INDUCTION MOTORS

1.      In a 3-phase squirrel cage induction motor, rotor conductors are shorted or opened.                                           

A:   Short circuited through end ring

2.      The rotor of a 3-phase wound rotor induction motor it provided with 1-phase or 3-phase winding.

A:   3-phase winding

3.      In an induction motor, if the air gap is increased, the power factor will ___

A:   reduce

4.      The rotor slots are slightly skewed in squirrel cage induction motor to reduce ___

A:   the magnetic flux and locking tendency of rotor

5.      The operation of induction motor is based on ___

A:   principle of mutual induction

6.      The rotor of an induction motor never runs at synchronous speed because then the relative speed between the rotating flux and rotor will be ___

A:   zero and hence torque will be zero

7.      An induction motor  is in general analogous to ___

A:   two winding transformer with secondary short-circuited

8.      The speed between starter and rotor fields, in an induction motor will be equal to ___

A:   zero

9.      A 3-phase induction motor at slip ‘s’. If  its two supply loads are interchanged, then its slip at that instant will be equal to ___

A:   ‘2s’

10.  In a 3-phase induction motor, the torque developed is maximum when the rotor circuit resistance per phase is equal to ___

A:   slip times the rotor leakage reactance per phase at stand still

11.  Breakdown torque of a 3-phase induction motor of negligible stator impedance is  ___ proportional to the rotor leakage reactance.

A:   inversely

12.  In a 3-phase motor iron loss mainly occurs in ___

A:   stator core

13.  In an induction motor slip depends on what rotor circuit parameters

A:   rotor i/p power and rotor Cu loss

14.  In a 3-phase induction motor, the mechanical power developed in terms of air gap power is ___

A:   (1-s) P_g

15.  The stator referred resistance in the equivalent circuit of an induction motor , representing mechanical o/p is ___

A:   r₂^/ ([1/s]-1)

16.  Blocked rotor test of an induction motor corresponds, in case of a transformer to ___

A:   short-circuit operation

17.  When the induction motor is stand still the slip is ___

A:   one

18.  The crawling in the induction motor is caused by ___

A:   harmonics developed in motor

19.  Pole charging method of speed control is used in ___ type of transformer.

A:   squirrel cage

20.  In v/f speed control of 3-phase induction motor with increase in frequency, the maximum torque and slip at which max torque occurs, would respectively ___

A:   increase and decrease

21.  In a 3-phase induction motor if supply voltage and frequency are reduced by the same ratio then slip at which max torque occurs is ___

A:   more and maximum torque remains constant

22.  The absolute speed of magnetic field in space of a 3-phase rotor fed induction motor is ___

A:   (N_s-N_r)

23.  Rotor resistance speed control is used in ___ type of induction motor.

A:   slip ring

24.  While breaking a 3-phase induction motor, its terminals were switched over to a dc source from ac supply, the method of breaking is___

         a) Regenerative braking                                            b) counter-current braking

        c) eddy-current braking                                             d) none

A:   [d]

25.  In a 3-phase induction motor, the regenerative braking occurs when ___

A:   the load is lowered by a hoisting machine