INDUCTION MOTORS QUESTION BANK

INDUCTION MOTORS QUESTION BANK

1.What are the 2 types of 3 phase induction motor?
A. Squirrel cage and slip ring induction motor.

 2. Write two extra features of slip ring induction motor?
A. Rotor has 3-phase winding. Extra resistance can be added in rotor circuit for improving PF with the help of three slip rings.

3.  Why an induction motor is called as rotating transformer?
A. The rotor receives same electrical power in exactly the same way as the secondary of a two winding transformer receiving its power from primary. That is why induction motor is called as rotating transformer.

4.  Why an induction motor never runs at its synchronous speed?
A. If the motor runs at sync. speed then there would be no relative speed between the two, hence no rotor emf, so no rotor current, then no rotor torque to maintain rotation.

5.  What are slip rings?
A. The slip rings are made of copper alloys and are fixed around the shaft insulating it. Through these slip rings and brushes rotor winding can be connected to external circuit.

6.  What are the advantages of cage motor?
A.Since the rotor has low resistance, the copper loss is low and efficiency is very high. On account of simple construction of rotor it is mechanically robust, initial cost is less, maintenance cost is less, simple starting arrangement.

7. Give the condition for maximum torque for 3 phase induction motor, when it is running?  A.The rotor resistance and reactance should be same for max.torque         i.e. R2=Sx2

8. List out the method for speed control of 3 phase cage type induction motor?
A.By changing supply frequency By changing no of poles By operating the two motors in cascade.

9. Name the two winding of single phase induction motor?
A.Running and starting winding.

10. What are methods available for making single phase induction motor a self starting?
A.By slitting the single phase, by providing shading coil in the poles.

11.  What is the function of capacitor in single phase induction motor?
A.To make phase difference between starting and running winding, to improve PF and to get more torque.

12.  State any 4 use of single phase induction motor?

A.Fans, wet grinders, vacuum cleaner, small pumps, compressors, drills.

13.  What kind of motors used in ceiling fan and wet grinders?
A.Ceiling fan - Capacitor start and capacitor run single phase induction motor, wet grinders - Capacitor start capacitor run single phase induction motor.

14.  What is the application of shaded pole induction motor?
A.Because of its small starting torque, it is generally used for small toys, instruments, hair driers, ventilators.etc.

15.  In which direction a shaded pole motor runs?
A.The rotor starts rotation in the direction from un shaded part to the shaded part.

16.  Why single phase induction motors have low PF?
A.The current through the running winding lags behind the supply voltage by large angle so only single phase induction motor has low power factor.

17.Differentiate between “capacitor start” & “Capacitor start capacitor run” single Phase induction motor (IM)?

 A. Capacitor start – capacitor run is connected series with starting winding, but it will be disconnected from supply when motor pick up its speed. Capacitor start capacitor run- starting winding and capacitor will not be disconnected from supply even though motor pickup its speed.

18.Explain why single phase induction motor is not a self starting one?

      A. When motor fed supply from single phase, its stator winding produces an alternating flux, which doesn‟t develops any torque.

 19.Define slip in an IM?

A.The slip of an IM is defined as the ratio of difference between sync. speed (Ns) and rotor speed (N) to the sync. speed. s= (Ns – N)/ Ns

      

       20.Define slip speed in an IM?

       A. The slip speed is defined as the difference in speed between the rotating magnetic field produced    by stator (Ns) and rotor speed (N).

21.What is the speed of the rotor field in space?

      A. The speed of the rotor field in space is speed of rotating field. 86. What is sync. speed in 3-phase IM? Ns = 120f/p Where f- supply frequency P- No of poles on the stator.

22.List the various methods of speed control of 3 phase IM? 

A. Types of stator side control

1.     Stator voltage control

2.     Stator frequency control

3.     v/f control

4.     pole changing method

23. In which type of motor can resistance be introduced in the rotor circuit? What is the effect of it? A.Slip ring IM. Effects:

1.     starting torque increased

2.     starting current decreased

3.     motor speed can be controlled

24.  Why the slots on the IM are usually skewed?

     A. In order to obtain a uniform torque, reduce the magnetic locking of the stator and rotor and reduce the magnetic humming noise while running.

     

    25.  What will be the effect when stator voltage and freq of a IM are reduced proportionally?

1.     Motor speed increases

2.     Maximum torque is constant

26. What is slip power recovery scheme?

A. Slip power can be returned to the supply source and can be used to supply an additional motor which is mechanically coupled to the main rotor. This type of drive is known as slip power recovery system and improves overall efficiency of the system.