BVRIT HYDERABAD College of Engineering for Women Department of Electrical and Electronics Engineering
Hand Out
Subject Name: Electrical Machine-III
Prepared by (Faculty(s) Name): Babita Gupta, Assistant Professor, EEE
Year and Sem, Department: III Year- I Sem, EEE
Unit – I: synchronous Machines & characteristics Important Points / Definitions: Synchronous machine is know as doubly excited machine. Synchronous machines can be operated as either generators or motors., The main parts of a synchronous machine are identified: rotor (field); stator (armature); prime mover ; exciter. An alternator which converts mechanical energy from a prime mover to AC electric power at specific voltage and frequency. It is also known as synchronous generator. The alternator work on the principle of faraday’s law of electromagnetic induction. The direction of the induced current can be determined by Flemming's right-hand rule. The armature winding in an alternator either open & closed type. Closed winding forms star connection in an alternator. There are different types of armature winding used in alternator. 1. Single phase and poly phase armature winding. 2.Concentrated winding and distributed winding 3.Full pitch coil winding and short pitch coil winding. 4.Intergral winding and fractional slot winding. There are two types of rotors in an alternator:- 1. Salient pole type. 2. Smooth cylindrical type.
The e.m.f equation of an alternator is Eph= 4.44fΦTphKcKd volts Synchronous generator always runs at synchronous speed i.e Ns = 120f/P. Harmonics which are generated in the e.m.f due to slotting is called slot harmonics. Different ways to eliminate the harmonics from generated voltage: 1. Distribution of armature winding 2.Chording 3.Fractional slot winding 4.Skewing 5.Large length of air gap. Effect of armature flux on the main flux affecting it’s value & the distribution is called armature reaction. The effect of armature reaction depends on the power factor i.e the phase relationship between the terminal voltage and armature current.
Questions
1. Define synchronous machine & its caharacteristics.
2. Distinguish between salient pole & non- salient type rotor
3. Explain construction & working principle of alternator 4. Derive emf equation of three phase of alternator 5. Define harmonics
6. What are the different ways to eliminate the harmonics in generated emf. 7. What is slot harmonics 8. What is expression for pitch factor 9. What is the expression for distribution factor 10. What is the effect of harmonics in generated emf of an alternator. 11. Which type of alternator is used in hydroelectric power station? 12. Which type of alternator is used in steam turbines?
Fill in the blanks / choose the Best: 1. When a turbine drives an alternator it is termed as turboalternator 2. A two pole alternator running at 1500 rpm will generate emf at 25 Hz. 3. A generator has two different windings, armature,field winding. 4. In a fractional-pitch winding the coil span is less than1800 electrical degrees. 5. In a synchronous generator, the armature is always a stationary member, and the field moving member. 6. High capacity generators are usually Hydrogen cooled. 7. The efficiency of a synchronous motor is more than that of an induction motor of same size and output rating. 8. The cost of a synchronous motor is High in comparsion to that of an induction motor of same size and output rating 9. Alternator operates on the principle of Electromagnetic induction 10. In synchronous machine stator frame is made of cast iron or welded steel plates 11. The slip rings employed in a three phase synchronous machine are insulated for very low voltage 12. The stator core of a synchronous machine is built up of silicon steel laminations.
Unit – II: Regulation of synchronous generator
Important Points / Definitions:
The voltage regulation is the percentage of voltage difference between no load and full load voltages with respect to its full load voltage.
Voltage regulation = Ef - Vt Vt Voltage regulation can be determined by direct method and indirect method. Direct method can be applicable for small rating of an alternator and it can be done by o.c and s.c test. Indirect method can be applicable for large rating of an alternator. Different method for finding voltage regulation of an alternator:- 1. Synchronous impedance method or E.M.F method 2. Ampere-turns method or M.M.F method 3.Zero power factor metod or potier triangle method 4. ASA modified form of M.M.F method Synchronous impedance method is called pessimistic method Ampere-turns method is called optimistic method
American standard Association method is superior to ZPF method because it consider drop due to Armature Reaction, Leakage Reactance and also consider Saturation. Emf generated in a synchronous motor is given by:
Potier triangle is drawn in between Open circuit characteristics and Zero power factor characteristics.
Slip test is used to determine the value of xd and xq Synchronous reactance is the combined reactance due to leakage flux and armature reaction. Questions
1. Define Voltage regulation.
2. At what power factor Zero Voltage regulation occurs 3. What is the use of Damper Windings? 4. Why ASA method is superior to ZPF method? 5. Define Slip test?
6. What is the relation between Xd &Xq for non salient pole machine 7. Which method is called pessimistic method? 8. Which method is called optimistic method? 9. What is meant by Quadrature and Direct axis reactance 10. What are the advantages of Slip Test? 11. Why do you conduct Slip-test 12. Salient pole Alternators are used in which type of power plants 13. What is meant by two axis reactance theory
Fill in the blanks / choose the Best: 1. Synchronous impedance method is called pessimistic method . 2. Ampere-turns method is called optimistic method
3. Slip test is used to determine the value of xd and xq 4. Slip test is used for non-salient pole machine. 5. Salient pole machine has non-uniform air gap. 6. Salient pole alternators are used in Hydro-electric plants. 7. Zero power factor method of an alternator is used to find its voltage regulation 8. During slip test for maximum current, the armature mmf aligns along d-axis. 9. The frequency of voltage generated in an alternator depends on no. of poles 10. In synchronous impedance method the value of synchronous method is more than normal value 11. Synchronous reactance is the combined reactance due to leakage flux and armature reaction. 12. The leakage reactance of a three phase alternator is determined by performing open-circuit and zero power factor test.
Unit – III: Parallel operation of synchronous generator
Important Points / Definitions: The process of connecting an alternator in parallel with another alternator or with common bus- bars is called the synchronization. Bus bars are copper rods which operate at constant voltage & frequency. The synchronous machine which is to be synchronized is called incoming machine. Synchronization can be done by lamp method in case of single phase motor. A synchroscope is a special device used for synchronizing the machine more accurately. An alternator synchronised to the infinite bus delivers lagging power to the bus when the field is overexcited. When two alternators are operating in proper synchronism,the synchronism power will be zero. synchonizing current circulating in the local circuit of alternator operating in parallel which brings the alternator in synchronism once they are out of it. Necessary condition to be satisfied for synchronising an incoming alternator to an already operating alternator should have: 1.Same voltage magnitude. 2.Same frequency. 2. Same phase sequence.
Questions
1. Define Synchronism. 2. Define infinite busbar. 3. What conditions must be fulfilled before an alternator can be connected to an infinite bus.? 4. Why parallel operation of alternators is necessary? 5. What are the advantages of connecting alternators in parallel. 6. What are the various methods of synchronization of alternators. 7. Define significance of transient and subtransient reactances in an alternator? 8. Explain the effect of change in excitation on the parallel operation of two alternators? 9. Define synchronizing torque and synchronizing power? 10. Define load sharing between two alternators. 11. Define synchonizing current?
Fill in the blanks / choose the Best: 1. For parallel operation , the frequencies of two machines should be same 2. When two alternators are operating in proper synchronism,the synchronism power will be zero. 3. An alternator synchronised to the infinite bus delivers lagging power to the bus when the field is overexcited. 4. The process of connecting an alternator in parallel with another alternator or with common bus-bars is called the synchronization. 5. Two- bright and one dark lamp method is used for synchronising of three phase alternator. 6. For successful parallel operation of two alternators should have high reactance in comparsion to resistance. 7. Transient response analysis is done for stable systems. 8. Synchronizing torque will come into operation whenever there is difference in the magnitude of voltages. 9. A stationary alternator should not be connected to the live bus-bar it will get short circuited. 10. If the prime mover of an alternator supplying load to an infinite bus suddenly shut down then it will continue to run as synchronous motor in the same direction.
Unit – IV: Synchronous Motors Important Points / Definitions: Electrical motor in general is an electro-mechanical device that converts energy from electrical energy to mechanical energy. Synchronous motors are inherently not self starting. They require some external means to bring their speed close to synchronous speed to before they are synchronised. To start the synchronous motor 1. Using induction motor 2. Using D.C. machine 3.Using slip ring induction motor 4.Using Damper winding. Synchronous motor always start at synchronous speed i.e
An overexcited synchronous motor operate at leading power factor, under-excited synchronous motor operate at lagging power factor and normal excited synchronous motor operate at unity power Due to sudden application of load the rotor has to search or hunt for its new equilibrium position. This phenomenon is know as hunting in synchronous motor. Causes of Hunting in Synchronous Motor 1.Sudden change in load. 2.Sudden change in field current. 3.A load containing harmonic torque. 4.Fault in supply system.
Questions
1. Define synchronous motor. 2. Why synchronous motor is not self starting motor? 3. What is hunting & how it can be suppressed. 4. What are the different methods of starting the synchronous motor? 5. What is synchronous condenser & it’s application? 6. Define two reaction theory of synchronous motor? 7. What is the difference between synchronous motor & induction motor. 8. What are the application of synchronous motor? 9. What are the uses of damper winding in synchronous motor. 10. What are the salient features of synchronous motor. 11. What are the different types of synchronous motor. 12. What is V curves & inverted V curves. 13. How can we run a synchronous motor as synchronous condenser. 14. Define critical excitation 15. What happens to the pf if the field current is gradually increased
Fill in the blanks / choose the Best:
1. The torque of a synchronous motor is less sensitive to change in supply voltage. 2. In a synchronous motor the magnitude of Eb can be varied by increasing DC excitation. 3. A synchronous motor is to be floating when it runs on no load. 4. In a synchronous motor, for maximum power the load angle should be 900 5. In a synchronous motor, the angle between the rotating stator flux and rotor poles is known as torque angle. 6. In a synchronous motor operating at zero power factor lagging with respect to excitation voltage, the armature reaction mmf is magnetizing 7. A synchronous motor working on leading power factor and not coupled to any mechanical load is called synchronous condenser 8. In a synchronous motor, when the back emf generated on no load is approximately equal to the applied voltage, then the percentage of excitation is 100% 9. When the direction of field excitation of a synchronous motor is reversed, the motor will run in the same direction. 10. An under-excited synchronous motor has a lagging power factor. 11. The cost of a synchronous motor is more than the cost of an induction motor of the same size. 12. The power factor of a synchronous motor can be controlled by varying the excitation. 13. The synchronous motor operating underexcited will have lagging power factor 14. The field of a synchronous motor is excited from a DC source. 15. An overexcited synchronous motor draws current at leading power factor 16. In synchronous motors, the pull-out torque is usually 125 to 300% of full load torque. 17. A synchronous motor has a tendency to hunt. 18. A synchronous motor draws minimum current from three phase supply mains at unity power factor. 19. A synchronous motor working at leading power factor without mechanical load is known as synchronous condenser. 20. The duration of sudden short- circuit test on a synchronous motor is about one second.
Unit – V: Single phase motors & special machines
Important Points / Definitions:
Single phase induction motors is also know as asynchronous motors.
Single Phase Induction Motor is not Self Starting. Single Phase Induction Motor is based on double field revolving theory. Types of single phase induction motor are: 1. Split phase induction motor. 2. Capacitor start inductor motor. 3. Capacitor start capacitor run induction motor. 4.Shaded pole induction motor. 5.Permanent split capacitor motor. Single phase induction motor are simple in construction, reliable and economical for small power rating . Single phase induction motor are simple, robust, reliable and cheaper for small ratings. They are available up to 1 KW rating. Single phase induction motor has low starting torque. The efficiency of single phase induction motors is less as compared to three phase induction motor. Split phase induction motors have low starting current and moderate starting torque. Split phase induction motors used for fans , grinders,centrifugal pumps, washing machines etc. Capacitor start induction motors have high starting torque. Capacitor start induction motors used for compressors, conveyors, grinders, fans, blowers. Shaded pole induction motors have low starting torque and low power factor. Shaded pole induction motors used for small fans, toy motors, hair dryers etc. Anti-symmetric pole-zero patterns for every pole in the left half of S-plane, there is a zero in the mirror image position with respect to imaginary axis. A transfer function which has one or more zeros in the right half of the S-plane is known as non- minimum phase transfer function. A transfer function which has least (minimum) phase angle range for a given magnitude curve is called a minimum phase transfer function. Universal motor is a series wound motor which operates at the same speed and output can be either dc or ac of same voltage. AC series motor rotate in the same direction regardless of the polarity of the supply.
Questions
1. What is double field revolving theory? 2. Why a normal.c series motor can not work satisfactorily when connected across a.c supply. Write a note on reluctance motor and its applications. 3. Which are the various types of stepper motor? 4. What is step angle and state the expression for it. 5. Define stepper motor. 6. Why are small fractional horse power a.c. series motor called universal motors. 7. Why do we use compensating winding in a.c series motor? 8. Define universal motor and its application. 9. What are the different types of stepper motor. 10. What are the different types of single pahse induction motor.
Fill in the blanks / choose the Best:
1. Resistance start single phase induction motor is commonly known as split phase motor. 2. The capacitor usually used in a capacitor-start induction motor is electrolytic type. 3. In a shaded pole motor the necessary phase splitting is produced by induction . 4. Reluctance motor is split phase motor with properly designed salient poles and operates at synchronous speed. 5. In a single phase induction motor, the starting torque is zero 6. Fractional horse power motors are usually single phase type. 7. The rotating part of a single phase ac motor is called the rotor 8. Single phase induction motor is based on double field revolving field theory 9. Capacitor- start capacitor-run induction motor is basically a two phase motor. 10. Capacitor- start capacitor-run motor are used in refrigerator. 11. In a shaded pole induction motor,locked rotor current is slightly more than full-load current 12. The speed of a universal motor is usually reduced by using gearing. 13. Speed of universal motor is independent of frequency of supply. 14. A repulsion motor is equipped with commutators and brushes. 15. In a single phase repulsion motor power factor is high at high speed