Electrical Engineering Practice MCQ : THREE PHASE INDUCTION MOTORS (Part 1)

Here are some practice MCQ related to THREE PHASE INDUCTION MOTORS.








1. Which of the following component is usually fabricated out of silicon steel ?

(a) Bearings

(b) Shaft

(c) Statorcore

(d) None of the above

Ans: c

2. The frame of an induction motor is usually made of

(a) silicon steel

(b) cast iron

(c) aluminium

(d) bronze

Ans: b

3. The shaft of an induction motor is made of

(a) stiff

(b) flexible

(c) hollow

(d) any of the above

Ans: a

4. The shaft of an induction motor is made of

(a) high speed steel

(b) stainless steel

(c) carbon steel

(d) cast iron

Ans: c

5. In an induction motor, no-load the slip is generally

(a) less than 1%

(b) 1.5%

(c) 2%

(d) 4%

Ans: a

6. In medium sized induction motors, the slip is generally around

(a) 0.04%

(b) 0.4%

(c) 4%

(d) 14%

Ans: c

7. 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

8. In case the air gap in an induction motor is increased

(a) the magnetising current of the rotor will decrease

(b) the power factor will decrease

(c) speed of motor will increase

(d) the windage losses will increase Ans: b

9. Slip rings are usually made of

(a) copper

(b) carbon

(c) phospor bronze

(d) aluminium

Ans: c

10. A 3-phase 440 V, 50 Hz induction motor has 4% slip. The frequency of rotor e.m.f. will be

(a) 200 Hz

(b) 50 Hz

(c) 2 Hz

(d) 0.2 Hz

Ans: c

11. In Ns is the synchronous speed and s the slip, then actual running speed of an induction motor will be

(a) Ns

(b) s.N,

(c) (l-s)Ns

(d) (Ns-l)s

Ans: c

The efficiency of an induction motor can be expected to be nearly

(a) 60 to 90%

(b) 80 to 90%

(c) 95 to 98%

(d) 99%

Ans: b

13. The number of slip rings on a squirrel cage induction motor is usually

(a) two

(b) three

(c) four

(d) none

Ans: d

14. The starting torque of a squirrel-cage induction motor is

(a) low

(b) negligible

(c) same as full-load torque

(d) slightly more than full-load torque

Ans: a

15. 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

16. 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

17. The term 'cogging' is associated with

(a) three phase transformers

(b) compound generators

(c) D.C. series motors

(d) induction motors

Ans: d

18. 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

19. An induction motor with 1000 r.p.m. speed will have

(a) 8 poles

(b) 6 poles

(c) 4 poles

(d) 2 poles

Ans: b

20. The good power factor of an induction motor can be achieved if the average flux density in the air gap is

(a) absent

(b) small

(c) large

(d) infinity

Ans: b

21. An induction motor is identical to

(a) D.C. compound motor

(b) D.C. series motor

(c) synchronous motor

(d) asynchronous motor

Ans: d

22. The injected e.m.f. in the rotor of induction motor must have

(a) zero frequency

(b) the same frequency as the slip fre-quency

(c) the same phase as the rotor e.m.f.

(d) high value for the satisfactory speed control Ans: b

23. 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

24. The crawling in the induction motor is caused by

(a) low voltage supply

(b) high loads

(c) harmonics develped in the motor

(d) improper design of the machine

(e) none of the above

Ans: c

25. The auto-starters (using three auto transformers) can be used to start cage induction motor of the following type (a) star connected only

(b) delta connected only

(c) (a) and (b) both

(d) none of the above

Ans: c

26. The torque developed in the cage induction motor with autostarter is

(a) k/torque with direct switching

(6) K x torque with direct switching

(c) K2 x torque with direct switching

(d) k2/torque with direct switching Ans: c

27. When the equivalent circuit diagram of doouble squirrel-cage induction motor is constructed the two cages can be considered (a) in series

(b) in parallel

(c) in series-parallel

(d) in parallel with stator

Ans: b

28. It is advisable to avoid line-starting of induction motor and use starter because

(a) motor takes five to seven times its full load current

(b) it will pick-up very high speed and may go out of step

(c) it will run in reverse direction

(d) starting torque is very high

Ans: a

29. Stepless speed control of induction motor is possible by which of the following methods ?

(a) e.m.f. injection in rotor eueuit

(b) Changing the number of poles

(c) Cascade operation

(d) None of the above

Ans: b

30. Rotor rheostat control method of speed control is used for

(a) squirrel-cage induction motors only

(b) slip ring induction motors only

(c) both (a) and (b)

(d) none of the above

Ans: b

31. In the circle diagram for induction motor, the diameter of the circle represents

(a) slip

(b) rotor current

(c) running torque

(d) line voltageAns: b

32. For which motor the speed can be controlled from rotor side ?

(a) Squirrel-cage induction motor

(b) Slip-ring induction motor

(c) Both (a) and (b)

(d) None of the above

Ans: b

33. If any two phases for an induction motor are interchanged

(a) the motor will run in reverse direction

(b) the motor will run at reduced speed

(c) the motor will not run

(d) the motor will burn

Ans: a

34. An induction motor is

(a) self-starting with zero torque

(b) self-starting with high torque

(c) self-starting with low torque

(d) non-self starting

Ans: c

35. The maximum torque in an induction motor depends on

(a) frequency

(b) rotor inductive reactance

(c) square of supply voltage

(d) all of the above

Ans: d

36. In three-phase squirrel-cage induction motors

(a) rotor conductor ends are short-circuited through slip rings

(b) rotor conductors are short-circuited through end rings

(c) rotor conductors are kept open

(d) rotor conductors are connected to insulation Ans: b

37. In a three-phase induction motor, the number of poles in the rotor winding is always

(a) zero

(b) more than the number of poles in stator

(c) less than number of poles in stator

(d) equal to number of poles in stator Ans: d

38. DOL starting of induction motors is usually restricted to

(a) low horsepower motors (b) variable speed motors

(c) high horsepower motors

(d) high speed motors

Ans: a

39. 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

40. The 'crawling" in an induction motor is caused by

(a) high loads

(6) low voltage supply

(c) improper design of machine

(d) harmonics developed in the motor Ans: d

41. 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

42. The 'cogging' of an induction motor can be avoided by

(a) proper ventilation

(b) using DOL starter

(c) auto-transformer starter

(d) having number of rotor slots more or less than the number of stator slots (not equal) Ans: d

43. If an induction motor with certain ratio of rotor to stator slots, runs at 1/7 of the normal speed, the phenomenon will be termed as

(a) humming

(b) hunting

(c) crawling

(d) cogging

Ans: c

44. Slip of an induction motor is negative when

(a) magnetic field and rotor rotate in opposite direction

(b) rotor speed is less than the syn-chronous speed of the field and are in the same direction

(c) rotor speed is more than the syn-chronous speed of the field and are in the same direction

(d) none of the above

Ans: c

45. Size of a high speed motor as compared to low speed motorfor the same H.P. will be

(a) bigger

(b) smaller

(c) same

(d) any of the above

Ans: b

46. A 3-phase induction motor stator delta connected, is carrying full load and one of its fuses blows out. Then the motor

(a) will continue running burning its one phase

(b) will continue running burning its two phases

(c) will stop and carry heavy current causing permanent damage to its winding

(d) will continue running without any harm to the winding Ans: a

47. A 3-phase induction motor delta connected is carrying too heavy load and one of its fuses blows out. Then the

motor

(a) will continue running burning its one phase

(b) will continue running burning its two phase

(c) will stop and carry heavy current causing permanent damage to its winding

(d) will continue running without any harm to the winding Ans: c

48. Low voltage at motor terminals is due to

(a) inadequate motor wiring

(b) poorely regulated power supply

(c) any one of the above

(d) none of the above

Ans: c

49. 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

50. Slip ring motor is recommended where

(a) speed control is required

(6) frequent starting, stopping and reversing is required

(c) high starting torque is needed all above features are required
Ans: d