Most Asked Objective Questions or MCQ on Polyphase Induction Motors for Electrical Engineering

Multiple Choice Questions (MCQ) on Polyphase Induction Motors for Electrical Engineering


MCQ on Polyphase Induction Motors for Electrical Engineering
1. Which of the following component is usually fabricated out of silicon steel ? 
(a) Bearings 
(b) Shaft 
(c) Stator core 
(d) None of the above 

Ans: c

2. The shaft of an induction motor is made of 
(a) stiff 
(b) flexible 
(c) hollow 
(d) any of the above 

Ans: a

3. The shaft of an induction motor is made of 
(a) high speed steel 
(b) stainless steel 
(c) carbon steel 
(d) cast iron 

Ans: c

4. In an induction motor, no-load the slip is generally 
(a) less than 1% 
(b) 1.5% 
(c) 2% 
(d) 4% 

Ans: a

5. In medium sized induction motors, the slip is generally around 
(a) 0.04% 
(b) 0.4% 
(c) 4% 
(d) 14% 

Ans: c

6. In squirrel cage induction motors, the rotor slots are usually given slight skew in order to 
(a) reduce windage losses 
(b) reduce eddy currents 
(c) reduce accumulation of dirt and dust 
(d) reduce magnetic hum 

Ans: d

7. Slip rings are usually made of 
(a) copper 
(b) carbon 
(c) phosphor bronze 
(d) aluminum 

Ans: c

8. The number of slip rings on a squirrel cage induction motor is usually 
(a) two 
(b) three 
(c) four 
(d) none 

Ans: d

9. A double squirrel-cage induction motor has 
(a) two rotors moving in oppsite direction 
(b) two parallel windings in stator 
(c) two parallel windings in rotor 
(d) two series windings in stator 

Ans: c

10. Star-delta starting of motors is not possible in case of 
(a) single phase motors 
(b) variable speed motors 
(c) low horse power motors 
(d) high speed motors 

Ans: a

11. In case of the induction motors the torque is 
(a) inversely proportional to (Vslip) 
(b) directly proportional to (slip)2 
(c) inversely proportional to slip 
(d) directly proportional to slip 

Ans: d

12. The injected e.m.f. in the rotor of induction motor must have 
(a) zero frequency 
(b) the same frequency as the slip frequency 
(c) the same phase as the rotor e.m.f. 
(d) high value for the satisfactory speed control 

Ans: b

13. Which of the following methods is easily applicable to control the speed of the squirrel-cage induction motor ? 
(a) By changing the number of stator poles 
(b) Rotor rheostat control 
(c) By operating two motors in cascade 
(d) By injecting e.m.f. in the rotor circuit 

Ans: a

14. The speed of a squirrel-cage induction motor can be controlled by all of the following except 
(a) changing supply frequency 
(b) changing number of poles 
(c) changing winding resistance 
(d) reducing supply voltage 

Ans: c

15. The power factor of an induction motor under no-load conditions will be closer to 
(a) 0.2 lagging 
(b) 0.2 leading 
(c) 0.5 leading 
(d) unity 

Ans: a

16. In an induction motor the relationship between stator slots and rotor slots is that 
(a) stator slots are equal to rotor slots 
(b) stator slots are exact multiple of rotor slots 
(c) stator slots are not exact multiple of rotor slots 
(d) none of the above 

Ans: c

17. If a 3-phase supply is given to the stator and rotor is short circuited rotor will move 
(a) in the opposite direction as the direction of the rotating field 
(b) in the same direction as the direction of the field 
(c) in any direction depending upon phase squence of supply 

Ans: b

18. Which type of bearing is provided in small induction motors to support the rotor shaft ? 
(a) Ball bearings 
(b) Cast iron bearings 
(c) Bush bearings 
(d) None of the above 

Ans: a

19. A 3-phase slip ring motor has 
(a) double cage rotor 
(b) wound rotor 
(c) short-circuited rotor 
(d) any of the above 

Ans: b

20. In a three-phase induction motor 
(a) iron losses in stator will be negligible as compared to that in rotor
(b) iron losses in motor will be neg¬ligible as compared to that in rotor 
(c) iron losses in stator will be less than that in rotor 
(d) iron losses in stator will be more than that in rotor 

Ans: d

21. The shape of the torque/slip curve of induction motor is 
(a) parabola 
(b) hyperbola 
(c) rectangular parabola 
(d) straigth line 

Ans: c

22. The stating torque of the slip ring induction motor can be increased by adding 
(a) external inductance to the rotor 
(b) external resistance to the rotor 
(c) external capacitance to the rotor 
(d) both resistance and inductance to rotor 

Ans: b

23. Blocked rotor test on induction motors is used to find out 
(a) leakage reactance 
(b) power factor on short circuit 
(c) short-circuit current under rated voltage 
(d) all of the above 

Ans: d

24. An induction motor can run at synchronous speed when 
(a) it is run on load 
(b) it is run in reverse direction 
(c) it is run on voltage higher than the rated voltage 
(d) e.m.f. is injected in the rotor circuit 

Ans: d

25. Which motor is preferred for use in mines where explosive gases exist ? 
(a) Air motor 
(b) Induction motor 
(c) D.C. shunt motor 
(d) Synchronous motor 

Ans: a

26. The torque developed by a 3-phase induction motor least depends on 
(a) rotor current 
(b) rotor power factor 
(c) rotor e.m.f. 
(d) shaft diameter 

Ans: d

27. When R2 is the rotor resistance, X2 the rotor reactance at supply frequency and s the slip, then the condition for maximum torque under running conditions will be 
(a) sR2X2 = 1 
(b) sR2 = X2 
(c) R2 = sX2 
(d) R2 = s2X2 

Ans: c

28. The purpose of using short-circuit gear is 
(a) to short circuit the rotor at slip rings 
(b) to short circuit the starting resistances in the starter 
(c) to short circuit the stator phase of motor to form star 
(d) none of the above 

Ans: a

29. A squirrel cage induction motor is not selected when 
(a) initial cost is the main consideration 
(b) maintenance cost is to be kept low 
(c) higher starting torque is the main consideration 
(d) all above considerations are involved 

Ans: c

30. Slip ring motor is preferred over squirrel cage induction motor where 
(a) high starting torque is required 
(b) load torque is heavy 
(c) heavy pull out torque is required 
(d) all of the above 

Ans: a

31. In a star-delta starter of an induction motor 
(a) resistance is inserted in the stator 
(b) reduced voltage is applied to the stator 
(c) resistance is inserted in the rotor 
(d) applied voltage per stator phase is 57.7% of the line voltage 

Ans: d

32. The starting torque of a three-phase induction motor can be increased by 
(a) increasing slip 
(b) increasing current 
(c) both (a) and (b) 
(d) none of the above 

Ans: c

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#MCQ on electrical engineering 
#electrical engineering mcq with answer 
#Polyphase Induction Motors mcq
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