device, which.transfer data in blocks, like for example, disk drivers. Such I//O device have block sized buffers as part of the buffer cache in memory. 13. What are some real world applications of CAN? Controller Area Networks are used in many different fields, the bulk of which are • Auto-motive industry • Factory Automation • Machine Control • Medical Equipment and devices,etc 14. List out the different standard versions for CAN. • Low speed CAN – 125 Kbps -11 bit identifier • Standard CAN 2.0A – 1Mbps -11 bit identifier

• External CAN 2.0B – 1Mbps – 29 bit identifier 15. Define PISO and SIPO. PISO: A shift register for a parallel input and serial output. It is used for serial bit reception in synchronous mode. SIPO: A shift registers for serial input and parallel output. It is used for serial bit transmission in synchronous mode. 16. Justify why embedded system design is so complex. Design Requirements:Embedded computers typically have tight constraints on both functionality and implementation. In particular, they must guarantee real time operation reactive to external events, conform to size and weight limits, budget power and cooling consumption, satisfy safety and reliability requirements, and meet tight cost targets.Real time/reactive operation, Small size, low weight, Safe and reliable, Harsh environment, Cost sensitivity, System-level requirements, Controlling physical systems, Power management. 17. What is I2C? The I2C module provides an interface between the TCI648x/C6472 device and other devices compliant with the I2C-bus specification and connected by way of an I2C-bus. External components attached to this 2-wire serial bus can transmit and receive up to 8-bit wide data to and from the device through the I2C module. I²C (Inter-Integrated Circuit) is a multi-master, multi-slave, single-ended, serial computer bus invented by Philips Semiconductor, known today as NXP Semiconductors, used for attaching low-speed peripherals to computermotherboards and embedded systems. 18. What is USB? Where it is used?( MAY 2015) (DEC 2014) Universal Serial Bus (USB) is an industry standard developed in the mid-1990s that defines the cables, connectors and communications protocols used in a bus for connection, communication, and power supply between computers and electronic devices. Short for Universal Serial Bus, USB is a standard that was introduced in 1995 by Intel, Compaq,Microsoft and other computer companies.USB 1.x is an external bus standard that supports data transfer rates of 12 Mbps and is capable of supporting up to 127 peripheral devices. 19. Why device drivers are necessary for interfacing a device with processor? (DEC 2014) In computing, a device driver (commonly referred to as a driver) is a computer program that operates or controls a particular type of device that is attached to a computer. A driver provides a software interface to hardware devices, enabling operating systems and other computer programs to access hardware functions without needing to know precise details of the hardware being used. A driver typically communicates with the device through the computer bus or communications subsystem to which the hardware connects. When a calling program invokes a routine in the driver, the driver issues commands to the device. Once the device sends data back to the driver, the driver may invoke routines in the original calling program. Drivers are hardware-dependent and operating-system-specific. 20. How SPI is differed from other serial interfaces?(MAY 2016) SPI(Synchronous Peripheral Interface) is a serial bus for interconnecting ICS. • It has a full duplex communication and addressing for each slave device is not needed. • It allows 8 bit to transmit and receive and it supports full master mode and slave mode. 21.What is the need for Device Driver.(MAY 2016) (DEC 2016)

A device driver is software for controlling, receiving and sending byte or a stream of bytes from or to a device.Eg.USB device driver program to communicate with PC.

22. What are the two actions take place in an SPI clock cycle? • The master sends a bit on the MOSI line which the slave reads from the same line. • The slave sends a bit on the MISO line and the master reads it from that same line. 23. How do we classify I/O devices? (Dec 2017) I/O devices are of three types namely sensory input, control output and data transfer devices 24. Draw the framework of I2C Bus? (Dec 2017).

25. What is mean by I/O bus or peripheral bus? Processors also need to communicate with peripherals, that is external input and output devices and this data pathway is called as the I/O bus or peripheral bus. 26. Mention few serial bus communication protocols. (May 2017) I2C, CAN, USB & firewire (IEEE1394) are widely used serial bus communication protocols. 27. List the features of CAN and SPI serial interfaces. (DEC 2016) (May 2017) Features of CAN 1. It is an Interconnection Network 2. Standard Version (i) Low Speed CAN – 125Kbps – 11bit identifier (ii) Standard CAN 2.0A – 1Mbps – 11 bit identifier (iii) Extended CAN 2.0B – 1 Mbps – 29 bit identifer Features of SPI 1. It is synchronous and full duplex serial data transfer protocal 2. It is a single Master, multi slave system.

UNIT – III EMBEDDED FIRMWARE DEVELOPMENT ENVIRONMENT PART A 8. What is EDLC? Embedded product development life cycle is an Analysis-Design-Implementation based standard problem solving approach for embedded product development. The first approach is to figure out and the last approach is to develop it. 9. What is the purpose of using EDLC? EDLC is essential for understanding the scope and complexity of the work involved in any embedded product development. It defines the interaction and activities among various groups of product development sector including project management, system design and development. 10. What are the various testing performed in a product development? Unit testing, Integration testing and System testing. 11. What is Smoke Testing? Non Exhaustive test to ensure that the crucial requirements for the product are functioning properly.This test is not giving importance to the finer details of different functionalities

12. Define Data flow graph? (Dec 2017) Data flow graph model is a data driven model in which the program execution is determined by data.This model emphasizes on the data and operations on the data which transforms the input data to output data. 13. What is control Data flow graph? CDFG model is used for modeling applications involving conditional program execution.This model contains both data operations and control operations. 14. What is the purpose of State machine cycle? (Dec 2017) The state machine cycle is used for modeling reactive or event driven embedded systems whose processing behavior are dependent on state transitions. 15. What is sequential program model? It acts as a conventional procedural programming,The program instructions are iterated and executed conditionally and the data gets transformed through a series of operation.FSMs are good choice for sequential program modeling. 16. Define Conceptualization. It a product concept development phase involves in scope of concept, Performs cost benefit analysis, feasibility study, prepares project management and risk management plans. 17. What is type checking? It is essential to make a program less prone to errors.For e.g, it does not allow arithmetic operations to be performed on char data types, also enables the usage of ’+’ for concatenation. 18. Define Finite State Machine(FSM). A Finite State Machines (FSM) is conceptual model to represent the discrete interactions in a system.A FSM is a conceptual model that represents how one single activity can change its behaviour over time, reaction to internally or externally triggered events. FSM is at a higher level than the actual software. This means it explains the behavior, but not how it can be implemented. It is an ideal representation; it doesn’t take time delays in account. 19. What do you mean by Re-entrant functions? It is usable by several tasks and routines synchronously (at the same time).This is because all the argument values are retrievable from the stack. 20. List the different phases of EDLC. (May 2017) Requirement analysis, Design, Development and test, Deployment and Maintenance. 21. What are the computational models commonly used in embedded system design? Data Flow Graph model, State Machine Model, Concurrent Process Model, Sequential Program Modeland Object Oriented Model. 22. What are the three primary objectives of EDLC? Ensure that high quality products are delivered to end user, Risk minimization and defect prevention in product development through project management, Maximize the productivity. 23. List out the requirements of Analysis phase? Functional capabilities like performance, Operational characteristics,Operational and Non- operational quality attributes, Product external interface requirements, Data requirements, User manuals, Operational and maintenance requirements, General assumptions. 24. What are the various testing performed in product development?

Unit testing, Integration testing, System testing, Load testing, Usability testing, Security testing, Smoke testing, Performance testing, User acceptance testing. 25. Differentiate between Simulator and Emulator in context of Embedded Systems.(MAY 2018) An emulator works by duplicating every aspect of the original device's behavior. It basically simulates all of the hardware the real device uses, allowing the exact same software to run on it unmodified. A simulator, on the other hand, sets up a similar environment to the original device's OS, but doesn't attempt to simulate the real device's hardware. Some programs may run a little differently, and it may require other changes . 26. Compare Data flow model and Finite state model. (MAY 2018) DFD - Also known as DFD, Data flow diagrams are used to graphically represent the flow of data in a business information system. DFD describes the processes that are involved in a systemto transfer data from the input to the file storage and reports generation.State Diagram - The behavior of an entity is not only a direct consequence of its inputs, but it also depends on its preceding state. 27. What are the drawbacks of Prototyping model? Deviations from extra cost and schedule due to requirements refinement, Increased project development, Minimal documentation on each prototype may create problems in backward prototype traceability, increased configuration management activities. 28. Define Prototyping Model Prototyping model is similar to the iterative model and the product is developed in multiple cycles.The only difference is that this model produces a more refined prototype of the product at the end of each cycle. 29. Define Finite state machine data path? FSMD architecture combines the controller architecture with data path architecture. It combines a controller with data path. The controller generates the control input whereas the data path processes the data. 30. What are hardware and software tradeoffs in embedded system design? Processing speed and performance, Frequency of change, Memory size and gate count and Reliability. 31. Mention different models used for development of an embedded system.(MAY 2016) Finite state machine, Petri net, Control and dataflow graph, activity diagram based UML model, Synchronous data flow graph, Timed Petri net and extended predicate/transition net and Multithreaded graph. 25. What are the processes involved in co-design.(MAY 2016) (DEC 2016) Selecting the model, Selecting the architecture, Controller architecture, Datapath architecture , Finite State Machine Datapath(FSMD), Complex Instruction Set Computing(CISC), Very Long Instruction Word(VLIW), Parallel Processing Architecture, Selecting the language, Partitioning System Requirements into hardware and software. 26. What is state machine model.(DEC 2016) (May 2017) The state machine model describes the system behavior with states, events, actions and transitions. The state machine model is used for modeling reactive or event driven embedded system whose processing behavior are dependent on state transitions.

UNIT–IV RTOS BASED EMBEDDED SYSTEM DESIGN PART A

1. Define Process. A process is a code that has its independent program counter values and an independent stack. It is a computational unit that processes on a CPU under the control of a scheduling kernel of an operating system. 2. Define Task. (MAY 2018) It is a computational unit or a set of codes, actions or functions that processes on a CPU under the control of a scheduling kernel of an operating system. Every task has a TCB. 3. What do you mean by Re-entrant functions? It is usable by several tasks and routines synchronously (at the same time).This is because all the argument values are retrievable from the stack. 4. What are the rules of Re-entrant functions? All the arguments pass the values &none of the arguments is a pointer whenever a calling function calls that function.When an operation is not atomic, that function should not operate on any variable, which is declared outside the function or which an interrupt service routine uses or which is a global variable but passed by reference and not passed by value as an argument into the function. The function that does not call any other function that is re- entrant. 5. What is a Semaphore? It is special variable used to take note of certain actions to prevent another task or event from proceeding. 6. What is the advantage of having multiple semaphores? Multiple semaphores are used so that different set of semaphores are shared among different set of tasks. 7. What is the disadvantage of cooperative scheduling? The disadvantage of cooperative scheduling is that a long execution of a low priority task makes a high priority task wait until it finishes. There is further disadvantage if the cooperative scheduler is cyclic but without a pre-defined time slices. 8. Define RTOS. Mention its functions. (May 2017) (DEC16) Operating system with real time task scheduling, interrupt latency control, synchronization of tasks with IPC’s predictable timing and synchronization behavior of the system. Functions: (i) Make the system convenient to use (ii) Organise and manage the system resources efficiently and correctly. 9. What is RPC? Remote procedure call is method used for connecting two remotely placed methods by first using a protocol for connecting the processes. It is used in the case of distributed tasks. 10. What are the parameters of TCB? It is memory block that holds information about the program counter, memory map, the signal(message)dispatch table, signal mask, task ID,CPU state(registers etc.), and a kernel stack(for executing system calls etc. 11. What is a mailbox? A message mailbox is an IPC queue that can be used only by a single destined task. 12. Differentiate a task from a thread. Task is kernel- controlled whereas threads are process-controlled. Task is a heavy-weight process whereas s thread is a light weight process and a task can call another task but a thread cannot call a task.

13. What is Dynamic program scheduling? The processor first analyzes the various tasks to be scheduled & then sets the timer based on it. 14. Define multithreading. (May 2017) The process by which one threads forks out many threads and a job is assigned to each thread. 15. What are counting semaphores? Unsigned integers that controls the blocking or running of codes of a task as well as of an accompanying task with which it shares value. 16. Mention the different task states. They are i) Idle ii) Ready iii) Running iv) Blocked (waiting) 17. What is priority inversion? A problem in which a low priority task inadvertently does not release the process for the high priority task. 18. When and where do we use spin locks? A spin lock successively tries to decrease the trial periods before finally blocking a task and does not block a running task instantly. 19. How are interrupt routines handled in RTOS environment? • Direct call to ISR by an interrupting source. • Direct call to RTOS by an interrupting source and temporary suspension of a scheduled task. • Direct call to RTOS by an interrupting source and scheduling of tasks as well as ISR’s by RTOS. 20. List the three methods by which fixed scheduling can be defined? Simulated annealing method, Heuristic method, Dynamic programming model. 21. What is meant by context switching? The process of pushing all of the registers at the beginning of an interrupt routine is known as saving the context and the process of popping them at the end are known as restoring the context. 22. Define interrupt latency. Interrupt latency refers to the amount of time it takes a system to respond to an interrupt. To keep interrupt latency low we should make interrupt routines short, Disable interrupts for only short periods of time 23. Distinguish between task and process. A program in execution is known as ‘process’. A program can have any number of processes. Every process has its own address space. Threads uses address spaces of the process. The difference between a thread and a process is, when the CPU switches from one process to another the current information needs to be saved in Process Descriptor and load the information of a new process. Switching from one thread to another is simple.A task is simply a set of instructions loaded into the memory. Threads can themselves split themselves into two or more simultaneously running tasks. 24. Compare user threads and kernel threads.(MAY 2018) User threads are supported and are implemented by a thread library at the user level. The library provides support for thread creation, scheduling, and management with no support from the kernel. When threads are managed in user space, each process needs its own private thread table to keep track of the threads in that process. Kernel threads are supported directly

by the operating system.The kernel performs thread creation, scheduling, and management in kernel space. 25. ComparePreemptive and Non-preemptive scheduling.(MAY 2016)(DEC2016) Under non-preemptive scheduling once the CPU has been allocated to a process, the process keeps the CPU until it releases the CPU either by terminating or switching to the waiting state. Preemptive scheduling can preempt a process which is utilizing the CPU in between its execution and give the CPU to another process. 26. Define Thread and Process.(MAY 2016) A thread otherwise called a lightweight process (LWP) is a basic unit of CPU utilization, it comprises of a thread id, a program counter, a register set and a stack. It shares with other threads belonging to the same process its code section, data section, and operating system resources such as open files and signals. Process is a computational unit that processes on a CPU under the control of a scheduling. Kernel of an OS. It has a process structure, called Process control block. A process defines a sequentially executing program and its state. 27. State the main differences between RTOS and conventional OS. The most common operating system for personal computer include Windows from Microsoft, OS X from Apple, and the wide variety of Linux variants that can be obtained from their respective developers. What most people do not know are Real-time Operating Systems or generally referred to by the acronym RTOS. These are operating systems that are used for more specialized applications that demand response that is as close to real time as possible. The most significant difference between the two is in how they approach each task. Standard operating systems focus on doing as much computation in the shortest span of time while RTOSes emphasize on having a predictable response time. 28. Define Semaphore signalling. (DEC2017) The simplest use for a semaphore is signaling, which means that one thread sends a signal to another to indicate that something has happened. Signaling makes it possible to guarantee that a section of code in one thread will run before a section of code in another; in other words, it solves the serialization problem. 29. What do you understand by real-time scheduling ? . (DEC2017) A real-time scheduling System is composed of the scheduler, clock and the processing hardware elements. In a real-time system, a process or task has schedulability; tasks are accepted by a real-time system and completed as specified by the task deadline depending on the characteristic of the scheduling algorithm.

UNIT –V EMBEDDED SYSTEM APPLICATION DEVELOPMENT PART A 1. Give examples for sophisticated embedded systems. • Embedded system for wireless LAN and for convergent technology devices • Embedded system for real time video and speech or multimedia processing systems • Security products and high speed network security. • Embedded sophisticated system for space lifeboat(NASA’s X-38) 2. List out the factors to be considered while writing embedded application programs. The various factors that must be considered while writing embedded application program are:Throughput, Response, Testability, Debug ability, Reliability, Memory space, Program installation, Power consumption, Cost

3. Define Spin Phase. In the second phase of washing, water is pumped out from the tub and the inner tub uses centrifugal force to wring out more water from the clothes by spinning at several hundred rotations per minute(RPM). This is called a “Spin Phase”. 4. Which type of memory is more suitable for embedded system? Justify your answer? The RAM family includes two important memory devices: static RAM (SRAM) and dynamic RAM (DRAM). The primary difference between them is the lifetime of the data they store. SRAM retains its contents as long as electrical power is applied to the chip. If the power is turned off or lost temporarily, its contents will be lost forever. DRAM, on the other hand, has an extremely short data lifetime-typically about four milliseconds. This is true even when power is applied constantly. Memories in the ROM family are distinguished by the methods used to write new data to them (usually called programming), and the number of times they can be rewritten. This classification reflects the evolution of ROM devices from hardwired to programmable to erasable-and-programmable. A common feature of all these devices is their ability to retain data and programs forever, even during a power failure. 5. Define Local Interconnect Network (LIN). LIN bus is a single master multiple slave communication interface.LIN is low speed, single wire communication interface with support for data rates upto 20Kbps and is used for sensor/actuator interfacing.LIN bus follows the master communication triggering technique to eliminate the possible bus arbitration problem that can occur by the simultaneous talking of different slave nodes connected to a single interface bus. 6. Give some applications where LIN bus is used in automobile systems. LIN bus is employed in applications like mirror controls, fan controls, seat positioning controls, Window controls and position controls where response time is not a critical issue. 7. What is meant by Low Speed Electronic Control Units? Low Speed ECUs are deployed in applications where response time is not critical. They are generally built around low cost microprocessors/ microcontrollers and digital signal processors. 8. Give some applications of Low Speed ECUs. Audio controllers, passenger and driver door locks, door glass controls, etc. are examples for Low Speed ECUs. 9. Explain Development board? Development board is a hardware in which we are having all the possible input and output device and serial and parallel ports for porting the downloadable image file in to the target hardware. 10. What do you mean by software timer? A software Timer is software that executes and increases or decreases a count variable on an interrupt on a timer output or on a real time clock input. 11. Define protocol. A way of transmitting messages on a network by using software that adds additional bits such as the starting bits, header, address of source and destination, error control and ending bits. A protocol suite may have multiple layers and each layer or sub layer, its protocol before a message transmits on a network. 12. Explain handshake signals.

The signals exchanged before the actual data transfer or storing the bits at the port buffer or the signals to setup or end the communication between source and destination. 13. Define microarchitecture. When processor architecture refers specifically to the architectural instruction sets and programmers model, the term microarchitecture refers specifically to the implementation of those architectures. A processor may have CISC architecture with an RISC microarchitecture implementation. 14. List some applications of embedded system.(May 2016)(Dec 16) (May 2017)(Dec 2017) • Consumer electronics, e.g., cameras, camcorders, etc. • Consumer products, e.g., washers, microwave ovens, etc. • Automobiles (anti-lock braking, engine control), etc., • Industrial process controllers & avionics/defense applications • Computer/Communication products, e.g., printers, FAX machines, etc. • Emerging multimedia applications & consumer electronics. 15. What are the events involved in smart card application.(May 2016)(Dec 16) Step 1:Receive from the host, on card installation, the radiation of carrier frequency or clock signals in case of contact with the card. Extract charge for the system power supply for the modem, processor, memories and port IO device. Step 2: Execute codes for a boot up task on reset task. The code begins to execute from the main and the main creates and initiates this task and starts the smart OS. 16. Define Software of Soft modem. A software modem or a soft modem is a modem with minimum hardware capacities, designed to host computers resources to perform most of the task performed by a dedicated hardware in a traditional modem. 17. List the advantages of software modem. It can be of easily upgradable of newer modem. It can be programmed other than a modem, for example it could emulate an answering machine or a signal generator.It is less cost and less weight.Low power Consumption.It can be used as a portable modem in laptops and PDAs. 18. Define Engine control unit. Engine Control units control the operating parameters, to make sure that the engine gets proper inputs. Thus, they help protect the engine against damage. Engine control unit is the brain of the engine. 19. List the applications of Micro Controller operating Systems. (May 2018) 1. Round Robin Scheduling. 2. For Managing tasks, 3. For kernels, 4. Managing memory and time 20. What is meant by high speed ECU? High speed ECUs are deployed in critical control units requiring fast response, like fuel injection systems, antilock brake system, engine control, electronic throttle, steering controls, transmission control and central control unit. 21. Define MOST Bus. The MOST bus is targeted for automotive audio/video equipment interfacing, used primarily in European cars. A MOST bus is a multimedia fiber optic point to point network implemented in a star, ring or daisy chained topology over optical fiber cables. The MOST bus specifications define the physical layer as well as the application layer, network layer and media access control. MOST bus is an optical fiber cable connected between the Electrical

Optical Converter(EOC) and Optical Electrical Converter (OEC), which would translate into the optical cable MOST bus.

22. Draw the system components in the smart card. (May 2017)

23. What are the basic requirement while designing an embedded system ? (Dec 2017) The basic requirements while designing the embedded system are hardware architecture, suitable software and proper signal processing.

EE8002- DESIGN OF ELECTRICAL APPARATUS UNIT-I DESIGN OF FIELD SYSTEM AND ARMATURE PART-A Major considerations in Electrical Machine Design 1. What is design? What are the conditions for an optimum design? Design may be defined as the creative physical realization of theoretical concepts. Engineering design is the application of science, technology and invention to produce machines to perform the specified task with optimum economy and efficiency. A design is said to be optimum when the following conditions are satisfied: • Minimum loss (or) Maximum efficiency • Minimum cost • Minimum volume • Minimum weight 2. Mention the different circuits available in a machine. • Magnetic circuit • Electric circuit • Dielectric circuit • Thermal circuit • Mechanical parts 3. What are the major design considerations? (M’13, Dec 2015 & May 2018) The major considerations to evolve a good design are: • Cost: Low initial cost and reasonable operating cost. • Durability: Quality of lasting for a long time • Compliance:Suffering with performance criteria as laid down in the specifications. • Weight: Lower 4. What are the limitations in design? • Saturation • Temperature rise • Stress on insulation • Efficiency • Mechanical precision of air gap • Commutation • Power factor • Specifications • Stress & strain on rotating parts and bearings. 5. What are the major design factors? The major design factors of electrical machines are as follows Magnetic circuit or the flux path: Should establish required amount of flux using minimum MMF. The core losses should be less. Electric circuit or windings: Should ensure required EMF is induced with no complexity in winding arrangement. The copper losses should be less. Insulation:

Should ensure trouble free separation of machine parts operating at different potential and confine the current in the prescribed paths. Cooling system or ventilation: Should ensure that machine operates at the specified temperature. Machine parts: Should be robust. Materials for Electrical apparatus 6. What are the different conducting materials used in rotating machines? (D’18) The most common conducting materials used in rotating machines are Copper – usually used in windings of electric machines Aluminum – substitute for copper due to low cost - Armature and field winding Iron and Steel- Pole core and Armature core Alloys of copper – Bronze – brush holders, commutator segment, cage windings Copper silver alloy - bearings. 7. Mention the requirements of highly conducting materials. • Highest possible conductivity (least resistivity) • Least possible temperature coefficient of resistance • Adequate mechanical strength, in particular, high tensile strength and degree of flexibility (absence of brittleness) • Rollability and drawability • Good weldability and solderability • Adequate resistance to corrosion 8. What is a soft magnetic material? Soft magnetic materials are easy to magnetize and demagnetize. These materials are used for making temporary magnets. Soft magnetic materials with narrow hysteresis loop are called soft magnetic material. 9. What are the causes of failure of insulation? a. Weak points in the insulator b. Effect of aging and mechanical fatigue c. Mica migration d. Tracking. 10. List out the classification of the resistivity materials. Resistivity materials are classified as Nickel, Sliver, and Iron. • The first group consists of materials used in precision measuring instruments and in making standard resistances boxes. • The second group consists of materials from which resistance elements are made for all kinds of rheostats and similar control devices. • The third group consists of materials suitable for making high temperature elements for electric furnaces, heating devices and loading rheostats. 11. What are the different types of magnetic materials according to their degree of magnetism? (May 2011, Dec 2011 & May 2014) Magnetic materials are classified in to three groups as: Ferromagnetic materials • Relative permeability (µr) >1 • Values depend upon the magnetizing force Paramagnetic materials • Relative permeability (µr) is only slightly greater than unity. • The value of susceptibility is positive for these materials

Diamagnetic materials • Relative permeability (µr) is only slightly less than unity. • In paramagnetic and diamagnetic materials, the values of permeability is independent of the magnetizing forces 12. What are the electrical properties of insulating materials? (M’17,’19) • High dielectric strength, sustained at elevated temperature. • High resistivity or specific resistance. • Low dielectric hysteresis. • Good thermal conductivity. • High degree of thermal stability i.e., it should not deteriorate at high temperature. • Ability to withstand moisture, chemical attack, heat and other conditions of proposed service. Design of Magnetic circuits – Magnetising current – Flux leakage – Leakage in Armature. 13. What is a magnetic circuit? How is the mmf of a magnetic circuit determined? The magnetic circuit is the path of magnetic flux. The mmf of the circuit creates flux in the path against the reluctance of the path. The equation which relates flux, mmf and reluctance is given by, mmf Flux = Re luc tan ce The mmf of a magnetic circuit determined is determined as follows: • The magnetic circuit is split into convenient parts (section) which may be connected in series or parallel. Then the reluctance, flux density and mmf for every section of the magnetic circuit is estimated. • The summation of mmf for all section in series gives the total mmf for the magnetic circuit. 14. What are the constituents of magnetic circuits of DC Machine? (D’18) • The various elements in the flux path of salient pole machine are poles, pole shoes, air gap, armature teeth, armature core and yoke. • The various elements in the flux path of non- salient pole machines are: Stator core, Stator teeth, Air gap, Rotor teeth and Rotor core. 15. Write any two similarities & difference between magnetic and electric circuits. Similarities: • In an electric circuit, the emf circulates current in a closed path, similarity in a magnetic circuit, the mmf creates flux in a closed path. • In electric circuit the flow of current is opposed by resistance of the circuit. Similarity in a magnetic circuit the creation of flux is opposed by reluctance of the circuit. Differences: • When the current flows in a circuit the energy is spent continuously, whereas in magnetic circuit the energy is needed only to create the flux but not to maintain it. • Current actually flows in a magnetic circuit, whereas the flux does not flow in a magnetic circuit but it is only assumed to flow.

16. What is meant by magnetic circuit calculation? Magnetic circuit calculation involves two types of problems as follows: • It is required to determine the mmf needed to establish a desired flux at a given point in a magnetic circuit. • The flux or flux density is unknown and is required to be determined for a given geometry of the magnetic circuit and specified mmf. The magnetic circuit is split up into convenient parts which may be connected in series or parallel. The flux density is calculated in every part and mmf per unit length, ‘at’ is found by consulting B-at curves. The calculations of mmf in the magnetic parts are simple whereas in case of air gap and tapered teeth, the calculations are complex. 17. Write ohm’s law for magnetic circuit. The equation relating mmf, flux and reluctance of a magnetic circuit can be called ohm’s law of magnetic circuit. The ohm’s law of magnetic circuit can be stated as follows: The mmf of a magnetic circuit is directly proportional to flux established in it provided no part of the magnetic circuit is saturated. The constant of proportionality is the reluctance of the magnetic circuit.,i.e., mmf α flux, mmf=flux x reluctance 18. What is importance of magnetizing current? The total mmf required for an electric machine for excitation of winding is decided by the magnetizing current. The value of exciting or magnetizing current depends upon the total mmf required, the no. of turns in the exciting current and upon the way in which the winding is distributed. 19. How is the magnetic curve made use of in the design of electrical machines? (or) For magnetic circuit calculations, B-at curves are generally used. Why?

The magnetization curve is a graph showing the relation between the magnetic field intensity (H) and the flux density (B) of magnetic material. It is used to estimate the mmf required for flux path in the magnetic material and it is supplied by the manufactures of stampings or laminations. In electrical machine design the magnetization curve is used to determine the mmf per metre of magnetic path in any part of the machine. First the flux density in any part of the machine is calculated and then for this value of flux density (B) the

value of mmf per metre (at) is determined from magnetism curve (B-at curve). 20. List the various steps involved in the estimation of mmf for a section of magnetic circuit. The various steps in the estimation of mmf for a section of magnetic circuit are: • Determine the flux in the concerned section. • Calculate the area of cross section of the section. • Calculate the flux density in the section. • From B-at curve of the magnetic material, determine the mmf per metre (at) for the calculate flux density. • The mmf of the section is given by the product of length of the section and mmf per metre 21. Define gap contraction factor for slots & ducts. The gap contraction factor for slots (Kgs) is defined as the ratio of reluctance of air- gap in machines with slotted armature to the reluctance of air-gap in machines with smooth armature. Reluctance of air gap in machines with slotted armature K = gs Reluctance of air gap in machines with smooth armature The gap contraction factor for ducts (Kgd) is defined as the ratio of reluctance of air-gap in machines with ducts to the reluctance of air-gap in machines without ducts. Reluctance of air gap in machines with ducts K = gd Reluctance of air gap in machines without ducts 22. Define total gap contraction factor. The total gap contraction factor (Kg) is defined as the ratio of reluctance of air-gap in machines with slotted armature & ducts to the reluctance of air-gap in machines with smooth armature & without ducts. Reluctance of air gap in machines with slotted armature & ducts K = g Reluctance of air gap in machines with smooth armature & ducts The total gap contraction factor is equal to the product of gap contraction factor for slots and ducts. 23. What is carter’s coefficient? Write down the carter’s coefficient of d.c. machine. (N/D’15, M’19) • The carter’s coefficient is a parameter that can be used to estimate the contracted or effective slot pitch in case of armatures with open or semi enclosed slots. It is a function of the ratio of w0 / lg where w0 is slot opening and lg is air-gap length. The carter’s coefficient is also used to estimate the effective length of armature when ducts are employed. In this case it will be a function of wd / lg .-where wd is width of the ducts and lg is air-gap length. • In electrical machine design the carter’s coefficient is used to estimate the air-gap expansion (or contraction) factor for slots and ducts 24. Write down the importance of gap contraction factor for slots and ducts in the design of magnetic circuits. • The slots and ducts in the armature (or stator & rotor) of electrical machine increases the reluctance of air gap, which in turn increases the mmf required for air-gap. The gap contraction factor represents the increase in air-gap length.

• Hence with the knowledge of gap contraction factor the mmf required for air-gap can be estimated without calculating the increase in reluctance due to slots & ducts. 25. Write the expression for gap contraction factor for slots and ducts. y Gap contractio n factor for slots, K gs = s y − K W s cs s y Gap contractio n factor for ducts, K gd = s L − K cd ndWd Where, ys =Slot pitch, Ws = Width of slots ,Kcs =Carter's coefficient for slots L - Length of armature, Kcd =Carter's coefficient for ducts, nd =Number of ducts, Wd =Width of each duct 26. Write the expression for reluctance of air gap in machines with smooth armature and slotted armature. Reluctance of air gap in machines with smooth armature is given by,

lg Rg = 0 Lys Reluctance of air gap in machines with slotted armature is given by,

lg Rg = 0 L(ys −Ws ) Where, ys =Slot pitch, L - Length of armature, Ws = Width of slots lg= length of the air gap. 27. Define copper space factor of a coil. (M/J’15, A/M’19). Copper space factor (Sf) of coil is defined as the ratio of conductor area and the area of cross section of the coil. Copper Sf = Conductor area/ Area of cross section of the coil Where, conductor area= Number of turns X Area of cross section of the coil 28. Define field form factor. (M/J’12) (N/D’16) Field form factor is defined as the ratio of average gap density over the pole pitch to maximum flux density in the air-gap.

Bav Field form, factorK f = Bg

29. Define air-gap expansion factor (Kg). The air gap expansion factor is defined as the ratio of reluctance of air gap in machine with slotted armature & duct to the reluctance of air gap in machine with smooth armature and without ducts. Reluctance of air gap in machines with slotted armature & duct Air gap expansion factor (K ) = g Reluctance of air gap in machines with smooth armature and without ducts 30. What is the effect of salient poles on the air-gap mmf? Write an expression to find the total mmf required to be produced by each pole.

In salient pole machines, the length of air-gap is not constant over the whole pole pitch. Hence the effective air-gap length is given by Kglg where kg is the gap contraction factor. Also, for calculating the maximum mmf, the maximum gap density Bg at the centre of the pole is considered instead of average gap density.

mmfforairgapBklin 800, 000 = AT ggg Total mmf produced by each pole = mmf for air gap+ mmf for teeth+ mmf for pole & pole shoe+ mmf for yoke+ mmf for armature core. 31. List the three methods used for estimating the mmf for teeth. What are the problems encountered in estimating the mmf for teeth? The following are the three methods used for estimating the mmf for teeth are • Graphical method • Three ordinate method (Simpson’s rule) • Bt 1/3 method The problems encountered in estimating the mmf for teeth are: • The flux density in the different section of the tooth is not uniform. • The slot provides another parallel path for the flux, shunting the tooth. 32. Give the relation between apparent flux density and real flux density. The real and apparent flux density are related by the equation Real flux density Breal = Bapp - µ0 at real (ks -1) -7 Where, ks= Lys/wtLi, Bapp - Apparent flux density , µ 0 = 4π *10 - Permeability of free space, atreal - at for Breal of tooth, L=length of armature, Li - Net iron length of armature, y s - slot pitch, wt - Width of tooth 33. Distinguish between real and apparent flux densities in a DC machine. (N/D’12,15,16) Real flux density Apparent flux density Real flux density is defined as the Apparent flux density is defined as ratio of total flux in a slot pitch to the ratio between actual flux in a the tooth area. tooth to the tooth area. Breal= φi/Ai Bapp= φs/Aa= (φi + φa)/ Ai Where, Where, φi= flux passing through iron φs=flux over one slot pitch (tooth) over a slot pitch φa= flux passing through air (slot) Ai = area of iron path of tooth over a slot pitch Real flux density is due to the Apparent flux density is due to the actual flux flowing through a total flux that has to pass through the tooth. tooth. Real flux density in teeth is less Apparent or the total flux density is than apparent flux density, since higher than real flux density some of the flux pass through the slot.

34. What is meant by unbalanced magnetic pull? (M/J’12) The unbalanced magnetic pull is the radial force acting on the rotor due to non uniform air gap around armature periphery. The undesirable effects of unbalanced magnetic pull are: • Saturation of magnetic materials due to reduction in air-gap.

• Excessive vibration and noise due to unbalanced radial forces. The techniques employed to overcome the unbalanced magnetic pull are: • The length of the rotor is kept small and diameter is made higher. • Ball bearings are employed and rotor is dynamically balanced. • The combinations of rotor and stator slots which produce vibrations are avoided. 35. What is magnetic leakage (or leakage flux) and leakage coefficient? Mention the importance of leakage coefficient in the design of magnetic circuits. The leakage flux (Φl) is the flux passing through the unwanted path. The leakage flux will not help either in the transfer or conversion of energy. The leakage coefficient (CΦ) is defined as the ratio of total flux (Φt) to useful flux (Φu). Importance: • Leakage coefficient is used to estimate the leakage flux. Since the leakage flux affects the performance of transformers and rotating machine, the knowledge of leakage flux is essential. • The leakage flux affects the excitation demand, regulation, forces on the winding under short circuit conditions, commutation, stray load losses and circulating currents is transformers tank walls 36. What is fringing flux? Bring out the difference between leakage flux and fringing flux. The bulging of magnetic path at the air gap is called fringing. The fluxes in the bulged portion are called fringing flux. Difference between leakage flux and fringing flux: Leakage flux Fringing flux The leakage flux is not useful for energy It is useful flux which takes part conversion. in the energy conversion. The leakage flux flows in the But the fringing flux flows in the unwanted path magnetic path. The effect of leakage flux on machine The fringing flux increases the performance is accounted by leakage slot reactance. reactance. 37. List some leakage fluxes available in the rotating machine. The different types of armature or stator leakages fluxes are • Slot leakage flux • Zigzag leakage flux • Harmonic leakage flux • Peripheral leakage flux • Tooth top leakage flux • Overhang leakage flux • Skew leakage flux 38. How can you minimize the magnetic leakage in DC machines? In DC machines, the leakage flux passing in non- useful paths affects the field excitation. The excitation has to be increased to compensate for loss of flux. Most of the leakage fluxes are flowing through air gap of the machine. If the air-gap of the machine is kept as low as possible, these leakage fluxes can be minimized. The

harmonic leakage can be minimized by balancing the stator and rotor currents. The slot leakage can be minimized if the width of the slot is more than the width of tooth. 39. List the factors that are affected in rotating machines by leakage flux. The leakage flux affects the following performances indices of electrical machines. • Excitation demand of salient pole machines • Performance of ac machine depends on the leakage reactance • Forces between the windings of generators and transformers • Voltage regulation of generators and transformers • Communication condition in dc machine • Stray load losses • Circulating currents in transformers tank walls Design of lap winding and wave winding 40. Define winding pitch and commutator pitch. Winding pitch (Y) is defined as the distance between the starts of two consecutive coils measured in terms of coil sides. Commutator pitch (Yc) is defined as the distance between the commutator segments to which the two ends (start and finish) of a coil are connected. It is measured in terms of commutator segment. 41. What is front pitch and back pitch? The front pitch (Yf) is the distance between two coil sides connected to the same commutator segment. The distance between top and bottom coil sides of a coil measured around the back of the armature is called back pitch (Yb). Both front pitch and back pitch is measured interns of coil sides 42. What is equalizer connection? The equalizer connection are low resistance copper conductors employed in lap winding to equalize the induced emfs in parallel paths. The difference in induced emf in parallels is due to slight asymmetry in flux per pole in dc machines. 43. How will select the back and front pitch for a lap winding? The back pitch should be nearly equal to coil sides per pole and it should be an odd integer. Back pitch (Yb) = No. of coil sides / No.of poles + K = (2C/p ) + K K is a constant which will make 2C/p as an odd integer. For lap winding, winding pitch Y= Yb – Yf, Always winding pitch is 2 in lap winding Front pitch Yf = Yb – Y = Yb – 2 44. What is simplex and multiplex winding? • In simplex lap winding the number of parallel paths is equals to the number of poles and in simplex wave winding the number of parallel paths is two. • In multiplex winding the number of parallel paths will be multiples of simplex winding • In duplex winding the number of parallel paths will be double that of simplex winding and in triplex winding the number of parallel paths is thrice that of simples winding and so on. 45. How equalizer connection is made?

In armature winding the points at same potential under ideal conditions are connected to same equalizer rings. The connection are made using copper conductors usually in the form of rings and so they are called equalizer ring 46. Why equalizer connections are not needed in wave winding? In simplex wave winding there are only two parallel paths. The conductors forming a parallel path will be distributed equally under all poles. Hence both the parallel paths are equally affected by the asymmetry in the magnetic circuit and so there is no circulating current. Therefore, there is no necessity for equalizer connections 47. What is split coil? The split coils will have more than two coil sides, when all the top coil sides of a coil are lying in one slot and their corresponding bottom coil sides are accommodated in two different slots then the coil is called split coil 48. What is progressive and retrogressive winding? • In wave winding, ff after one round of the armature the coil falls in a slot right to its starting slot the winging is called Progressive wave winding. • If after one round of the armature the coil falls in a slot left to its starting slot the winging is called Retrogressive wave winding. 49. What is the purpose of dummy coil? The wave winding is possible only with particular number of conductors and slots combinations. It is not always possible to have the standard stampings in the winding shop consist of the number of slots according to the design requirements. In such cases dummy coils are employed. This coil is placed in the slots to give the machine the mechanical balance but they are not electrically connected to the rest of the winding 50. Mention the two types of armature winding used in DC machine and compare (May 2018) S. No. Lap winding Wave winding

1 Number of parallel path Number of parallel path (A)= Poles (P) (A)= 2 Always 2 Number of brush sets required is Number of brush sets required is equal to number of poles always equal to two

3 Preferable for high current, Preferable for high voltage, low voltage capacity low current capacity generators generators 4 Normally used for generators of Preferred for generators of capacity more than 500A capacity less than 500A

UNIT-II DESIGN OF TRANSFORMERS PART A Construction 1. List the various types of transformers.

The transformer can be classified based on construction, applications, frequency range, number of winding and type of connection Based on construction the transformers are classified as: • Core type • Shell type Based on applications the transformers are classified as: • Distribution transformers • Instrumental transformers • Power transformers • Electronic Transformers • Special Transformers Based on frequency range the transformers are classified as: • Power frequency transformer • Wide band transformer • Audio frequency transformer • Narrow band transformer • Pulse transformer • UHF transformer Based on number of windings the transformer are classified as; • Auto transformer • Two winding transformer Based on the type of construction the transformers are classified as: • Single Phase transformer • Three Phase transformer 2. What are the salient features of distribution transformers? The salient features of distribution transformers are: • The distribution transformers will have low iron loss and higher value of copper loss. • The capacity of transformers will be up to 500KVA. • The transformers will have plain walled tanks or provided with cooling tubes or radiators. • The leakage reactance and regulation will be low. 3. Write about distribution & power transformers and mention its use? Distribution Transformer: A distribution transformer is a transformer that provides the final voltage transformation in the electric power distribution system, stepping down the voltage used in the distribution lines to the level used by the customer. If mounted on a utility pole, they are called as pole-mount transformers. If the distribution lines are located at ground level or underground, distribution transformers are mounted on concrete pads and locked in steel cases, thus known as pad-mount transformers. Uses: The transformers used at load centers to step down distribution voltage to a standard service voltage required for consumers. Power Transformer: The transformers used in sub stations and generating stations for step down or step

up the voltage are called power transformers Uses: • In generating station, the power transformers are used to step –up the voltage to a higher level required for primary transmission. • In substations the power transformers are used to step down the voltage from to a lower level required for secondary transmission. 4. How does the design of distribution transformer differ from that of a power transformer? • The distribution transformers are designed to have low iron loss and higher copper loss, whereas in power transformers, the copper loss will be lesser than iron loss. • The distribution transformers are designed to have the maximum efficiency at a load much lesser than full load, whereas the power transformers are designed to have maximum efficiency at or near full load • In distribution transformer the leakage reactance is kept low to have better regulation, whereas the power transformer in leakage reactance is kept high to limit the short circuit current. 5. What is transformer bank? A transformer bank consists of three independent single phase transformers with their primary and secondary windings connected either is star or delta. 6. What is yoke section of a transformer? What is its use? The sections of the core which connect the limbs are called yoke. The yoke is used to provide a closed path for flux. 7. Mention the uses of distribution transformer. The distribution transformers are used at load centres to step down the transmission line voltage to a standard voltage required for consumers. 8. Why is low voltage winding placed near the core in transformers? • The winding & core are both made of metals and so on insulation has to be placed between them. The thickness of insulation depends upon voltage rating of the winding. • In order to reduce the insulation requirement, the low voltage winding is placed near the core. 9. Write the advantages of shell type transformers over core type transformers. (Or) Distinguish between core and shell type transformers. (N/D’13) (M/J’16) (M/J’12) S.no Core Type Shell Type 1 The winding encircles the The core encircles the most part of core. the winding. 2 Cylindrical type coils are Generally, multi layer disc type or used. sandwich coils are used. Easy in design and Comparatively complex. 3 construction. Has low Mechanical 4 High Mechanical Strength Strength 5 Reduction of leakage Reduction of leakage reactance is reactance is not easily highly possible. possible.

6 The assembly can be easily It cannot be easily dismantled for dismantled for repair work. repair work. 7 Better heat dissipation from Heat is not easily dissipated from winding. (natural cooling) winding since it is surrounded by core. (no natural cooling) Has longer mean length of It is not suitable for EHV requirements. core and shorter length of coil 8 turn. Hence best suited for EHV (Extra High voltage) requirements. 10. List the advantages of star connected transformers. • The phase voltage is smaller, and hence the major insulation required between windings and earth is smaller. • The number of turns is smaller with the result the amount of insulation used is smaller • In case of star connection, both phase and line voltages are available for four wire supply. This is useful for distribution purposes at lower voltages where both 1-phase and 3-phase loads have to be supplied. 11. Mention the advantage and disadvantages of 3-Phase transformers. Advantages of 3-Phase transformers: • A three-phase transformer is lighter, occupies lesser space, cheaper and more efficient than a bank of 1-phase transformers. The reason for lower cost of 3-Phase transformers: savings in cost of iron core of the tank and oil of the bushings • In case of 3-Phase transformers, there is only one unit to install and operate. Hence installation and operational costs are smaller for 3-Phase units. Disadvantages of 3-Phase transformers: • It is more difficult to transport a 3-Phase transformer as the weight per unit is more • In this event of a fault in any phase of a 3-Phase transformer, the fault is transferred to the other two phases. Therefore, the whole unit needs a replacement. 12. What is the range of current density used in the design of transformer windings? The choice of current density depends on the allowable temperature rise, copper loss and method of cooling. The range of current density for various types of transformers are given below. δ=1.1 to 2.2 A/mm2- For distribution transformers δ=1.1 to 2.2 A/mm2- For small power transformers with self-oil cooling δ=2.2 to 3.2 A/mm2- For large power transformers with self-oil cooling or air-blast δ=5.4 to 6.2 A/mm2- For large power transformers with forced circulation of oil or with water cooling coils 13. List the optimum quantities for transformer design. Transformer may be designed to make one of the following quantities as minimum: • Total volume • Total weight • Total cost • Total losses

In general, these requirements are contradictory and it is normally possible to satisfy  only one of them. All these quantities vary with ratio r = m .Thus r is the AT controlling factor for the above-mentioned quantities. Overall dimensions – design of yoke, core and winding for core and shell type transformers Estimation of No-load current 14. What are the advantages of using higher flux density in core? a. Reduction in core and yoke section b. Reduction in mean length of LV and HV winding c. Reduction in mean cost. 15. What are the disadvantages of using higher flux density in core? a. Increased magnetizing current & Iron loss b. Saturation of magnetic material c. Low efficiency d. Increased temperature Rise. 16. Why is the area of yoke of a transformer usually kept 15-20% more than that of core? The area of yoke is taken as 15 to 25 % larger than that of core of transformers using hot rolled silicon steel. This reduces the value of flux density obtained in the yoke, thereby resulting in the reduction of iron loss and the magnetizing current. In order to compensate for reduction in flux, the number of turns has to be increased. Therefore, the core area is not increased. The section of the yoke may be taken as rectangular or it may be stepped. In rectangular section yokes, the depth of the yoke is equal to the depth of the core. In stepped section, the depth of the yoke is equal to the width of the largest stamping. 17. Why is low voltage winding placed near the core in transformers? • The winding & core are both made of metals and so on insulation has to be placed between them. The thickness of insulation depends upon voltage rating of the winding. • In order to reduce the insulation requirement, the low voltage winding is placed near the core. • The low voltage winding is placed closer to the core because the high voltage winding requires more space due to its MV insulation. If designers put the LV winding outside the MV winding, the tank would be larger because the MV winding size would be about the same size. More insulating oil would be required, and the leakage reactance would be significantly higher. Remember, higher leakage reactance means that the voltage regulation (more aptly voltage drop) under load will be higher, which is not desirable. 18. What are the advantages of stepped cores? (M’15, D’14, 17, M’18, D’18) For same area of cross section, the stepped cores will have lesser diameter of circumscribing circle than square cores. This results in reduction in length of mean turn of the winding with consequent reduction in both copper cost and copper loss. 19. Why are the cores of large transformers built-up of cross section? (M15) The excessive leakage fluxes produced during short circuit and over loads develop severe mechanical stress in the coils. On circular coils these forces are radial and

there is no tendency for the coil to change its shape but on the rectangular coils the forces are perpendicular to the conductors and tend to deform the coil in circular. 20. Why the efficiency of transformer is so high? (May 2016) Since, transformer is a Static device (i.e. no moving parts in it) hence it is the most efficient machines ever made by man. However, in rotating machines there are various losses like friction and windage, losses in commutator etc. So, yes, Transformers are highly efficient devices. 21. Why is transformer yoke designed for low flux density? (Dec 2015) The transformer yoke is designed for low flux density because, higher flux density gives rise to the following disadvantages: • Increased magnetizing current and iron losses, • Saturation of magnetic material • Lower efficiency due to higher no-load losses • Higher temperature rise of transformers. 22. How the magnetic curves are used for calculating the no-load current of the transformer? (May2017) The B-H curve can be used to find the mmf per metre for the flux densities in yoke and core. The loss curve can be used to estimate the iron loss per kg for the flux densities in yoke and core. 23. What are the factors affecting the choice of flux density of core in a transformer? (May2011) (i) Core and yoke area (ii) Overall size and weight of the transformer (iii) Magnetizing current (iv) Iron loss (v) saturation and temperature rise. 24. What is meant by stacking factor? (May2012 & Dec2014) In transformers, the core is made up of laminations and the laminations are insulated from each other by a thin coating of varnish. Hence when the laminations are stacked to form the core, the actual iron area will be less than the core area. The ratio of iron area and total core is called stacking factor. The value is usually 0.9. Area of cross section of iron in the core Stacking factor, S = f Area of cross section of the core 25. How is iron losses reduced in transformers? (Dec 2018) Hence to reduce the iron losses of a transformer we have to reduce hysteresis and eddy current losses. Hysteresis losses are reduced by using a core material of smaller loop area and adding a small amount of silicon in core material. Whereas, to reduce eddy current losses, we increase resistivity of the core material. 26. Give the necessary expression for yoke design of transformers. For rectangular section of yokes, Area of yoke A y =Depth of yoke x height of yoke

ADHy= y y

Ay-(1.15 to 1.25) Agi for transformers using hot rolled steel Ay =Agi for transformer using grain-oriented steel 27. Write the expression for no load current of a single and three phase transformer.

Under no load condition (i.e., secondary of the transformer is open), the primary draws only small amount of current from the supply. This small (negligible) value of current is called as no load current. The no load current of a transformer has two components. They are magnetizing component(Im) and loss component(Il). The magnetizing current depends on the mmf required on the mmf required to establish the desired flux. The loss component of no- load current depends on iron loss. 22 No load current IIIo=+() m l 28. Define window space factor. (A/M’13,N/D’16, A/M’19) The window space factor is defined as the ratio of copper area in the window of the total window area. It depends upon the relative amounts of insulation and copper provided which in turn depends upon the voltage rating and output of transformers. Conductor area in window Window space factor,K= w Total area of window Temperature rise in Transformers & Design of Tank and cooling tubes of Transformers. 29. What are the cooling methods used for dry type transformers? The different methods of cooling of transformers are: • Air natural (AN) • Air Blast (AB) • Oil Forced (OF) • Oil Natural (ON) • Oil Forced Air Natural (OFAN) • Oil Forced Air Forced (OFAF) • Oil Natural Air Forced (ONAF) • Oil Forced Water Forced (OFWF) • Oil Natural Water Forced (ONWF) 30. How is the heat dissipation improved by the provision of cooling tubes? How is Heat dissipated in a transformer? (N/D’16) • If the temperature rise as calculated with plain wall tank exceeds the specified limits, it can be reduced by the provision of cooling tubes. • The cooling tubes will improve the circulation of oil. The circulation of oil is due to more effective pressure heads produced by columns of oil in tubes. • The cooling tubes increases the heat dissipation surface area and it can improve the oil circulation and thereby an additional 35 % tube dissipation becomes effective for convection. 31. What are the methods by which heat dissipation occurs in a transformer? (M/J’14) Heat dissipation in a transformer occurs by, • Conduction • Convection and • Radiation 32. Classify the transformer according to cooling methods. (M’19) According to cooling methods the transformers can be classified as follows • Air cooled Transformers- In this method the transformer is allowed to cool by natural

airflow surrounding it. • Oil cooled transformers-This method is used for oil immersed transformers. In this method, the heat generated in the core and winding is transferred to the oil • Water Cooled Transformers- forced water flow is used to dissipate hear from the heat exchangers. 33. Transformers with oil cooling should not be used in mining areas. Why? The high rating transformers are used for special applications like mines. The oil used for such high rating transformer cooling is inflammable (i.e, can easily set on fire), it has a high coefficient of volume expression with temperature. Hence leakage of cooling oil may create fire accidents in mines. Therefore, oil cooled transformers are not used in mines. 34. How will you fix the tank dimensions based on overall dimensions of transformer frame? • The dimensions of the tank are decided by the dimensions of the transformers frame and clearance required on all the sides. • The clearance on the sides depends on voltage and power rating of the winding. • The clearance at the top depends on the oil height above the assembled transformer and the space for mounting the terminals and tap changing transformers. • The clearance at the bottom depends on the space required for mounting the transformer frame inside the tank. 35. What are the factors to be considered for selecting the cooling method of a transformer? (N/D’12) What are the types of coolant used in transformers? The factors to be considered for selecting the cooling method of a transformer are: • kVA rating of transformer • Size of transformer • Area of application (site condition) The types coolants used in transformers are: • Air and Oil The transformers using air as a coolant are called dry type transformer while use oil as the coolant are called oil immersed transformers . 36. List the methods of transformer cooling using external heat exchangers. The method of cooling using external heat exchangers • The medium in contact with the windings • The circulation of the coolant in contact with the windings • The medium used in the external heat exchanger and • The circulation of the coolant in the external heat exchanger 37. Give the functions of transformer oil. The transformer oil has two prime functions: • To create an acceptable level of insulation in conjunction with insulated conductors and coils • To provide a cooling medium capable of extracting quantities of heat without deterioration as an insulating medium • Transformer oil consist of mainly four generic class of organic compounds namely, Aromatics, Paraffins, Napthenes and Olefines. 38. What is bushing?

• In electric power, a bushing is an insulated device that allows an electrical conductor to pass safely through a (usually) earthed conducting barrier such as the wall of a transformer or circuit breaker. • The bushings consists of a current carrying part in the form of a conductor rod, bus or cable , a porcelain cylinder installed in a hole in the transformer cover and used for isolating the current carrying part

UNIT-III DESIGN OF DC MACHINES PART-A Construction - Output Equations – Main Dimensions 1. What are the main dimensions of rotating machine? The main dimensions of a rotating machine are the • Armature diameter or stator bore, D • Armature or stator core length, L. 2. What is output equation? The equation which relates the power output to the main dimensions (D and L), specific loadings (Bav and ac) and speed (n) of a machine is known as output equation. 3. Write the output equation of DC machine and output co-efficient. (D’16, M’13) 2 Pa = C0 D L n , in kW 2 -3 3 C0 = π Bav ac x 10 in kW/m – rps where Pa – power developed in armature of dc machine C0 – output co-efficient D-Diameter of the machine, L-Length of the machine, n-Speed in rps Bav-Specific Magnetic Loading, ac – Specific Electric Loading 4. What is square pole criterion? The square pole criterion states that for a given flux and cross-section area of pole, the length of mean turn of field winding is minimum, when the periphery forms a square. This implies that the length L must be approximately equal to pole arc, ie) L = b = ψ τ 5. State the parameters governing slot utilization factor or slot space factor. • Voltage rating • Thickness of insulation • No. of conductors per slot • Area of cross-section of the conductor • Dimensions of conductor 6. List the different types of slots that are used in rotating machines. • Parallel sided slots with flat bottom • Tapered slots with flat bottom • Parallel sided slots with circular bottom • Tapered slots with circular bottom • Circular slots 7. List the factors governing the length of armature core in dc machines. (D’14) • Cost

• Ventilation • Voltage between adjacent commutator segments • Specific magnetic loading Choice of specific loadings – Selection of number of poles 8. What are the factors to be considered for the choice of specific magnetic loading? • Flux density in teeth • Frequency of flux reversals • Size of machine 9. What are the factors to be considered for the choice of specific electric loading? • Temperature rise • Size of machine • Speed of machine • Voltage • Armature reaction • Commutation 10. What is meant by magnetic loading? (May 2012) Total amount of flux available in air gap of armature pheriphery is called magnetic loading.It is denoted by Bav. The value lies between 0.4 to 0.8 Tesla for a DC machine 11. What are the effects of higher specific electric loading in dc machines? • Reduction in armature size • Reduction in magnetic material and cost. • Armature copper losses are increased. 12. Mention any two guiding factors for the choice of number of poles. (Dec 2011, May 2013, May 2014 & Dec 2016) • Frequency of flux reversal • Weight of iron • Weight of copper • Length of commutator. • Labour charge • Flash Over • Distribution of Distorted field form 13. Why square pole is preferred? (May-2016, 2015, Dec-2013) If the cross-section of the pole body is square than the length of the mean turn of field winding is minimum. Hence to reduce the copper requirement a square cross- section is preferred for the poles of dc machine. 14. Define specific electric and magnetic loading (Dec 2017 & Dec 2018) Specific Electric Loading: The total amount of ampere conductors available at the armature periphery per unit length is called specific electric loading. It is given by ac = Izz/πD, A/m, where Iz= current hrough conductor, z – no.of conductors, D- diameter of the core. Specific Magnetic loading : The average flux density over the air gap of a machine is known as specific magnetic loading. Bav = Total flux around the air gap/Area of

flux path at the air gap = PΦ/πDL, where P- no.of poles, Φ- flux per pole, D- diameter of the core , L- armature core length 15. State the advantages of having larger number of poles in DC Machines. (D’17) The following are the advantages of having larger number of poles in Dc machines. There is a reduction in (i) Weight of armature core and yoke (ii) Cost of armature and field conductors (iii) Overall length and diameter of the machine (iv) Length of commutator Design of Armature – Design of commutator and brushes Design of Armature main dimensions 16. Mention the factors that govern the choice of number of armature slots in a dc machine. • Slot pitch (mechanical difficulties) • Cooling of armature conductors • Flux pulsations • Commutation • Cost 17. What are the factors to be considered for deciding the slot dimensions? • Flux density in tooth • Flux pulsations • Eddy current loss in conductors & Reactance voltage 18. Discuss the parameters governing the length of commutator. The length of the commutator depends upon the number of brushes and clearance between the brushes. The surface area required to dissipate the heat generated by the commutator losses is provided by keeping sufficient length of commutator 19. What are the factors that influence the choice of commutator diameter?(M’19) • Peripheral speed • Peripheral voltage gradient should be limited to 3 V/mm • No. of coils in armature 20. What are the materials used for brushes in DC machines? • Natural graphite • Hard carbon • Electro graphite • Metal graphite 21. Find out the cross-sectional area of armature conductor of a four pole wave wound DC machine with armature current 200 A and armature winding current density 4.5 A/mm2. Current per conductor = 200/2 = 100 A Cross sectional area of conductor = 100/4.5 = 22.2 mm2. 22. What is reactance voltage? The reactance voltage is the induced voltage due to leakage flux. It is not beneficial. The reactance voltage will affect commutation and produce sparking in DC machines. The reactance voltage will increase the regulation in case of transformers and rotating machines.

23. State the parameters governing slot utilization factor or slot space factor. • Voltage rating • Thickness of insulation • No. of conductors per slot • Area of cross-section of the conductor • Dimensions of conductor 24. Name any two methods to reduce armature reaction. (May 2011) (i) Increasing the length of air gap at pole tips (ii) Increasing reluctance of pole tips (iii) Compensating winding (iv) Interpoles 25. What is meant by peripheral speed? Write the expression for peripheral speed of a rotating machine.(May 2012 & Dec 2013) The peripheral speed is a translational speed that may exist at the surface of the rotor, while it is rotating. It is translational speed equivalent to the angular speed at the surface of the rotor. peripheral speed Va=πDrn in m/sec. 26. What is meant by magnetic loading? (May 2012) Total amount of flux available in air gap of armature pheriphery is called magnetic loading.It is denoted by Bav. The value lies between 0.4 to 0.8 Tesla 27. Mention the factors governing the length of armature core in a dc machine? (Dec 2014) • Cost • Ventilation • Voltage between adjacent Commutator segments • Specific Magnetic Loading 28. Why dc motors are preferred in general? (Dec 2014) i. Speed control over a wide range both above and below the rated speed read ii. High starting torque iii. Accurate steep less speed with constant torque iv. Quick starting, stopping, reversing and acceleration and v. Free from harmonics, reactive power consumption 29. Write down the expression for brush friction loss. (May2017) Pbf= µPbABVc, Watts 2 Where, Pb= Brush contact pressure on commutator, N/m 2 AB= Total contact area of all brushes,m AB=p Ab , for lap AB= 2Ab, for wave Vc= Peripheral speed of commutator,m/sec 30. What factors decides the minimum number of armature coils? (M’18) • Slot Pitch • Cooling of armature conductors • Flux Pulsations • Commutation • Cost

UNIT-IV DESIGN OF INDUCTION MOTOR PART-A Construction - Output equation of Induction motor – Main dimensions

1. Write down the output equation for 3-phase induction motor (Dec2011, May2013 &May2014) The equation for input kVA is considered as output equation in induction motors. 2 The input kVA , Q=C0D Lns in kVA C0= output coefficient D,L-main dimensions ns= speed in rps. The rating of induction motor is sometimes given in horse power. This rating refers to the power output at the shaft of the motor. kVA input , Q=(HP x 0.746)/(η x cos φ) 2. Why is an induction motor also called as a rotating transformer? (Dec 2018) The principle of operation of induction motor is electromagnetic induction, which is similar to that of a transformer. The stator winding is equivalent to primary of a transformer and the rotor winding is equivalent to short circuited secondary of a transformer. In transformer the secondary is fixed but in induction motor it is allowed to rotate. The difference is that, the normal transformer is an alternating flux transformer, while the induction motor is a rotating flux transformer. Hence the induction motor is also called as a rotating transformer 3. Write down the equation for output coefficient in an induction motor. (M’19) The output coefficient of induction motor can be expressed as, -3 3 C0= 11kwsBav ac x 10 in kVA/m Where, C0= output coefficient Bav – Average flux density ac- ampere conductors Kws- winding factor The value of output coefficient depends on the value of specific electric loading (ac) and specific magnetic loading (Bav) 4. Name the different types of induction motor. How do they differ from each other? The two different types of induction motor are: • Three phase induction motor ✓ Squirrel cage induction motor and ✓ Slip ring induction motor • Single phase induction motor The stator is identical for both types but they differ in the construction of rotor. The squirrel cage rotor has copper or aluminium bars mounted on rotor slots and short circuited at both ends by end rings. The slip ring rotor carries a three-phase winding. One end of each phase is connected to a slip ring and other ends are star connected. 5. List the advantages of slip ring rotor over cage rotor. • The main advantage of a slip ring induction motor is that its speed can be controlled easily. • "Pull-out torque" can be achieved even from zero rpm. • It has a high starting torque when compared to squirrel cage induction motor. Approximately 200 - 250% of its full-load torque. • A squirrel cage induction motor takes 600% to 700% of the full load current, but a slip ring induction motor takes a very low starting current approximately 250% to 350% of the full load current

6. Why is fractional slot winding not used in induction motor? (May 2019) Windings with fractional number of slots per pole per phase create asymmetrical mmf distribution around the air gap and favour the creation of noise in the motor. Therefore fractional slot windings are not used in the stator of induction motors. The total number of slots is also a multiple of number of phases. So, integral slot windings are used. In integral slot winding, the total number of slots is chosen such that the number of slots per pole per phase is an integer 7. How can a 3-phase induction motor be designed for various design features? The ratio of core length to pole pitch (ration L/τ) for various design features are: Minimum cost- 1.5 to 2 Good power factor-1.0 to 1.25 Good efficiency -1.5 Good overall length -1 For best power factor the pole pitch τ is chosen such that,  = 0 . 1 8L The value of L/τ lies between 0.6 and 2 depending upon the size of machine and the characteristics desired. For normal design, the diameter should be chosen that the peripheral speed does not exceed about 30m/s. 8. What is slot space factor? The slot space factor is the ratio of conductor (or copper) area per slot and slot area. It gives an indication of the space occupied by the conductors and the space available for insulation. The slot space factor for induction motor varies for 0.25 to 0.4. High voltage machines have lower space factors owing to large thickness of insulation. Copper area per slot Slot space factor = Area of each slot

Choice of specific loadings – Design of squirrel cage rotor and slip ring rotor 9. List the factors to be considered for the choice of specific magnetic loading. The choice of specific magnetic loading depends on: • Power factor – With large values of Bav results in magnetizing current will be high, which results in poor power factor • Iron loss - With large values of Bav increased iron loss and decreased efficiency • Over load capacity • With large values of Bav provides, large values of flux per pole, the turns per phase will be less and so the leakage reactance will be less. Lower value of leakage reactance results in higher overload capacity 10. What are the factors to be considered for the choice of specific electric loading? The choice of specific electric loading depends on: • Copper loss • Temperature rise- Large values of ac results in higher copper losses and higher temperature rise • Voltage rating –Machine with high voltage rating smaller values of ac should be preferred. Since for high voltage machines the space required for insulation is large. • Over load capacity- For high overload capacity lower values of ac should be selected

11. Comment on the selection of values of overload capacity of induction motor. What is its impact if the “ac” value is higher? A large value of ampere conductors would result in large number of turns per phase. This would mean that, the leakage reactance of the machine becomes high and the diameter of the circle diagram is reduced resulting in reduced value of overload capacity. Therefore higher the values of a.c, lower would be the overload capacity. Hence the value of ampere conductors per metre depends upon the size of the motor, voltage of stator winding, type of ventilation and the overload capacity desired. It varies between 5000 to 45000 ampere conductors per meter 12. Mention the factors to be considered for the choice of number of slots of an induction machine. (May 2015) The factors to be considered for choice of number of slots of an induction machine are • Slot loading- slot loading should not exceed 750 amp. conductor • Slot pitch - The slot pitch should lie between 15mm and 25mm • Type of winding ➢ For integral slot winding the stator slots should be a multiple number of slots per pole per phase. ➢ For double layer winding, the conductors per slot should be even. Harmonic torque- Certain combinations of stator and rotor slots give rise to harmonic torques which results in crawling and cogging. To avoid these undesirable effects the difference between stator and rotor slots should not be equal 13. List the factors to be considered for estimating the length of air gap in induction motor. The following factors are to be considered for estimating the length of air-gap: • Power factor • Overload capacity • Pulsation loss • Unbalanced magnetic pull • Cooling 14. What happens when the air-gap of an induction motor is doubled?(D’16) (i)If the air gap of an induction motor is doubled then the mmf and magnetizing current approximately doubles. Because in induction motor the air gap requires large mmf when compared to rest of the magnetic circuit. (ii)Also the increase in air gap length increases the overload capacity, offers better cooling, reduces noise and reduces unbalanced magnetic pull. 15. Give the different formulas for calculating the length of air gap in induction motors. • For small induction motor,

lg =+0.2 2 DLmm • Alternate formula for small induction motor

l=0.125 + 0.35 D + L + 0.015 V mm ga • Alternate formula to use

l D=+ m m0 . 2 g • For machines with journal bearings

lDmm=−1.60.25 g Where, D, L – main dimensions expressed in metre Va= peripheral speed in metre per second. 16. Give the reasons for the rise of imbalanced magnetic pull in induction motors. The design of frames has special significance in the case of induction motor of large dimensions on account of the relatively small air gaps used in these machines. For induction motors, the frame should be strong and rigid both during the construction and after assembly of the machine. This is because the length of air gap is very small and if the frame is not rigid, the rotor will not remain concentric with stator giving rise to imbalanced magnetic pull 17. Write down the rules for selecting rotor slots of an squirrel cage IM. (May/June16) (Nov/Dec15) The following general rules should be concerning the choice of rotor slots for squirrel cage machines • The number of rotor slots should never be equal to the stator slots, but it must be either smaller or larger. • The difference between stator and rotor slots should not be equal to 3p to avoid synchronous cusps. • The difference between the number of stator and rotor slots should not be equal to 3p for 3 phase machines in order to avoid magnetic locking. • The difference between number of stator slots and rotor slots should not be equal to 1.2, (p+1) or (p+2) to avoid noise and vibrations 18.Where is mush winding used? (or) Where is basket winding used in Induction motors? (May15) The mush windings are used in small induction motors of ratings less than 5 HP having circular conductors. This is a single layer winding where all the coils have the same coil span (unlike concentric winding where the coils have different span). For small induction motors of the slip ring type, it is normal practice to use mush windings for rotor housed in semi closed slots. The coils are roughly formed outside the machine and dropped one by one, into the previously insulated slots. For large motors, a double layer bar type wave winding is used. This winding has generally two bars per slot. The bars are pushed through partially closed slots and are bent to shape at the other end. 19. Show a relation between D and L for best power factor. (May 2016) Foir best power fator the relation between D and L is as follows: τ = 0.18L where τ- pole pitch, L=Length of stator core. 20. What are the factors to be considered for selecting the number of slots in induction machine stator? (May 2012)(Dec 2017) The factors to be considered for selecting the number of slots is tooth pulsation loss, leakage reactance, magnetizing current, iron loss and cost. Also, the number of slots should be multiple of slots per pole per phase for integral slot winding.

21.Why rotor slots are skewed? i.) To reduce noise and vibrations, ii.) To avoid tendency of cogging, iii.) To reduce torque defects 22. What are the effects of increased air gap? i.) Increased magnetizing comment, ii.) Reduced PF, iii.) Improved cooling, iv.) Reduced noise. 23.How can crawling be prevented by design in an induction motor? (M’14) Crawling is a phenomenon in which the induction motor cannot accelerate upon its full speed but continues to run at a speed lesser than sub synchronous speed. This action is due to the fact that, flux wave produced by a stator winding is not purely sinusoidal. Instead, it is a complex wave consisting of a fundamental wave and odd harmonics like 3rd, 5th, 7th etc. Slotting produces harmonics of the order 6Aq± 1 in a three-phase machine, where A is any integer. Therefore, to prevent crawling, it is necessary to avoid the values of rotor slots exceeding stator slots by about 15-30%. 24. What is meant by cogging? (or) Why in an induction motor, the number of stator slots should never be equal to the number of rotor slots? (Dec 2018) When the number of rotor slots is equal to the number of stator slots, the speeds of all the harmonics produced by the stator slotting, coincides with the speed of corresponding rotor harmonics. Thus, the harmonics of every order would try to exert synchronous torques at their corresponding synchronous speeds and the machine would refuse to start. This is known as cogging. Therefore, the number of stator slots should never be equal to the number of rotor slots in an induction motor. 25. What happen if the airgap length of Induction Motor is doubled? (M’18) • The permeability of the magnetic circuit rotor-to-stator will decrease. • The magnetizing inductance of the motor thus decreases. • The magnetizing current will increase. This will cause a poorer power factor at all loads. • The magnetic flux in the air gap will decrease and leakage fluxes will increase. This will cause a reduction in the maximum available torque. 26. What is unbalanced magnetic pull in Induction Motor? (Dec 2017) A net radial force created due to change in position of the rotor at this unstable point pulls the rotor further out of alignment; this force is known as unbalanced magnetic pull (UMP). 27. Write down the rules for selecting rotor slots of squirrel cage induction motor. (Dec 2015 & May 2017) ➢ S1  S2 to avoid cogging ➢ S1-S2  +p, +2p,+5p to avoid crawling ➢ S1-S2 +1,+2 or +(p+1), +(p+2) to avoid noise and vibration ➢ S1-S2 +3p to avoid magnetic locking 1 2 ➢ Generally, rotor slots are selected by q − q = 1, , 1 2 3 3 ➢ Number of rotor slots, S2=3q2p. Where, q1- stator slots per pole per phase q2- rotor slots per pole per phase

28. List the advantages of using open slots. (Dec 2014 & Dec2016) • When open slots are used, winding coils can be formed and insulated completely before they are inserted into the slots. Also the windings are reasonably accessible, when individual coils must be replaced or serviced in the field. • On the other hand the coil must be taped and insulated after they are placed in the slots for machines with semi enclosed slots. • Open slots are used where it is desired to complete the coils outside the armature and drop them into the slots. The use of open slots avoids excessive slot leakage, thereby reducing the leakage reactance. 29. How are slip rings made in an induction motor? • The slip rings are made of either brass or phosphor bronze. They may be pressed together on a body of reinforced thermo-setting resin carried on a mild steel hub. The slip rings are located either between the core and the bearing or on the shaft extension. • In the latter case the shaft is made hollow to allow the three connections from rotor to slip rings to pass through bearings. Magnetic leakage calculations – Operating characteristics : Magnetizing current - Short circuit current 30. Define dispersion coefficient of an induction motor. The dispersion coefficient is defined as the ratio of magnetizing current to ideal short circuit current. Dispersion coefficient, σ =Im/Isci Where, Im -Magnetizing current Isci -Ideal short circuit current, Isci= Es/Xs Es= stator phase voltage Xs= total leakage reactance of the motor referred to stator 31. List the factors that are affected by change in the value of dispersion coefficient in induction motors. The dispersion coefficient has its effect on the following factors in an induction motor. • Maximum Power factor • Overload capacity • Air gap length • Number of poles and Frequency 32. List the components of no load current. The no load current I0 of an induction motor is made up of two components. They are: • Magnetizing current( Im ) and • Loss component of current (Il) The magnetizing current is 900 out of phase with voltage while the loss component is in phase with the voltage. 33. How is the loss component calculated in an induction machine? The calculation of loss component of no load current involves the determination of no load loss.

Iron loss consists of • hysteresis and eddy current loss in teeth and cores, • surface loss in teeth due to variation of air gap density, • tooth pulsation loss due to variation of teeth density • Friction and windage loss 34. What is meant by Ideal short circuit current? (May 2015) The two major factors which influence the power factor of an induction motor are: • Magnetizing current and • Ideal short circuit current Ideal short circuit current is defined as the current drawn by the motor at standstill if its resistance is neglected. If the ideal short circuit current of a machine is large it indicates that its leakage reactance is small. A small value of leakage reactance means that the power factor of the machine is good. Thus a large value of short circuit current (a small value of leakage reactance) indicates a good power factor 35. Where do the additional copper losses occur in induction motors? How can you reduce it? With a sinusoidal voltage impressed across the terminals of the motor, the additional copper losses are due to, higher order harmonics and due to skin effect. These additional losses owing to, higher order harmonics, occur mainly in the windings of squirrel cage rotor. Additional copper losses may be reduced by ✓ Chording the stator winding ✓ Skewing the rotor ✓ Having a proper slot combination Advantages: A large gap length results in higher overload capacity better cooling, reduction in noise and reduction in unbalanced magnetic pull. Disadvantage: High valve of magnetizing current. 36. What are the factors to be considered for selecting the number of slots in induction machine stator? (May 2012)(Dec 2017) The factors to be considered for selecting the number of slots is tooth pulsation loss, leakage reactance, magnetizing current, iron loss and cost. Also the number of slots should be multiple of slots per pole per phase for integral slot winding. 37. Why fractional slot winding is not used for induction motor? (Dec 2014) Fractional slot winding creates non uniform flux density distribution in air gap and it leads to torque ripples. 38. What are ranges of efficiency and power factor in induction motor? (May 2017) Squirrel cage motors Efficiency=0.72 to 0.91 Power factor= 0.66 to 0.9 Slip ring motors Efficiency=0.84 to 0.91 Power factor= 0.7 to 0.92 39. Name the losses that occur in 3phase Induction Motor? (May 2018) • Core loss in the stator and the rotor. • Stator and rotor chopper losses. • Friction and windage loss.

UNIT – V DESIGN OF SYNCHRONOUS MACHINES PART A Output equations – choice of specific loadings – Design of salient pole machines 1. Give the output equation. -3 2 Q = (11 Bav ac Kw x 10 ) D ns Where, Bav = Specific Magnetic loading; ac= Specific electric loading Kw= Winding factor; D= Diameter of stator L= length 2. Name the two types of synchronous machines. Based on the construction the synchronous machines are classified as, • Salient pole machines • Cylindrical rotor machines 3. What is run away speed of Synchronous Machine?(M’11, ‘12, D’13’17’18) Speed at which the prime mover would have if suddenly unloaded when working at its rated load is known as runaway speed. 4. List the disadvantages of high Bav. i. Higher losses and temperature rise ii. Reduced efficiency iii. increased transient short circuit. 5. What is critical speed of alternator? When the rotor of the alternator has an eccentricity, it may have a deflection while rotating. This deflection will be maximum at a speed called critical speed. When a rotor with eccentricity pass through critical speed, severe vibrations are developed 6. Why alternators are rated in kVA? (May-2015) The kVA rating of ac machine depends on power factor of load. The power factor in turn depends on the operating conditions. The operating conditions differ from place to place. Therefore the kVA rating is specified for all machines 7. How is cylindrical pole different from salient pole in a synchronous machine? (May 2015, Dec 2018) i.) Cylindrical pole are non-projecting pole whereas the salient pole machines are projecting pole. ii.) Cylindrical rotor construction is used for turbo alternators which are driven by high speed steam or gas turbines whereas the salient pole construction is used for generators driven by hydraulic turbine since these turbines operate at relatively low speeds. 8.What are the factors to be considered for the choice of specific magnetic loading in synchronous machines?(M’16, Dec-2015,May 2019) • Iron loss • Stability • Voltage rating • Parallel operation

• Transient short circuit current 9. What are the factors to be considered for the choice of specific electric loading in synchronous machines? • Copper loss • Synchronous reactance • Temperature rise • Stray load losses • Voltage rating 10. State the factors for separation of D and L for cylindrical rotor machine. (D’16) The separation of D and L in cylindrical rotor machine depends on the following factors. • Peripheral speed • Number of poles. • Short Circuit Ratio (SCR) Short circuit ratio – Estimation of air gap length –Design of damper winding 11. Define SCR of a Synchronous machine. (May 2013, May 2014, May 2016 & Dec 2015, May 2019) The SCR is defined as the ratio of field current required to produce rated voltage on open circuit to field current required to calculate rated current at short circuit. 12.What are the effects of SCR on machine performance? (May 2017) For high stability and low regulation, the value of SCR should be high, which requires large airgap. When the length of airgap is large, the mmf requirement will be high and so the field system will be large. Hence the machine will be costlier. 13. What are the advantages of large airgap in synchronous machines? • Reduction in armature reaction • Better cooling • Small value of regulation • Lower tooth pulsation loss • Higher value of stability • Less noise • Smaller unbalanced magnetic pull 14. List the influence of airgap on the performance of the synchronous machine. (Dec-2013) The synchronous machines usually have large airgap. With increase in airgap length the following factors decreases. • Armature reaction • Regulation • Noise • Tooth pulsation loss • Unbalanced magnetic pull • Sensitivity to load variation 15. What are advantages of low values of SCR? i. Cost of control equipments is reduced ii. Reduction in size of machines iii. Reduction in overall cost of machines

16. What are the disadvantages of low value of SCR? i.Poor stability limit ii.Poor voltage regulation iii. Unsatisfactory parallel operation 17. Give expression for armature ampere turns/ pole. ATa =1.35 Tph . Iph. Kw / P Tph = turns/phase; Iph = current/Phase : Kw = winding factor, P= Pair of poles. 18. How the value of SCR affects the design of alternator? (May 2012) For high stability and low regulation the value of SCR should be high, which requires larger airgap. When the length of airgap is large, the mmf requirement will be high and so the field system will be large. Hence the machine will be costlier. 19. What is the limiting factor for the diameter of synchronous machines? (Dec 2013) The limiting value of peripheral speed is 175 m / sec for cylindrical rotor machines and 80 m/sec for salient pole machines. 20. Write the expression for air gap length in cylindrical rotor machine? For smooth cylindrical rotor the air gap length, -6 lg = 0.5 (SCR) ac τ Kf x 10 / Kg Bavg where, SCR-short circuit ratio, ac- specific electric loading, τ- pole pitch, Kf – form factor, Kg- gap contraction factor, Bavg- average flux density 21. What is the effect of high value of dispersion coefficient? The effect of having high value of dispersion coefficient are as follows: Poor power factor, Reduced over – load capacity, reduced output power. 22. Give the need for damper winding in synchronous machine? (M’11,18) The primary function of damper winding in a synchronous machine is to prevent the phenomenon called hunting. If load on a synchronous machine is changed suddenly, its power angle has to change accordingly. However, change in power angle doesn't occur smoothly. The rotor starts swinging around the new power angle 23. How the dimensions of induction generator differ from that of an induction motor? (May 2015) Dimensions of induction generator and induction motor are same. Energy conversion process is reversible. Therefore, induction motor can operate as induction generator. Design of rotor- Design of field winding – Armature design- Design of turbo alternators 24. Mention the factors that govern the design of field in an alternator. (M’16) i) No of poles and voltage across each field winding, ii) Ampere- turn per pole, iii) Copper loss in the field winding, iv) Dissipating surface of field coil, v) Specific loss dissipation and allowable temperature rise. 25. Mention the factors to be considered for the selection of number of armature slots. (Dec 2014) i. Balanced windings ii. Cost iii. Hot spot temperatures iv. Leakage reactance v. Torque ripples vi. Flux density in iron. 26. Determine the total number of slots in the stator of alternator having 4poles, 3phase, 6 slots per pole for each phase? (Dec 2016) Ss=3pqs, where p=4 poles, qs=6 ;Ss=3*4*6=72 27. State three important features of turbo alternator rotors. (M’17, D’17)

• The rotor of turbo alternators has large axial length and small diameters. • Damping torque is provided by rotor itself and so there is no necessity for additional damper winding. • They are suitable for high speed operations and so number of poles is usually 2 or 4.

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EE8005 - SPECIAL ELECTRICAL MACHINES UNIT – I STEPPER MOTORS PART - A 1. Define: Stepper motor? Stepper motor is a motor which rotates step by step and not by continuous rotation. When the stator is excited using a DC supply the rotor poles align with the stator poles in opposition such that the reluctance is minimum. 2. What are the advantages of Stepper motor? No feedback is normally required for either position control or speed control. Positional control is non – cumulative. Stepping motor is compatible with modern digital equipment. 3. Mention the different types of stepper motor. (Dec 2018) Variable Reluctance stepper motor (Single stack, Multi stack), Permanent magnet stepper motor, Hybrid stepper motor, Outer rotor stepper motor. 4. Define: Step Angle of stepper motor.( June 2016) A stepping motor rotates through a fixed angle for every pulse. The rated value of this angle is called the step angle and expressed in degrees. 5. Define: Holding torque and Detent torque of stepper motor.(Dec 2018) Holding torque is defined as the maximum static torque that can be applied to the shaft of an excited motor without causing continuous rotation. It is defined as the maximum static torque that can be applied to the shaft of an unexcited motor without causing continuous rotation.

6. Define: Resolution of stepper motor. It is defined as the accuracy of positioning of the rotor pole at a particular step angle with respect to stator pole. Most modern drives implement micro stepping to increase resolution and motion smoothness. Most common stepper resolution is 200 full steps per revolution but when driven for example with 16-microstep drive, the resulting resolution is 1600 steps per revolution (1/1600 revolutions). 7. Define: Pull – in torque of stepper motor. They are alternatively called the starting characteristics and refer to the range of frictional load torque at which the motor can start and stop without losing steps for various frequencies in a pulse strain. 8. Define: Pull – out torque of stepper motor.

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These are alternatively called the slewing characteristics. After the test, motor is started by a specified driver in the specified excitation mode in the self-starting range; the pulse frequency is gradually increased; the motor will eventually run out of synchronism. The relation between the frictional load torque and the minimum pulse frequency with which the motor can synchronize is called pull – out characteristics. 9. Define: Slewing frequency of stepper motor. This is defined as the maximum frequency (stepping rate) at which the loaded motor can run without losing steps is alternatively called the maximum slewing frequency. The range of stepping rate which the motor follows without missing a step indicates that the motor is started and synchronized. This area of operation of the stepper motor is called slew range. The motor is said to be operating in slewing mode. 10. Define: Stepping frequency of stepper motor. The speed of rotation of a stepping motor is given in terms of the number of steps per second and the term stepping rate is often used to indicate speed. Stepper motors have a natural frequency of operation. When the excitation frequency matches the resonance, steps may be missed and stalling is more likely to occur. 11. Define: Maximum starting torque of stepper motor. This is alternatively called as maximum pull – in torque and is defined as the maximum frictional load with which the motor can start and synchronize with the pulse train of frequency as low as 10 Hz. 12. Mention the features of stepper motor. The various features of stepper motor are small step angle, High positioning accuracy, High torque inertia ratio, Stepping rate, Pulse frequency. 13. Why interleaving is done in a stepper motor? Interleaving is done in the stepper motor to decrease the step angle and thus increasing the resolution. Interleaving is half stepping, when coil pairs are energized simultaneously in order to get double resolution. 14. Explain: VR type stepper motor. It is a basic type of stepping motor in which the motor step by step rotation is achieved when the rotor teeth and stator teeth are in alignment such that the magnetic reluctance is minimized and this state provides a rest or equilibrium position. 15. Explain: PM type stepper motor. A stepping motor using permanent magnet in the rotor for step movement is called a permanent magnet motor. The Permanent Magnet Stepper Motor has a stator construction similar to that of the single stack variable

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reluctance motor. The rotor consists of permanent magnet poles of high retentivity steel and is cylindrical in shape. The concentrating windings on diametrically opposite poles are connected in series to form a two phase winding on the stator. 16. What are the different modes of excitation? (Nov 17) Stepper drives control how a stepper motor operates; there are three commonly used excitation modes for stepper motors, full step, half step and micro stepping. These excitation modes have an effect on both the running properties and torque the motor delivers. 17. In what way closed loop control is advantageous when compared to open loop control in stepper motor. Closed loop control is more accurate, oscillatory motions are avoided for certain speed ranges, Speed remains constant for high inertial load, follows the input pulses at stepping frequency are some of the advantages over open loop control. But it is costly and complex. 18. Calculate the stepping angle for a 3 phase, 24 pole permanent magnet stepper motor.(Dec 12) Step angle β = 360/ (no. of stator phases * no. of rotor teeth) =5º. 19. Define torque constant of a stepper motor. The torque constant of the stepper motor is defined as the initial slope of the torque current curve of the stepper motor. The torque constant, Kt, of a motor is a very useful parameter for sizing and controlling motors showing a linear speed / torque relationship. 20. What is the function of driver circuit in stepper motor? The stepper motor is a digital device that needs binary signals for its operation. The power driver is essentially a current amplifier, since the sequence generator can supply only logic but not any power. 21. Give the difference between single and multi-stack stepping motors. Sl.No Single Stack Stepper Motor Multi Stack Stepper Motor 1 The number of stator poles The stator and rotor have should be different that of same number of poles and the rotor poles in order to same pole pitch. have self-starting capability and bidirectional rotation. 2 In single stack each and It is used to obtain small every stator pole carries a step sizes. It consists of m field coil. identical single stack variable reluctance motor with the rotor mounts on

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the single shaft.

22. Distinguish the half step and full step operation of a stepper motor. (Nov 17) HALF STEP SL.NO FULL STEP OPERATION OPERATION 1 It is defined as the It is the one phase on mode alternate one phase on operation .It means at that time and two phases on mode only one winding is energized. operation. 2 Rotor rotates on each By energizing one stator step angle is half of the winding the rotor rotates at full step angle. some angle.it s full step operating. 23. Define the micro stepping mode of stepper motor. Micro stepping means, the step angle of the VR stepper motor is very small. It is also called mini stepping. It can be achieved by two phases simultaneously as in 2 phases on mode but with two currents deliberately made unequal. 24. Name the various driver circuits used in stepper motor. (June 2016) Driver circuits for stepper motor are broadly classified into Unipolar and Bipolar driver circuits. Based on the supply voltage given to stator windings they are classified as L/R driver circuit, Chopper drive circuit, H bridge drive circuit. 25. Define: Maxwell’s stress. The Maxwell stress tensor (named after James Clerk Maxwell) is a symmetric second-order tensor used in classical electromagnetism to represent the interaction between electromagnetic forces and mechanical momentum. It is defined as curving of magnetic lines of force at the end of the poles of the stator when rotor rotates. 26. Define Lead angle. (Dec 2016) The angle difference between the phase to be de-energized to bring the stepper motor to the position of equilibrium (stopping the motor) and energization of next phase winding to start the motor during closed loop operation is known as lead angle. The relation between the rotor’s present position and the phase to be excited specified in terms of lead angle. 27. What is the need for suppressor circuits in stepper motor? (Dec 2016)

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The suppressor circuits are needed to ensure the fast decay of current through the winding when it is turned off. When the transistor is turned off a high voltage builds up to Ldi/dt and this voltage may damage the transistor. There are several methods of suppressing this spike voltage and protecting the transistor like diode suppressor, diode-resistor suppressor, zener diode suppressor, etc.

28. Draw the equivalent circuit of winding in stepper motor. (April 17)

29. What are the applications of stepper motor? (April 17, May 2019) *Industrial Machines – Stepper motors are used in automotive gauges and machine tooling automated production equipment. *Security – new surveillance products for the security industry. *Medical – Stepper motors are used inside medical scanners, samplers, and also found inside digital dental photography, fluid pumps, respirators and blood analysis machinery. *Consumer Electronics – Stepper motors in cameras for automatic digital camera focus and zoom functions. And also have business machines applications, computer peripherals applications. 30. Draw the block diagram of the drive system of a stepping motor. (May 2019)

A stepper motor system consists of three basic elements, often combined with some type of user interface (host computer, PLC or dumb terminal): Indexers: The indexer (or controller) is a microprocessor capable of generating step pulses and direction signals for the driver. In addition, the

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indexer is typically required to perform many other sophisticated command functions. Drivers: The driver (or amplifier) converts the indexer command signals into the power necessary to energize the motor windings. There are numerous types of drivers, with different voltage and current ratings and construction technology. Not all drivers are suitable to run all motors, so when designing a motion control system the driver selection process is critical. Stepper motors: The stepper motor is an electromagnetic device that converts digital pulses into mechanical shaft rotation. Advantages of step motors are low cost, high reliability, high torque at low speeds and a simple, rugged construction that operates in almost any environment. The main disadvantages in using a stepper motor is the resonance effect often exhibited at low speeds and decreasing torque with increasing speed. UNIT – II SWITCHED RELUCTANCE MOTORS (SRM) PART – A 1. What is switched reluctance motor? The switched reluctance motor is a doubly salient, singly excited motor. This means that it has salient poles on both the rotor and the stator, but only one member (usually the stator) carries windings. The rotor has no windings, magnets, or cage windings, but is built up from a stack of salient-pole laminations. 2. What are the advantages of Switched Reluctance motor?( June 2016, April 17, Dec 2018) Rotor is simple and it tends to have a low inertia. The stator is simple to wind. In most applications the bulk of the losses appear on the stator, which is relatively easy to cool. Due to the absence of magnet the maximum permissible rotor temperature may be higher than in PM motors. Under fault conditions the open circuit voltage and short circuit current are zero or varying small. Extreme by high speeds are possible. 3. What is the difference between Switched Reluctance motor and variable reluctance stepper motor? Switched Reluctance motor Variable reluctance stepper motor Conduction angle for phase Stepper motor is usually fed current is controlled and with a square wave of phase synchronized with the rotor current without rotor position position, usually by means of a feedback. shaft position sensor. The SRM is designed for high It is usually designed with a

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speed. limited speed. Closed loop control is necessary Closed loop control is required for high frequency operation

4. What are the applications of Switched Reluctance motor? (Dec 2016, April 17) SRM motors are used in Precision position control system for Robotics and Low power servo motor. High-speed motors have the advantage of high power density, which is an important issue of traction motors in electric vehicles (EV). 5. Give the basic features or characteristics of Switched Reluctance motor. The switched reluctance motor is a doubly-salient, singly-excited motor. This means that it has salient poles on both the rotor and the stator but only one member (usually the stator) carries windings. The rotor has no windings, magnets, or cage windings, but is built up from a stack of salient-pole laminations. Low inertia and simple manufacturing, Losses appear only on the stator and easy to cool, No magnets and so permissible rotor temperature is higher than in PM motors, Torque is independent of the polarity of phase current. Reduction in no of semiconductor devices in controller Open circuit voltage and Short circuit current are zero or very small under faulty condition, Immune from shoot through failure, High starting torque, extremely high speeds possible. 6. What are the disadvantages of a Switched Reluctance motor? The absence of free PM excitation imposes the burden of excitation on the stator windings and the controllers and increases the per unit copper losses,  is limited, torque/ampere is limited, Non uniform nature of the torque production which leads to torque ripple and may contribute to acoustic noise. 7. Mention different modes of operation of SRM. (Nov 17) The different modes of operation of Switched Reluctance motor are Low speed operating mode and High Speed operating mode. Monitoring of exciting current during low speed operation is essential because of long duration of each phase period and needs chopping of energization to restrict each phase current within the semiconductor ratings. Moreover, the developed torque is controlled by varying the average phase current. During high speed operation, current control is not essential because the inductance of the winding and the motional back emf induced restrict the excitation to single pulses of current. Torque is controlled by optimal positioning of these pulses rather than the current level.

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8. Mention the applications of micro stepping VR stepper motor? Micro stepping operation mode of VR stepper motor is used in following applications. Printers, digital cameras, CNC machines, toys, robotics, fine position control systems. 9. Write the relations between the speed and fundamental switching frequency. f=nNr=(r.p.m./60)Nr Hz , Nr=No. of rotor poles, If there are q phases there are qNrsteps per revolution and the step angle or stroke is given by ε=2π/(qNr) rad. The number of stator poles usually exceeds the number of rotor poles. 10. What is co-energy? In the ψ –i curve of a motor, the area between the curve and horizontal i axis is the co-energy W’ and the other part is the stored field energy Wf. Co-energy is expressed in the same units as energy and is especially useful for calculation of magnetic forces and torque in rotating machines. 11. Give the expression for torque of a Switched Reluctance motor. The torque is given by T = [∂W’/∂θ] i=constOr by T = [∂Wf / ∂θ] ψ=const. With magnetic saturation negligible and with ψ–i curve straight line,ψ=Li, 2z W’=Wf=(½)Li , 2 T = (½)i dL/dθNmwhere T is the torque, L is the inductance,Wf is the stored field energy. 12. Why rotor position sensor is essential for the operation of Switched Reluctance motor? (Dec 2016) It is normally necessary to use a rotor position sensor for communication and speed feedback. The turning ON and OFF operation of the various devices of power semiconductor switching circuit are influenced by signals obtained from rotor position sensor.

13. Define: Chopping mode of operation of Switched Reluctance motor. In this mode, also called low – speed mode, each phase winding gets excited for a period which is sufficiently long. As the speed is low in position control, the Switched reluctance motor operates in PWM mode. Soft-chopping control is used in motoring mode while hard- chopping control is applied in braking mode to hold on the load torque and to provide stable operation at zero speed. 14. What are the types of power controllers used for Switched Reluctance motor? (May 2019) The power semiconductor switching circuits used are 1. Two power semiconductor switching devices per phase and two diodes. 2. (n+1) power semiconductor switching devices (n+1) diodes. 3. Phase winding using

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bifilar wires. 4. Split-link circuit used with even-phase number. 5. C-dump circuit. 15. What are the two types of current control techniques? The two types of current control techniques of Switched Reluctance motor are Hysteresis type control and PWM type control. Hysteresis type control is used to compensate contact bounce in switches or noise in an electrical signal. Pulse width modulation (PWM) is a powerful technique for controlling analog circuits with a microprocessor's digital outputs. PWM is employed in a wide variety of applications, ranging from measurement and communications to power control and conversion. 16. What is the step angle of a 5 phase Switched Reluctance motor and commutation frequency in each phase for the speed of 6000 rpm. SRM having 10 stator poles and 4 rotor poles. Solution: Step angle ()=(2π/qNr)=(360°/5*4)=18°. Commutation frequency at each phase=(Nr*ω)/2π=(4*6000)/60=400Hz.[ω = 2πN]. 17. What are the merits of Dump C – Converter? The Dump C-Converter topology uses lower number of switching devices and has only one switch voltage drop, the converter has full regenerative capability, and there is faster demagnetization of phases during commutation.

18. What are the merits of split power supply Converter? The merits of split power supply converter are it requires lower number of switching devices, there is faster demagnetization of phases during commutation. 19. What are the merits of classic converter or power controller in SRM? Control of each phase is completely independent of the other phases; the energy from the off going phase is feedback to the source, which results in useful utilization of the energy. 20. Why SR machines popular in adjustable speed drives? Rotor is simple and it tends to have a low inertia, The stator is simple to wind, In most applications the bulk of the losses appear on the stator, which is relatively easy to cool, Due to the absence of magnet the maximum permissible rotor temperature may be higher than in PM motors, Under fault conditions the open circuit voltage and short circuit current are zero or varying small, Extreme by high speeds are possible. 21. Mention some position sensors used in switched reluctance motor. (Dec 2018)

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Switched reluctance motor is always operated with closed loop control. Normally we have to use a rotor position sensor for commutation and speed feedback. Here the phase windings are energized by using power semiconductor circuit. Some of the position sensors are Optical encoder, resolver, Speed sensors and Hall Effect sensor. 22. What is the significance of closed loop control in switched reluctance motor? (June 2016) Switched reluctance motor is always operated with closed loop control. Normally we have to use a rotor position sensor for commutation and speed feedback. Here the phase windings are energized by using power semiconductor circuit. He turning on and off operation of the various semiconductor devices is influenced by signals obtained from rotor position sensor. It is the main significance of closed loop control in SR motor. 23. Give the advantages of sensorless operation of switched reluctance motor. The advantages of sensorless operation of switched reluctance motor are Low cost, Reliable and it avoids additional cost size. 24. List out the advantages and disadvantages of the converter circuit with two power semiconductor devices and two diodes per phase. (Nov 17) Advantages • The converter has low number of switching devices which reduces the cost of the converter. • The converter is able to freewheel during the chopping thus reducing the switching frequency and losses. Disadvantages • The common switches conduct for all phases and thus have higher switching stress. • Disability to magnetize a phase while the off going phase is still demagnetizing which results in high torque ripple during commutation. 25. What is Hysteresis current control? Hysteresis current controller derives the switching signals of the inverter power switches in a manner that reduces the current error. The switches are controlled asynchronously to ramp the current through the inductor up and down so that it follows the reference. 26. Define Energy Ratio (ER). (April 17) The energy ratio is defined as, ER = W/ W + R Where W is the energy converted into mechanical work and R is the energy returned to the source using a regenerative converter. The term

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energy ratio is analogous to the term power factor used for AC machines. 27. Draw the torque –speed characteristics of SRM. (April 17)

28. State the principle of operation of switched reluctance motor. (May 2019) The SRM has wound field coils as in a DC motor for the stator windings. The rotor however has no magnets or coils attached. It is a solid salient- pole rotor (having projecting magnetic poles) made of soft magnetic material (often laminated steel). When power is applied to the stator windings, the rotor's magnetic reluctance creates a force that attempts to align the rotor pole with the nearest stator pole. In order to maintain rotation, an electronic control system switches on the windings of successive stator poles in sequence so that the magnetic field of the stator "leads" the rotor pole, pulling it forward. Rather than using a mechanical commutator to switch the winding current as in traditional motors, the switched-reluctance motor uses an electronic position sensor to determine the angle of the rotor shaft and solid state electronics to switch the stator windings, which enables dynamic control of pulse timing and shaping. In an SRM the rotor magnetization is static (a salient 'North' pole remains so as the motor rotates) while an induction motor has slip, and rotates at slightly less than synchronous speed. SRM's absence of slip makes it possible to know the rotor position exactly, allowing the motor to be stepped arbitrarily slowly. UNIT – III PERMANENT MAGNET BRUSHLESS D.C. MOTORS PART - A 1. Why adjustable speed drives are preferred over a fixed speed motor? The common reasons for preferring an adjustable speed drives over a

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fixed speed motor are: Energy saving e.g. Fan or pump flow process, Velocity and position control e.g. Electric train, portable tools, washing machine, Amelioration of transients: Starting and stopping of motors produce sudden transients. It can be smoothened using adjustable speed drives. 2. What is the structure of an adjustable speed drive system? An adjustable speed drive consists of an electric motor and controller that is used to adjust the motor's operating speed. The combination of a constant-speed motor and a continuously adjustable mechanical speed- changing device might also be called an adjustable speed drive. The general structure of a motion control system or drive consists of the following elements: The load, the motor, the power electronic converter; and the control. 3. Write briefly about the construction and types of a Brushless DC machines. 1) Brushless PM machines are constructed with the electric winding on the stator and PMs on the rotor. There are several conventional PM machine configurations and other more novel concepts conceived in recent years to improve performance.2)The configuration of a PM machine and the relationship of the rotor to the stator determine the geometry and the shape of the rotating magnetic field. PM machines in which the magnetic flux travels in the radial direction are classified as radial-flux machines. 3) They are cylindrical in shape, and the rotor is usually located inside the stator but can also be placed outside the stator. PM machines in which the magnetic flux travels in the axial direction are classified as axial-gap machines. They can have multiple disk or pancake-shaped rotors and stators. The stator-rotor-stator configuration is typical. 4. What are the advantages of PM machine? (May 2019) 1) In general, PM machines have a higher efficiency as a result of the passive, PM-based field excitation. PM machines have the highest power density compared with other types of electric machines, which implies that they are lighter and occupy less space for a given power rating.2)The amount of magnet material that is required for a given power rating is a key cost consideration. The cost of magnet material is high compared with the cost of the other materials used in electric motors, and design attributes that minimize the required amount of magnet material are important considerations in motor selection.3) The stators of PM machines are generally fabricated in the same manner as induction machine stators; however, modifications are sometimes necessary, such as the design of a stator lamination to accommodate high flux density.

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5. What are the types of PM machines? The types of Permanent magnet machines are 1. Interior PM Machine 2. Surface mounted PM machine. A type of motor that has a rotor embedded with permanent magnets is called the IPM (interior permanent magnet) type. Compared with the SPM (surface permanent magnet), this type of motor can reduce the risk of a magnet being peeled off by centrifugal force, and take advantage of reluctance torque. The IPM type allows various structures for embedding permanent magnets. 6. What are the differences between mechanical and electronic commutator? (Dec 2016) MECHANICAL ELECTRONIC COMMUTATOR COMMUTATOR 1. Commutator arrangement is 1. Commutator arrangement is located in the rotor. located in the stator. 2. Shaft position sensing is 2. It requires a separate rotor inherent in the arrangement position sensor. 3. Sparking takes place. 3. There is no sparking. 4. It requires regular 4. It requires less maintenance. maintenance. 5. Sliding contacts between 5. No sliding contacts. commutator and brushes. 7. What is meant by permeance coefficient? The line drawn from the origin through the operating point is called load line and absolute value of its slope normalized to µ0 is called permeance coefficient. Permeance coefficient= µrec((1+PrlRg)/(PmoRg))where µrec=relative recoil permeability,Rg=air gap reluctance,Pmo=internal leakage permeance,Prl=normalized rotor leakage permeance. 8. Discuss briefly about the types of Permanent Magnets used in electrical machines. (April 17, May 2019) PM strength and other key properties: The various types of PMs include the following: Alnico—a family of magnets made from aluminum, nickel, and cobalt characterized by excellent temperature stability, high residual induction, and enough energy for a number of industrial and commercial applications. Ceramic—a hard, low-cost ferrite made of barium and strontium ferrite with excellent stability. Ceramic magnets tend to be brittle, hard, and resistant to corrosion. 9. What is commutation? Commutation is the process in which generated alternating current in the

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armature winding of a dc machine is converted into direct current after going through the commutator and the stationary brushes. it will takes place, the coil undergoing commutation is short circuited by the brush. 10. Draw the magnetic equivalent circuit of PMBLDC motor.

11. Compare brushless DC motor with P.M. commutator motor.(Dec 2018) Brushless DC motor P.M. Commutator motor 1. No Brushes. Maintenance 1. Commutator based DC problems (RFI, sparking, ignition machines need carbon and fire accidents) eliminated. brushes, so sparking and wear and tear is un avoidable. 2. More cross sectional available 2. Armature winding is inside for armature windings. and the magnet is on the Conduction of heat through the stator outside. frame is improved. 3. Increase in electric loading is 3. Efficiency less. possible, providing a greater specific torque. Higher efficiency. 4. Space saving, higher speed 4. Commutator restricts possible, with reduced inertia. speed.

5. Maximum speed limited by 5. Magnet is on the stator. No retention of magnet against problem. centrifugal force. 6. Shaft position sensor is a must. 6. Not mandatory. 7. Complex electronics for 7. Simple controller.

12. Give the emf and torque equations of the square wave BLDC motor.(April 17,Nov 17)

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The emf equation is given by E = kφω and the torque equation is given by T = kφIwhere k is the armature constant depending on the number of turns in series per phase in the armature winding, ω is rotor speed in rad / sec and φ is the flux ( mainly contributed by the Permanent Magnet on the rotor). I is the load current. 13. What is meant by demagnetization in PM-BLDC motor? In the absence of externally applies ampere turn, the magnets operating point is at the intersection of demagnetization curve and the load line.

Demagnetization curve 14. Write the principle of operation of PM-BLDC motor. (Nov 17) When a D.C supply is switched on to the motor the armature winding draws a current. The current distributes with the stator armature winding depends upon rotor position and the devices turned on. As per faradays law of electromagnetic induction a emf is dynamically induced in the armature conductors. This back emf as per lenz law opposes the cause. As a result developed torque reduces. Finally the rotor attain the steady speed. 15. Compare 120 degree and 180 degree operation of BLDC motor. The 180 degree magnetic arc motor uses 120 degree mode of inverter operation. The motor with 120 degree magnetic arc uses 180 degree mode of inverter operation. In 180 degree mode of inverter has 1.5 times copper losses but produce same torque with only 2/3 of magnetic material. Motor operation is less efficient. 16. Give the expression for self and mutual inductances of a BLDC motor. 2 Self inductance is given by Lg=(ψ/i)=(πμ0 N lr1)/(2g”)where g”= g’+lm/μrec,g’= Kcg,N=Number of conductors in the slot, I = current, lm = magnet length in radial direction , g’ = air gap, g” = air gap including radial thickness of the magnet, μrec = relative recoil permeability, Mutual inductance is given by Mg=-(1/3)Lg.

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17. What are the types of sensors used with PMBLDC motors? Hall Effect sensors are most commonly used for speed, position sensing with PMBLDC motors. Optical Disc based sensors are also used. Presently rotor position sensors are avoided by using alternative methods called as Sensorless control methods, which uses terminal emf measurement, third harmonic voltage measurement, flux estimation and neuro – fuzzy techniques etc. 18. Write the dynamic equations of the PMBLDC motor. The dynamic model equations of PMBLDC motor is given by dia/dt=(van–Ria –ea(θ))/L dib/dt=(vbn –Rib –eb(θ))/L dic/dt=(vcn–Ric –ec(θ))/L dω/dt=[ Te–Tl –Bω]/J dθ/dt = Pω/2 where the Torque developed is given by Te=(e(θ)ia+ea(θ)ib+ea(θ)ic)/ω,TL=Load torque applied is the coefficient of friction and J is the moment of inertia. 19. What are the relative merits and demerits of brush less DC motor drives? (Dec 2016) Merits: Commutator less motor, Specified electrical loading is better, Heat can be easily dissipated, No sparking takes place due to brush, Source of EMI is avoided. Demerits: Above 10 kW, the cost of magnet is increase, Due to centrifugal force the magnet may come out. 20. What are the difference between conventional DC motor and PMBLDC motor? (April 17) DC PMBLDC Brushes are present. Brushes are not present. Sparking may occur due to Sparking will not occur as brush. brush is not present. Brushes tend to produce RF1 problem does not occur. RF1. There is a need for brush No need of brush maintenance. maintenance. 21. What are the various kinds of permanent magnets? There are basically three different types of permanent magnets which are used in small DC motors; Alnico magnets, Ferrite or ceramic magnet, and Rare - earth magnet (samarium – cobalt magnet). 22. What is meant by multiphase brushless motors? A multi-phase brushless motor including a stator having a plurality of drive coils each corresponding to a specific phase and a rotor having a

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plurality of field magnet poles of successively alternating polarity. The stator further has a plurality of Hall generators for detecting the positions of the rotor and a speed sensor for detecting the rotational speed of the rotor. 23. Give the uses of sensors in motors. It is used to identify the position of the rotor and it is also used to excite the coils in proper manner. For instance, a motor is an electrical machine used to create motion. The device converts electricity (electrical energy) into motion (mechanical energy). Typically performed by rotating an object. Rotational sensor: A sensor that measures the turning movement of a wheel for purposes of calculating distance traveled. 24. List some applications of PMBLDC motor. (June 2016) Fans, Pump drive, Traction and Hydraulic power steering, precision high speed spindle drivers for hard disk drivers etc.,. 25. Why brushless permanent magnet motor is called as electronically commutated motor? (June 2016, Dec 2018) The switching instants of the individual transistor switches, Q1 – Q6 with respect to the trapezoidal emf wave are synchronized with the rotor. So switching the stator phases synchronously with the emf wave make the stator and rotor mmfs rotate in synchronism. Thus, the inverter acts like an electronic commutator that receives switching logical pulses from the rotor position sensor. This is why a BLDC drive is also commonly known as an electronically commutated motor (ECM). 26. How the demagnetization occurs in PMBLDC motor? During the normal operation of motor, when the torque and back emf are constant, if the field flux level becomes low, then demagnetization occurs. Demagnetization of the permanent magnets occurs to some extent whenever a current flows in the motor winding. 27. What are the classifications of BLPM dc motor? 1. BLPM square wave motor. 2. BLPM sine wave motor. 28. What are the two types of BLPM SQW DC motor? 1. 1800 pole arc BLPM square wave motor. 2. 1200 pole arc BLPM square wave motor. 29. What are the ways by which demagnetization can be limited in permanent magnet? There are several ways to limit the demagnetization. One way is to keep the current below the maximum value and another way is y use of pole shoes to a permanent magnet to collect the flux and then transfer it to the air gap.

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30. Define the energy product and maximum energy product of a permanent magnet. The absolute values of the product of the flux density and the field intensity at each points along the demagnetization curve is called energy product. The maximum value of the energy product is called maximum energy product and this quantity is one of the strengths of the permanent magnet. 31. What is electronic commutator? (April 17) Brushless motors rely on semiconductor switches to turn stator windings on and off at the appropriate time. The process is called electronic commutation, borrowing on terminology used for the mechanism in dc motors, called a commutator, which switches current from winding to winding, forcing the rotor to turn. UNIT – IV PERMANENT MAGNET SYNCHRONOUS MOTORS (PMSM) ) PART - A 1. Distinguish PM synchronous motor from BLPM DC motors. PMSM PM Brushless DC motor Sinusoidal or quasi–sinusoidal Rectangular distribution of distribution of magnetic flux in magnetic flux in the air gap the air gap Sinusoidal or quasi-sinusoidal Rectangular current waveforms current waveforms Quasi-sinusoidal distribution of It has concentrated stator stator conductors. (short windings pitched and distributed or concentric stator windings) 2. Explain in detail the vector control of permanent magnet synchronous motor. 1) BLPM SNW motor is usually employs for variable speed applications. For this we keep V/f constant and vary V and f to get the desired speed and torque. From the theory of BLPM SNW motor it is known that as the speed is varied from a very low value up to the corner frequency, the desired operating point of current is such that Id =0 and I is along q-axis. 2) When the frequency is more than the corner frequency it is not possible to make Id=0, due to voltage constraints. In such a case a better operating point for current is obtained with minimum I value after satisfying the voltage constraints. Controlling BLPM SNW motor taking into considerations the above mentioned aspects is known as vector control of BLPM SNW motor.

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3. What are the performances of PMSM when compared with BLDC motor? With equal r.m.s. phase currents the torque of the square wave motor exceeds that of sine wave motor by a factor 1.47. With equal peak currents the factor is 1.27. For the same flux-density flux per pole of a square wave motor exceeds that of a sine wave motor by a factor π/2. Square wave motor has a slightly better utilization of the peak current capability of the converter switches. In PMSM three devices conduct at a time (180 degree mode of inverter), where as in BLDC only two devices conduct at a time in 120 degree mode. 4. What is meant by field oriented control of PMSM? In general for field oriented control the stator currents are transformed into a frame of reference moving with the rotor flux. In the PMSM the rotor flux is stationary relative to the rotor. The rotor flux is therefore defined by the mechanical angle of rotation α,this is obtained from a rotor position sensor. Thus, the control is much easier to implement than in the case of induction motor. 5. What is meant by slot less motor? Slot less brushless DC motors represent a unique and compelling subset of motors within the larger category of brushless DC motors. They are called “slot less” because typical slotted brushless DC motors contain a stator core of laminated steel composed of slots separated by teeth around which copper wire is wound. In slot less motor, the stator teeth are removed and resulting space is partially filled with addition of copper. 6. Write the expression for the synchronous reactance of PMSM. The sum of the magnetizing and leakage reactance define synchronous reactance. X = X + X S M l , Xs = Synchronous reactance, XM = Magnetizing reactance, Xl = Leakage reactance. 7. What are the applications of PMBLDC and PMSM motors? PMBLDC: (Low rating application) turn table drives for record players, Hard disc drives, Low cost instruments, Small fans for cooling electronic equipment, (High rating application) Air craft, Satellite system, Traction system (in future). PMSM are used in low to medium power (up to several hundred HP) Applications, Fiber spinning mills, Rolling mills, Cement mills, Ship propeller, Electric Vehicles, Servo and robotic drives and Starters / generator for air craft engine. 8. What are the features of permanent magnet synchronous motor? (Dec

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2018) Robust, compact and less weight, No field current or rotor current in PMSM, unlike in induction motor, Copper loss due to current flow which is largest loss in motors is about half that of induction motor and High efficiency. 9. Explain the difference between synchronous motor and PMSM. Synchronous Motor PMSM 3 phase AC or six step voltage 3 phase sine wave ac or PWM or current source inverter is ac is used as supply. used as supply. This type of motor is used in Here it is used in low integral very large compressor and fan HP industries drives, fiber drives. spinning mills. 10. What are disadvantages of PMSM relative to the commutator motor? The disadvantages of PMSM relative to commutator motor are that it requires shaft position sensing and increased complexion in the electronic controller. Also speed dependence on the external load torque and the reduced dynamic performances are the other disadvantages. 11. What are the assumptions made in derivation of emf equation for PMSM? Flux density distribution in the air gap is sinusoidal, Rotor rotates with an uniform angular velocity of ωm( rad/sec), Armature winding consists of full pitched, concentrated similarly located coils of equal number of turns. 12. Why PMSM operating in self – controlled mode is known as commutatorless DC Motor? (May 2019) Load side controller performs somewhat similar function as commutator in a DC machine. The load side converter and synchronous motor combination functions similar to a DC machine. First it is fed from a DC supply and secondly like a DC machine. The stator and rotor field remain stationary with respect to each other at all speeds. Consequently, the drive consisting of load side converter and synchronous motor is known as commutator less DC motor. 13. Explain the distribution factor for PMSM. Distribution factor (Kd): Distribution factor is given by the ratio of the MMF performed in the concentrated windings compared to the distributed windings. Coils in the stator are displaced from each other by a certain electrical angle, each coil produce sinusoidal MMF with a shift angle β.

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where ‘m’ is no. of coils/pole/phase, β = angle of displacement 14. Write down the expressions for torque of a PMSM. T= 3 E I sin β / ωm N-m where ωmis the angular velocity, T is the torque produced, β is the torque angle or power angle, I is the current and E is the induced emf . 15. What are the features of closed- loop speed control of load commutated inverter fed synchronous motor drive? The features of closed loop speed control of load commutated inverter fed synchronous motor drive is that it has Higher efficiency, Four quadrant operation with regeneration braking, Higher power ratings and run at high speeds ( 6000 rpm). 16. Write down the emf expressions of PMSM. Eph = 4.44 f Фm KwTph volts. This is the rms value of induced emf per phase, where f = Frequency in Hertz, Фm = flux per pole, Kw = Winding factor, Tph = Turns per phase. 17. What is meant by self-control in PMSM drive? (April 17) • As the rotor speed changes the armature supply frequency is also changes proportionally so that the armature field always moves (rotates) at the same speed as the motor. • The armature and rotor field move in synchronism for all operating points. • Here accurate tracking of speed by frequency is realized with the help of rotor position sensor. 18. What are advantages and disadvantages of PMSM? Advantages: Runs at constant speed, No field winding, no field copper loss, better efficiency, High power density, Lower rotor inertia, Robust construction of rotor, No sliding contact hence requires less maintenance. Disadvantages: Loss of flexibilities of field flux control, Demagnetization effect and High cost. 19. Distinguish between self-control and vector control of PMSM?

S.No. Self-control Vector control

1. Dynamic performance is Dynamic performance is poor better

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2. Control circuit is simple Control circuit is complex

20. Brief-up the advantages of load commutation in permanent magnet synchronous motor. Commutation of thyristor by induced voltages of load is known as “load commutation”. Here, frequency of operation is higher and it does not require commutation circuits. It can operate at power levels beyond the capability of forced commutation. 21. Define the term load angle. The phase angle introduced between the induced emf phasor, E and terminal voltage phasor, V during the load condition of an Alternator is called load angle. In PMSM the load angle is the angle between stator field and rotor field when the machine is rotated at synchronous speed. It is represented as δ. 22. State the power controllers for PMSM. (Nov 17) Permanent magnet synchronous motors (PMSM) are typically used for high-performance and high-efficiency motor drives. High-performance motor control is characterized by smooth rotation over the entire speed range of the motor, full torque control at zero speed, and fast acceleration and deceleration. The various power controllers are PWM inverter using power MOSFETS with microprocessor control. PWM inverter using BJT’s with microprocessor control (up to 100 KW). 23. Write the advantages of optical sensors. Quite suitable for sinusoidal type motor as it is a high resolution sensor. The signal from the photodiode rises and falls quite abruptly and the sensor outputs are switched high or low so the switching points are well defined.

24. Write the disadvantages of optical sensors. Following are the disadvantages of Optical Sensor: ➨Susceptible to interference from environmental effects ➨Can be costly ➨Susceptible to physical damage 25. Classify the different types of PMSM. (Dec 2016, Nov 17, May 2019) a) General classification: There are two types of PMSM: 1) Surface mounted rotor: further classified as a) Projected type b) Insert type 2) Interior (or buried) rotor

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b) Classification based on rotor configuration: 1) Peripheral 2) Interior 3) Claw-pole 4) Transverse 26. Differentiate Square wave and sine wave motor. (Dec 2016) Features BLPM Square wave BLPM Sine wave motor motor

Flux density distribution

Flux per pole Փ = BgτlWb Փ = Bavτl ( ) ퟐ흅풓 ퟐ 푩̂ Փ τ = = ( ) πl Wb ퟐ풑 흅 푰풎 RMS value of ퟐ Irms= I = Irms= √ Id √ퟐ the line ퟑ current to the motor

27. Define synchronous reactance in PMSM. (May 2016) The synchronous reactance is the fictitious reactance employed to account for the voltage effects in the armature circuit produced by the actual armature leakage reactance and the change in the air gap flux caused by the armature reaction.

28. Draw the output phasor diagram of PMSM. (May 2016, Dec 2018)

UNIT – V OTHER SPECIAL MACHINES PART – A 1. What are SYNREL motors?

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Synchronous reluctance motor is similar to three–phase Synchronous motor except the rotor are demagnetized and made with saliency to increase the reluctance power. It is a motor which develops torque due to the difference in reluctance of the two axes, namely quadrature and direct axis. 2. What is the principle of operation of reluctance machine? 1) In reluctance machines, torque is produced by the tendency of the rotor to move to a position where the inductance of the excited stator winding is maximized (i.e., rotor tooth aligns with active stator phase to minimize reluctance).2) The rotor is typically constructed of soft magnetic iron shaped so as to maximize the variation of inductance with rotor position. 3) Opposite poles form a phase and the phases are magnetically independent of one another. The machines tend to be noisy; a characteristic that has limited their applications in the past and has also limited their use currently in vehicles. Research has been on-going for years in an attempt to address the noise issue, but little has been accomplished in actual noise mitigation. Reluctance machines are relatively low-cost machines, and they generally do not contain PMs.

3. What are the properties of Reluctance motor? Combined reluctance and magnet alignment torque, Field weakening capability, under excited operation for most loaded condition, High inductance, High speed capability and High temperature capability.

4. What are the various stator current modes used in synchronous reluctance motor? The various stator current modes used in synchronous reluctance motor are (i) Unipolar current modes (ii) Bipolar current modes. 5. Mention the applications of distributed anisotropy cage rotor of synchronous reluctance motor. (May 2019) These rotors are used for line – start (constant voltage and frequency) applications. Also used in processing of continuous sheet or film material, used in regulators and turntables, applied in wrapping and folding machines and it can be used in synchronized conveyors. 6. What is meant by reluctance torque? (Dec 2016, Dec 2018) The torque which is exhibited on the rotor due to the difference in Reluctance in the air gap (or) a function of angular position of rotor with respect to the stator coil is known as reluctance torque.

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7. What are the advantages and disadvantages of Synchronous reluctance motor? Advantages :Rotor is simple in construction i.e. very low inertia, Robust, Low torque, ripple, Can be operated from standard PWM AC Inverters, It can be also built with a standard induction motor, stator and windings. Disadvantages: It has poor power factor performance and therefore the efficiency is not as high as permanent magnet motor, The converter kVA requirement is high, The pull – in and pull – out torque of the motor are weak. 8. What are the types of Synchronous reluctance motor? Synchronous reluctance motor is classified into three types depending upon the construction of rotor. They are Salient type or Radial type rotor, Flat type or axial type rotor, Flux Barrier type or Laminated type rotor. 9. Write the torque equation of Synchronous reluctance motor. 2 T = (U / 2ωs) (1/Xq - 1/Xd) sin 2δ, U = Supply Voltage, Is be the supply current which has two components Id and Iq, Id = Direct axis current, Iq =  Quadrature axis current, s =Synchronous speed in rad/sec, Xd =Direct axis reactance, Xq =Quadrature axis reactance. 10. Skewing is required for Synchronous reluctance motor. Justify? At the time of starting, reluctance motor are subjected to logging due to the saliency of motor. This can be minimized by the skewing of the rotor parts. Reluctance motors are subject to cogging, since the locked rotor torque varies with the rotor position, but the effect may be minimized by skewing the rotor bars and by not having the number of poles. 11. What are the advantages of increasing Ld / Lq ratio in Synchronous reluctance motor? The advantages of increasing Ld / Lq ratio in synchronous reluctance motor are: (i) Motor power factor increases (ii) I2R losses reduced (iii) Reduced volt – ampere ratings of the inverter driving the machine. 12. Compare Synchronous reluctance motor and Induction motor. (Nov 17, May 2019) S.No. Synchronous reluctance motor Induction motor 1. Torque generation due to Torque generation due reluctance principle to Lorentz force 2. Runs at synchronous speed Runs at asynchronous speed 3. Better efficiency. Efficiency is low.

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4. Low cost. High cost. 5. High power factor. Low power factor. 6. Used for low and medium Used for high power power application. application.

13. Define: Magnetic flux. Magnetic flux is the number of magnetic field lines passing through a surface (such as a loop of wire). The magnetic flux through a closed surface (such as a ball) is always zero. Magnetic flux is sometimes used by electrical engineers designing systems with electromagnets or designing dynamos. 14. Define: Reluctance. The opposition offered to the magnetic flux by a magnetic circuit is called its reluctance. Reluctance is analogous to resistance. As resistance opposes electric current in an electric circuit that same work a reluctance does in a magnetic circuit. 15. Define: Permeance. In electromagnetism, permeance is the inverse of reluctance. In a magnetic circuit, permeance is a measure of the quantity of magnetic flux for a number of current-turns. It is a measure of the ease with which flux can be setup in a material. 16. List out any four features of synchronous reluctance motors. The four features of synchronous reluctance motors are (i) Better efficiency (ii) High cost (iii)Low power factor (iv)Used for low and medium power application. 17. Give some potential application of synchronous reluctance machine. (Dec 16, June 16, April 17) It is used for constant speed applications i.e. timing devices, signaling devices, recording instruments and phonograph, it is used in automatic processors such as in food processing and packaging industries. Used in high speed applications, Synthetic fiber manufacturing equipment, Wrapping and folding machines, synchronized conveyors. 18. Write the various design parameters of a synchronous reluctance motor. The various design parameters of a synchronous reluctance motor are Power factor, Copper loss, core loss, Cost and Efficiency. The reluctance synchronous motor with conventional rotor is simple, robust, requires simple controls but is characterized by low power factor kW/kVA ratio,

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low torque density and high torque pulsations. 19. Give the difference between synchronous reluctance motor and switched reluctance motor. S.No. Synchronous Switched reluctance motor reluctance motor 1. Single salient electric Doubly salient electric motor motor 2. Continuous rotation Designed for continuous rotation 3. Controller is not The step pulse are given by necessary. Hence it is external controller which uses cheap. rotor position sensors 20. What is a vernier motor? A Vernier motor is an unexcited (or reluctance Type) inductor synchronous motor. It is also named because it operates on the principle of a vernier. The peculiar feature of this kind of motor is that a small displacement of the rotor produces a large displacement of the axes of maximum and minimum permeance. When a rotating magnetic field is introduced in the air gap of the machine, rotor will rotate slowly and at a definite fraction of the speed of the rotating field. 21. Write the salient features of Vernier motor. The nature of a PM vernier motor is analytically surveyed, and substantial information such as main geometric factors affecting torque and the maximally obtainable torque for a given current are provided, leading to a new relation for torque per air gap volume. The peculiar feature of this motor is that a small displacement of rotor produces a large displacement of the axis of maximum and minimum permeance. 22. State the application of vernier motor. A vernier motor works as an electric gearing. This kind of motor is attractive in applications which require low speed and high torque and where mechanical gearing is undesirable. The stator of a vernier motor has slots and a distributed winding just like the stator of an ordinary poly phase induction motor. 23. What is the specialty of hysteresis motor? 1. Uniform torque from no load to full load 2. Rotor doesn‘t carry any winding, hence provide smooth noiseless operation. 3. It can run at synchronous speed. 24. Draw the voltage and torque characteristics of synchronous reluctance motor. (June 2016)

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Pull out torque

Pull in torque

25. What are the main differences between the axial and radial air gap motors? S.No. Radial airgap motors Axial airgap motors 1. High speed applications Low speed applications 2. Lamination is radial Lamination is axial 3. More mechanical strength Less mechanical strength 4. The radially laminated rotor Axially laminated rotor in has the best potential for general gives the best economic production. performance 27. Draw the torque – angle characteristics of synchronous reluctance motor. (Dec 2018)

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