Classification or Types of Motor
2 Overview of Three-Phase Induction Motor
Induction motors are used worldwide in many residential, commercial, industrial, and utility applications. Induction Motors transform electrical energy into mechanical energy. It can be part of a pump or fan, or connected to some other form of mechanical equipment such as a conveyor, or mixer. Introduction
• A induction machine can be used as e i t h e r a i n d u c t i o n g e n e r a t o r o r a induction motor. • Induction motors are popularly used in the industry • M a i n f e a t u r e s : c h e a p a n d l o w maintenance • Main disadvantages: speed control is not easy Induction Motor
5 Construction
The three basic parts of an AC motor are the rotor, stator, and enclosure. The stator and the rotor are electrical circuits that perform as electromagnets. Construction Parts of Induction motor
8 The other parts, which are required to complete the induction motor, are: • Shaft for transmitting the torque to the load. This shaft is made up of steel. • Bearings for supporting the rotating shaft. • One of the problems with electrical motor is the production of heat during its rotation. To overcome this problem, we need a fan for cooling. • For receiving external electrical connection Terminal box is needed. • There is a small distance between rotor and stator which usually varies from 0.4 mm to 4 mm. Such a distance is called air gap. Constructio n An induction motor has two main parts
1. Stator 2. Rotor Stat or Bearings Frame Stator Core Rotor
Stator winding Rotor winding Construction (Enclosure)
The enclosure consists of a frame (or yoke) and two end brackets (or bearing housings). The stator is mounted inside the frame.
Stator
Rotor
Air gap Construction (Stator construction)
The stator is the The stator core is made up of . Stator laminations are forming a .
Electromagnetism is the principle behind motor operation. , together with the steel core it surrounds, form an electromagnet. The stator windings are connected directly to the power source. Stator Frame
13 It is the outer most part of the three phase induction motor. Its main function is to support the stator core and the field winding. It acts as a covering and it provide protection and mechanical strength to all the inner parts of the induction motor. The frame is either made up of die cast or fabricated steel. Stator Core
14 The main function of the stator core is to carry the alternating flux. In order to reduce the eddy current loss, the stator core is laminated. These laminated types of structure are made up of stamping which is about 0.4 to 0.5 mm thick. All the stamping are stamped together to form stator core, which is then housed in stator frame. The stamping is generally made up of silicon steel, which helps to reduce the hysteresis loss occurring in motor.
Stator Winding or Field Winding
16 The slots on the periphery of stator core of the three phase induction motor carries three phase windings This three phase winding is supplied by three phase ac supply. The three phases of the winding are connected either in star or delta depending upon which type of starting method is used. The winding wound on the stator of three phase induction motor is also called field winding and when this winding is excited by three phase ac supply it produces a rotating magnetic field. Stat or Rotor
Types: 1. Squirrel-cage rotor 2. wound-rotor Rotor (Laminations) Construction (Rotor construction)
Induction motor types:
Rotor winding is composed of copper bars embedded in the rotor slots and shorted at both end by end rings Simple, low cost, robust, low maintenance
Rotor winding is wound by wires. The winding terminals can be connected to external circuits through slip rings and brushes. Easy to control speed, more expensive. Squirrel-cage rotor Squirrel Cage Rotor Squirrel Cage Rotor
23 The rotor of the squirrel cage three phase induction motor is cylindrical in shape and have slots on its periphery. The slots are not made parallel to each other but are bit skewed (skewing is not shown in the figure of squirrel cadge rotor beside) as the skewing prevents magnetic locking of stator and rotor teeth and makes the working of motor more smooth and quieter. Squirrel Cage Rotor
24 The squirrel cage rotor consists of aluminum, brass or copper bars (copper bras rotor is shown in the figure beside). These aluminum, brass or copper bars are called rotor conductors and are placed in the slots on the periphery of the rotor. The rotor conductors are permanently shorted by the copper or aluminum rings called the end rings. In order to provide mechanical strength these rotor conductor are braced to the end ring and hence form a complete closed circuit resembling like a cage and hence got its name as "squirrel cage induction motor". Advantages of squirrel cage induction rotor
25 Its construction is very simple and rugged. As there are no brushes and slip ring, these motors requires less maintenance. Applications: Squirrel cage induction motor is used in lathes, drilling machine, fan, blower printing machines etc Construction (Rotor construction)
Squirrel-Cage Rotor Wound-rotor(Slip Ring) Wound-rotor Motor
28 In this type of three phase induction motor the rotor is wound for the same number of poles as that of stator but it has less number of slots and has less turns per phase of a heavier conductor. The rotor also carries star or delta winding similar to that of stator winding. The rotor consists of numbers of slots and rotor winding are placed inside these slots. The three end terminals are connected together to form star connection. Wound-rotor Motor
29 Wound-rotor Motor
30 The three ends of three phase windings are permanently connected to these slip rings. The external resistance can be easily connected through the brushes and slip rings and hence used for speed control and improving the starting torque of three phase induction motor. The brushes are used to carry current to and from the rotor winding. Applications
31 Application of Slip Ring Induction Motor Slip ring induction motor are used where high starting torque is required i.e in hoists, cranes, elevator etc.
Applications of Squirrel Cage Induction Rotor We use the squirrel cage induction motors in lathes, drilling machine, fan, blower printing machines, etc 32
Slip ring or phase wound Squirrel cage induction motor Induction motor Construction is complicated due to presence of slip ring and Construction is very simple brushes The rotor consists of rotor bars The rotor winding is similar to which are permanently shorted the stator winding with the help of end rings Since the rotor bars are We can easily add rotor permanently shorted, its not resistance by using slip ring and possible to add external brushes resistance Due to presence of external Staring torque is low and resistance high starting torque cannot be improved can be obtained Slip ring and brushes are Slip ring and brushes are present absent Difference between Slip Ring and Squirrel Cage Induction Motor 33 Slip ring or phase wound Squirrel cage induction motor Induction motor The construction is complicated The construction is simple and and the presence of brushes robust and it is cheap as and slip ring makes the motor compared to slip ring induction more costly motor This motor is rarely used only Due to its simple construction 10% industry uses slip ring and low cost. The squirrel cage induction motor induction motor is widely used Rotor copper losses are high Less rotor copper losses and and hence less efficiency hence high efficiency Speed control by rotor Speed control by rotor resistance method is not resistance method is possible possible Slip ring induction motor are Squirrel cage induction motor is used where high starting torque used in lathes, drilling machine, is required i.e in hoists, cranes, fan, blower printing machines elevator etc etc Rotating Magnetic Field
When a 3 phase stator winding is to a 3 phase voltage supply, 3 phase current will , which also will 3 phase flux in the stator. These flux will rotate at a speed called a . The flux is called as Rotating magnetic Field Synchronous speed1: s2p0efed of rotating flux n = s p
Where; p = is the number of poles, and f = the frequency of supply Rotating Magnetic Field
35
When we apply a three-phase supply to a three-phase distributed winding of a rotating machine, a rotating magnetic field is produced which rotates in synchronous speed. 36
The magnetic flux produced by the current in each phase can be represented by the equations given below. 37 38 Principle of operation Three phase windings of stator are connected to three phase supply, so three phase magnetic fluxes are produced. Due to combination of three phase fluxes rotating magnetic flux is generated. This rotating magnetic field cuts the rotor windings and produces an induced voltage in the rotor windings. Due to the fact that the rotor windings are short circuited, for both squirrel cage and wound-rotor, and induced current flows in the rotor windings. The rotor current produces another magnetic field. A torque is produced as a result of the interaction of those two magnetic fields t ind =kBR ´ Bs
Where ind is the induced torque and BR and BS are the magnetic flux densities of the rotor and the stator respectively Induction motor speed At what speed will the IM run? Can the IM run at the synchronous speed, why? If rotor runs at the synchronous speed, which is the same speed of the rotating magnetic field, then the rotor will appear stationary to the rotating magnetic field and the rotating magnetic field will not cut the rotor. So, no induced current will flow in the rotor and no rotor magnetic flux will be produced so no torque is generated and the rotor speed will fall below the synchronous speed When the speed falls, the rotating magnetic field will cut the rotor windings and a torque is produced Slip and Rotor Speed
The rotor speed of an Induction machine is different from the speed of Rotating magnetic field. The % difference of the speed is called slip. ns - nr s = OR nr =ns (1- s) ns
Where; ns = synchronous speed (rpm) nr = mechanical speed of rotor (rpm) under normal operating conditions, s= 0.01 ~ 0.05, which is very small and the actual speed is very close to synchronous speed. Note that : s =(ns - n)rpm Effect of slip in rotor parameters Note : 120 f At stator : n s = p n p \ f = s .....( i) 120 120 f At Rotor : n s - n r = p (n - n ) p \ f = s r .....( ii ) r 120
(ii ) ¸ (i) : f r = s. f Slip and Rotor Speed
When the rotor move at rotor speed, nr (rps), the stator flux will circulate the rotor conductor at a speed of (ns-nr) per second. Hence, the frequency of the rotor is written as:
fr =(ns - nr ) p =sf
Where; s = slip f = supply frequency Slip may be expressed as a percentage:
Where s is the slip Notice that : if the rotor runs at synchronous speed s = 0 if the rotor is stationary s = 1 Induction Motor Speed = Rotor speed
Effect of Slip on magnitude of rotor induced EMF
When rotor is at standstill s=1, relative speed is max amd max. emf get induced in the rotor, Let this emf is E2
As the rotor gains speed, the relative speed b/w rotor and RMF decreases and hence induced emf in rotor also Decreases as it is proportional to speed (Ns-N), let this emf be E2r E2r E2r Ns- N =s = E2 E2 Ns E2r =sE2 Power Stages in an Induction Motor
The input electric power fed to the stator of the motor is converted into mechanical power at the shaft of the motor. The various losses during the energy conversion are:
Fixed losses (i) Stator iron loss (ii) Friction and windage loss
Variable losses (i) Stator copper loss (ii) Rotor copper loss Power Stages in IM Power Stages in IM (i) Stator input, Pi = Stator output + Stator losses = Stator output + Stator Iron loss + Stator Cu loss (ii) Rotor input, Pr = Stator output It is because stator output is entirely transferred to the rotor through airgap by electromagnetic induction. (iii) Mechanical power available, Pm = Pr - Rotor Cu loss This mechanical power available is the gross rotor output and will produce a gross torque Tg. (iv) Mechanical power at shaft, Pout = Pm - Friction and windage loss Mechanical power available at the shaft produces a shaft torque Tsh. Pm - Pout = Friction and windage loss
INDUCTION MOTOR Double squirrel cage motor
Squirrel cage motors are the most commonly used induction motors, but the main drawback in them is their poor starting torque due to low rotor resistance. (Starting torque is directly proportional to the rotor resistance). One can not even add external resistance for starting of purposes, as the rotor bars are permanently short circuited. These drawbacks are removed by a double squirrel cage motor, which has high starting torque without sacrificing efficiency Double Squirrel-Cage Rotor Construction
It consists of two layers of bars, both short-circuited by end rings. The upper bars are small in cross-section and have a high resistance. They are placed near the rotor surface so that the leakage flux sees a path of high reluctance; consequently, they have a low leakage inductance. The lower bars have a large cross-section, a lower resistance and a high leakage inductance.
Double Squirrel-Cage Rotor Construction
At starting, rotor frequency is high and very little current flows through the lower bars; the effective resistance of the rotor is then the high resistance upper bars. A t n o r m a l l o w s l i p operation, leakage r e a c t a n c e s a r e negligible, and the rotor current flow s largely through the low resistance lower bars; t h e e f f e c t i v e ro t o r resistance is equal to that of the two sets of bars in parallel. The outer side rotor offers more resistance but poor inductive reactance. The higher valued rotor resistance results more torque to be developed at the starting. Working of double squirrel cage motor
At starting of the motor, frequency of induced emf is high because of large slip (slip = frequency of rotor emf / supply frequency). Hence the reactance of inner cage (2πfL where, f = frequency of rotor emf) will be very high, increasing its total impedance. Hence at starting most of the current flows through outer cage despite its large resistance (as total impedance is lower than the inner cage). This will not affect the outer cage because of its low reactance. And because of the large resistance of outer cage starting torque will be large. Double Squirrel Cage Motor
As speed of the motor increases, slip decreases, and hence the rotor frequency decreases. In this case, the reactance of inner cage will be low, and most of the current will flow through the inner cage which is having low resistance. Hence giving a good efficiency. Equivalent Circuit of Double Squirrel-Cage Rotor Construction
Equivalent circuit of a single- cage induction motor (with one rotor winding).
Equivalent circuit of a double- cage induction motor (two rotor windings). The no-load test
The efficiency of large motors can be determined by directly loading them and by measuring their input and output powers. For larger motors it may be difficult to arrange loads for them. Moreover power loss will be large with direct loading tests. Thus no load and blocked rotor tests are performed on the motors. No load test is performed when rotor rotates with synchronous speed and there is no load torque. This test gives the information regarding no-load losses such as core loss, friction loss and windage loss. This test is used to evaluate the resistance and impedance of the magnetizing path of induction motor. The no-load test Theory of No Load Test of Induction Motor
An ammeter measures the no-load current, and a voltmeter gives the normal rated supply voltage. The I2R losses on the primary side is neglected as they vary with the square of the current as we know that the no load current is 20-30% of the full load current, As the motor is running at no load, the total input power is equal to the constant iron loss, friction and windage losses of the motor. If, Vinl is the input line voltage Pinl is the total three-phase input power at the no load I0 is the input line current Vip is the input phase voltage Blocked-rotor Test
A blocked rotor test is normally performed on an induction motor to find out the leakage impedance. Apart from it, other parameters such as torque, motor, short-circuit current at normal voltage, and many more could be found from this test. Blocked-rotor Test
In block rotor test, the low voltage is applied so that the rotor does not rotate and its speed becomes zero and full load current passes through the stator winding. The slip is unity related to zero speed of rotor hence the load resistance becomes zero. Now, slowly increase the voltage in the stator winding so that current reaches to its rated value. At this point, note down the readings of the voltmeter, wattmeter and ammeter to know the values of voltage, power and current. The test can be repeated at different stator voltages for the accurate value. Blocked-rotor Test
Where cos is the power factor of the short circuit.
The equivalent resistance of the motor referred to the stator side is given by the equation shown below. Blocked-rotor Test
The equivalent impedance of the motor referred to the stator side is given by the equation shown below.
The equivalent reactance of the motor referred to the stator side is given by the equation shown below. Blocked-rotor Test
This test should be performed at the reduced frequency. In order to obtain the accurate results, the Blocked Rotor Test is performed at a frequency 25 percent or less than the rated frequency. The leakage reactances at the rated frequency are obtained by considering that the reactance is proportional to the frequency. However, for the motor less than the 20-kilowatt rating, the effects of the frequency are negligible, and the blocked rotor test can be performed directly at the rated frequency. Blocked-rotor Test
If normal voltage applied to the stator, short ckt current would be : Is ISN = ´ V 1 Vs Where Isn is short circuit current with normal volatge V1 Abnormal operation of 3 phase IM Abnormal operation of a three phase IM are due to Internal causes (e.g. short ckt in stator, overheating of bearing etc) or External causes: Mechanical overload Supply voltage changes Single phasing Frequency changes
According to national standards, a motor shall operate successfully on any volatge within ±10% of the nominal volatge and for any frequency within ± 5% of the nominal frequency. Unbalance In Induction Motor
Voltage unbalance can be more detrimental than voltage variation to motor performance and motor life. When the line voltages applied to a polyphase induction motor are not equal in magnitude and phase angle, unbalanced currents in the stator windings will result. A small percentage voltage unbalance will produce a much larger percentage current unbalance. Causes
Some of the causes of voltage unbalance are the following: 1. An open circuit in the primary distribution system.
2. A combination of single-phase and three-phase loads on the same distribution system, with the single-phase loads unequally distributed.
3. An open wye-delta system. Unbalance In Induction Motor a. Transformer loading varied 50 to 150%: The greatest unbalance occurs when a smaller transformer is lightly loaded and a larger transformer is overloaded. If a single-phase load varies over a large range, it is better to supply this phase with the larger transformer on the leading phase. b. Impedance of lines to the single-phase loads: The voltage and current unbalance ratios increase with the line impedances. Again, the unbalance ratios decrease as the motor is loaded more heavily. Unbalance In Induction Motor c. Impedance of the supply line to the motor: The voltage and current unbalance ratios dec re a s e w i t h a n i n c re a s e i n t h e l i n e impedance to the motor. However, this results in lower voltage at the motor and decreased motor torque and speed. d. Other parameters: Va r i a t i o n s i n t h e m a g n i t u d e o f transformer impedances, the power factor of s i n g l e - p h a s e l o a d s , a n d p r i m a r y l i n e impedances have minor effects (not more t h a n 3 % ) o n t h e p h a s e c u r r e n t s a n d unbalance ratios.