OBJECTIVE QUESTIONS 

1. The speed at which the rotating magnetic field produced by stator currents rotates in an induction motor is:

 (A)    Synchronous speed(B) Rotor speed(C) Greater than synchronous speed

 Ans. (A) 

2. 2.       A 3-phasde 50 Hz induction motor runs at speed of 940 RPM; the speed of the rotating magnetic field will be:

 (A) 940 RPM(B) 1000RPM(C) 1050 RPM(D) 1100 RPM

 Ans. (B) 

3. The no-load speed of an induction motor is of the order of  (A) 8%(B) 4%

(C) 2%(D) 6%

 Ans. (C)

 4. Iron losses in the rotor of a 3-phase induction motor are negligible, because  (A) (A)    Frequency of rotor emf is too low(B) (B)    Flux linking the rotor is of constant magnitude(C) (C)    Flux density in the rotor parts is too low 

Ans: (A) 

5. Full-load copper losses in a 3-phase 50 Hz 4-pole induction motor running at 1455 rpm are 300 W. The rotor input is

 (A) (A)    5 kW(B) (B)    10 kW(C) (C)    20 kW(D) (D)    50 kW 

Ans. (B) 

6. The no-load current of a 3-phase induction motor in terms of its full-load current is of the order of  (A) (A)    5%(B) (B)    10%(C) (C)    25%(D) (D)    50% 

Ans. (C) 

7. The power factor of a 3-phase induction motor at no load is approximately  (A) (A)   0.2(B) (B)    0.7(C) (C)    0.85(D) (D)    1

 Ans: (A) 

8.The maximum torque in a 3-phase induction motor occurs at a slip 

(A) (A)       R2/ X2

(B) (B)       X2/ R2

(C) (C)      R22/ X2

(D) (D)      R22/ X2

2

 Ans: (A) 

9. For high starting torque, the most 3-phase induction motor is  

(A) (A)    Squirrel-cage type(B) (B)    Slip-ring type(C) (C)    Deep bar squirrel-cage type(D) (D)    Double-cage induction motor

 Ans. (B) 

10. 10.     The starting torque in a single-phase induction motor is  (A) (A)   High(B) (B)    Low(C) (C)    Zero(D) (D)    Very low

 Ans. (C)

 11. 11.     The efficiency of a single-phase induction motor as compared to a 3-phase induction motor

rating is (A) (A)   Lower(B) (B)    Higher(C) (C)    Equal 

Ans: (A) 

12.12.     The starting torque of a 3-phase induction motor can be increased by increasing

 

a)        the rotor reactance

b)       the rotor resistance

c ) the stator resistance

 Ans. B

13. 13.  The maximum starting torque in the induction motor is developed when the rotor reactance is

 a) a)      half of the rotor resistanceb) b)       twice of the rotor resistancec) c)       equal to the rotor resistance

 Ans. C 

14. 14.  A change of 55 in the supply voltage of an induction motor will produce a change of approximately

 a) a)      5%b) b)       7%c) c)       10%d) d)       25%

in the rotor torque.Ans. D 

15. 15.     The transformation ratio of an induction motor can be found by 

a) a)       blocked rotor testb) b)       open circuit testc) c)       slip testd) d)       stator resistance test

 16. 16.     The crawling in the induction motor is caused by 

a) a)       improper design of the machineb) b)       low supply voltagec) c)       high loadsd) d)       harmonics developed in the motor.

 Ans. D 

17. 17.     Which one of the following motors is most suitable for signalling devices and timers? 

a) a)       dc series motorb) b)       ac series motorc) c)       induction motord) d)       reluctance motor

 Ans. D