INTRODUCTION Unit III-Three Phase Induction Motor: The three phase induction motor runs on three phase AC supply. It is an ac motor. The power is transferred by means of induction. So it is also called as rotating transformer. CONSTRUCTIONAL DETAILS OF THREE PHASE INDUCTION MOTOR Prepared by:p.priyadharshini AP/EEE - 1 -
Squirrel cage Rotor Prepared by:p.priyadharshini AP/EEE - 2 -
Phase -Wound Rotor Prepared by:p.priyadharshini AP/EEE - 3 -
PRINCIPLE OF OPERATION Prepared by:p.priyadharshini AP/EEE - 4 -
SLIP The difference between synchronous speed and rotor speed and expressed as a percentage of synchronous speed FREQUENCY OF ROTOR CURRENT ' ' ' 120 f 120 Also S NS N N P Dividing one by the other we get f f f NS N N S sf s f P PROBLEM: 01 Prepared by:p.priyadharshini AP/EEE - 5 -
Note Relation between torque and rotor power factor Prepared by:p.priyadharshini AP/EEE - 6 -
Starting Torque Prepared by:p.priyadharshini AP/EEE - 7 -
Rotor emf and reactance under running condition Prepared by:p.priyadharshini AP/EEE - 8 -
Rotor torque Prepared by:p.priyadharshini AP/EEE - 9 -
Condition for maximum torque Prepared by:p.priyadharshini AP/EEE - 10 -
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SYNCHRONOUS WATT PROBLEM: 01 Prepared by:p.priyadharshini AP/EEE - 12 -
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EQUIVALENT CIRCUIT OF INDUCTION MOTOR Prepared by:p.priyadharshini AP/EEE - 14 -
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LOSSES AND EFFICIENCY 1. LOSSES Prepared by:p.priyadharshini AP/EEE - 17 -
2. EFFICIENCY Prepared by:p.priyadharshini AP/EEE - 18 -
Power flow diagram for an induction motor Prepared by:p.priyadharshini AP/EEE - 19 -
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PROBLEM: 01 Prepared by:p.priyadharshini AP/EEE - 21 -
PROBLEM: 02 Prepared by:p.priyadharshini AP/EEE - 22 -
PROBLEM: 03 Prepared by:p.priyadharshini AP/EEE - 23 -
PROBLEM: 04 Prepared by:p.priyadharshini AP/EEE - 24 -
PROBLEM: 05 Prepared by:p.priyadharshini AP/EEE - 25 -
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SLIP-TORQUE CHARACTERISTICS Prepared by:p.priyadharshini AP/EEE - 27 -
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GENERATING AND BRANKING REGION Motoring Mode: In this mode of operation, supply is given to the stator sides and the motor always rotates below the synchronous speed. The induction motor torque varies from zero to full load torque as the slip varies. The slip varies from zero to one. It is zero at no load and one at standstill. From the curve it is seen that the torque is directly proportional to the slip. Generating Mode In this mode of operation induction motor runs above the synchronous speed and it should be driven by a prime mover. The stator winding is connected to a three phase supply in which it supplies electrical energy. Actually, in this case, the torque and slip both are negative so the motor receives mechanical energy and delivers electrical energy. Induction motor is not much used as generator because it requires reactive power for its operation Braking Mode In the Braking mode, the two leads or the polarity of the supply voltage is changed so that the motor starts to rotate in the reverse direction and as a result the motor stops. This method of braking is known as plugging. This method is used when it is required to stop the motor within a very short period of time. Prepared by:p.priyadharshini AP/EEE - 29 -
Effect of rotor resistance upon torque slip or torque speed relationship Prepared by:p.priyadharshini AP/EEE - 30 -
PROBLEM: 01 PROBLEM: 02 Prepared by:p.priyadharshini AP/EEE - 31 -
PROBLEM: 03 Prepared by:p.priyadharshini AP/EEE - 32 -
LOAD TEST Measure: Vl,IL,W1,W2 Prepared by:p.priyadharshini AP/EEE - 33 -
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NO-LOAD TEST Prepared by:p.priyadharshini AP/EEE - 35 -
BLOCKED ROTOR TEST Prepared by:p.priyadharshini AP/EEE - 36 -
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SEPERATION OF NO-LOAD LOSSES CIRCLE DIAGRAM Prepared by:p.priyadharshini AP/EEE - 38 -
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Maximum Quantities Prepared by:p.priyadharshini AP/EEE - 40 -
DOUBLE CAGE CONSTRUCTION Rotor of a double squirrel cage motor has two independent cages on the same rotor. The figure at left shows the cross sectional diagram of a double squirrel cage rotor. Bars of high resistance and low reactance are placed in the outer cage, and bars of low resistance and high reactance are placed in the inner cage. The outer cage has high 'reactance to resistance ratio' whereas, the inner cage has low 'reactance to resistance ratio' Working: 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 resistnace (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. 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. Prepared by:p.priyadharshini AP/EEE - 41 -
When the double cage motor is running at normal speed, frequency of the rotor emf is so low that the reactance of both cages is negligible. The two cages being connected in parallel, the combined resistance is low. SYNCHRONOUS INDUCTION MOTOR Prepared by:p.priyadharshini AP/EEE - 42 -
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Advantages Disadvantages Prepared by:p.priyadharshini AP/EEE - 44 -
TWO MARKS QUESTION AND ANSWERS 01 02 03 04 05 06 07 08 Prepared by:p.priyadharshini AP/EEE - 45 -
09 10 11 12 13 14 Sketch the torque slip characteristics of a squirrel cage 3 phase induction motor. Prepared by:p.priyadharshini AP/EEE - 46 -
15 Can induction motor runs at Synchronous speed? Why? No. If, for some reasons, the rotor speed equals synchronous speed, it will lead to absence of relative speed and hence the rotor emf and current will become zero and the machine will not be able to develop any mechanical torque to run as a motor. 16 Draw the approximate equivalent circuit of induction motor. 17 What is the circle diagram of an induction motor? When an induction machine operates on constant voltage and constant frequency source, the loci of stator current phasor is found to fall on a circle. This circle diagram is used to predict the performance of the Induction machine at different loading conditions as well as mode of operation. 18 What is meant by crawling in 3 phase induction motor? It is observed that 3 phase induction motors, especially the squirrel cage type, sometimes exhibit a tendency to run at a stable speed as low as one-seventh of their synchronous speed, N s and is unable to pick up to its normal speed. This phenomenon is known as crawling of Induction motor. 19 What is meant by cogging in 3 phase induction motor? Sometimes, even when full voltage is applied to the stator winding, the rotor of squirrel cage 3 phase Induction motors refuses to start and pickup speed. This happens when the number of stator slots, S s is equal to the number of rotor slots, s r and is due to magnetic locking between the stator and rotor teeth. This phenomenon is called as cogging or teeth locking. 20 How are the outer and inner cage windings arranged in double cage induction motors? The rotor of a double-cage Induction motor houses two squirrel-cage windings embedded in two rows of slots. The outer slots contain a high resistance, low leakage reactance cage winding and the inner slots contain a low resistance, high leakage reactance cage winding. Prepared by:p.priyadharshini AP/EEE - 47 -
21 What is an induction generator? When an induction machine is driven at a speed N r greater than its synchronous speed N s, i.e. at super-synchronous speed, the slip becomes negative, 0>S>-1 and the machine receives mechanical energy as the input through the shaft and delivers electrical energy as the output through the stator terminals. Induction machine operating in this mode is called induction generator. 22 List the advantages and disadvantages of induction generators. Advantages (i) Induction generators need less number of auxillary equipments (ii) They can be operated in parallel with other generators without hunting. (iii) Speed variation of prime-mover is less important. (iv) The generator has a self protective feature i.e. in case a short circuit fault occurs on its terminals, excitation fails and hence the machine stops generating current. (v) Very robust in construction requiring less maintenance. Disadvantages (i) (ii) (iii) Induction generators draw considerable amount of lagging reactive power from the supply for excitation and other operations. Efficiency is relatively poor. The generator can operated only at leading p.f. i.e the reactive power delivery cannot be controlled. 23 Under what condition, the slip in an induction motor is a) Negative b)greater than one. a) The slip of induction motor is negative when it runs more than the synchronous speed and acts as a generator. b) When the slip of induction motor is greater than one, it runs in a braking mode and it is rotated in opposite direction to the rotating magnetic field. 24 What are the two fundamental characteristics of a rotating magnetic field? 25 Write down the advantages of double cage rotor. Prepared by:p.priyadharshini AP/EEE - 48 -