110 TOP MOST POLYPHASE INDUCTION MOTORS – Electrical Engineering Multiple Choice Questions and Answers

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 frequency (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 voltage Ans: 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 starti (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 synchronous speed of the field and are in the same direction (c) rotor speed is more than the synchronous 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 motor for 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 (d) all above features are required Ans: d 51. As load on an induction motor goes on increasing (a) its power factor goes on decreasing (b) its power factor remains constant (c) its power factor goes on increasing even after full load (d) its power factor goes on increasing up to full load and then it falls again Ans: d 52. 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 53. It is advisable to avoid line starting of induction motor and use starter because (a) it will run in reverse direction (b) it will pick up very high speed and may go out of step (c) motor takes five to seven times its full load current (d) starting torque is very high Ans: c 54. The speed characteristics of an induction motor closely resemble the speedload characteristics of which of the following machines (a) D.C. series motor (b) D.C. shunt motor (c) universal motor (d) none of the above Ans: b 55. 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 56. A pump induction motor is switched on to a supply 30% lower than its rated voltage. The pump runs. What will eventually happen ? It will (a) stall after sometime (b) stall immediately (c) continue to run at lower speed without damage (d) get heated and subsequently get damaged Ans: d 57. 5 H.P., 50-Hz, 3-phase, 440 V, induction motors are available for the following r.p.m. Which motor will be the costliest ? (a) 730 r.p.m. (b) 960 r.p.m. (c) 1440 r.p.m. (d) 2880 r.p.m. Ans: a 58. A 3-phase slip ring motor has (a) double cage rotor (6) wound rotor (c) short-circuited rotor (d) any of the above Ans: b 59. The starting torque of a 3-phase squirrel cage induction motor is (a) twice the full load torque (b) 1.5 times the full load torque (c) equal to full load torque Ans: b 60. Short-circuit test on an induction motor cannot be used to determine (a) windage losses (b) copper losses (c) transformation ratio (d) power scale of circle diagram Ans: a

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