Monday, September 14, 2020

Chapter 8: Induction Motor-Page 6

  Chapter 8: Induction Motor

51. 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


... Answer is (B)

 

 
 

52. A 500 kW, 3-phase, 440 volts, 50 Hz, A.C. induction motor has a speed of 960 r.p.m. on full load. The machine has 6 poles. The slip of the machine will be
A.0.01
B.0.02
C.0.03
D.0.04

... Answer is (D)

 
 
 

53. The complete circle diagram of induction motor can be drawn with the help of data found from
A. no-load test
B. blocked rotor test
C. stator resistance test
D. all of the above

... Answer is (D)

 

 
 

54. The torque of a rotor in an induction motor under runing condition is maximum
A. at the unit value of slip
B. at the zero value of slip
C. at the value of the slip which makes rotor reactance per phase
D. at the value of the slip which makes the rotor reactance half of the rotor resistance

  ... Answer is (C)

 

 
 
55. What will happen if the relative speed between the rotating flux of stator and rotor of the induction motor is zero?
A. The slip of the motor will be 5%
B. The rotor will not run
C. The rotor will run at very high specd
D. The torque produced will be be very large
  ...
Answer is (B)

 
 
 

56. In a three-phase induction motor
A. power factor at starting is high as compared to that while running
B. power factor at starting is low as compared to that while running
C. power factor at starting in the same as that while running
D. power factor remains through out constant

  ... Answer is (B)

 

 
 

57. Insertion of resistance in the rotor of an induction motor to develop a given torque
A. decreases the rotor current
B. increases the rotor current
C. rotor current becomes zero
D. rotor current reamins same

... Answer is (D)

 
 

58. For driving high inertia loads best type of induction motor suggested is
A. slip ring type
B. squirrel cage type
C. any of the above
D. none of the above

... Answer is (A)

 
 

59. Temperature of the stator winding of a three-phase induction motor is obtained by
A. resistance rise method
B. thermometer method
C. embedded temperature method
D. all above methods

 
  ... Answer is (D)

 
 

60. 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

  ... Answer is (A)

 

 

Page 6 of  17

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Sunday, September 13, 2020

Chapter 8: Induction Motor-Page 5

  Chapter 8: Induction Motor

41. 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


... Answer is (A)

 

 
 

42. 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 negligible 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

... Answer is (D)

 
 
 

43. In case of 3-phase induction motors, plugging means
A. pulling the motor directly on line without a starter
B. locking of rotor due to harmonics
C. starting the motor on load which is more than the rated load
D. interchanging two supply phases for quick stopping

... Answer is (D)

 

 
 

44. Which is of the following data is required to draw the circle diagram for an induction motor?
A. Block rotor test only
B. No load test only
C. Block rotor test and no-load test
D. Block rotor test, no-load test and stator resistance test

  ... Answer is (D)

 

 
 
45. In three-phase induction motors sometimes copper bars are placed deep in the rotor to
A. improve starting torque
B. reduce copper losses
C. improve efficiency
D. improve power factor
  ...
Answer is (A)

 
 
 

46. In a three-phase induction motor
A. power factor at starting is high as compared to that while running
B. power factor at starting is low as compared to that while running
C. power factor at starting in the same as that while running
D. power factor remains through out constant

  ... Answer is (B)

 

 
 

47. The value of transformation ratio of an induction motor can be found by
A. open-circuit test only
B. short-circuit test only
C. stator resistance test
D. none of the above

... Answer is (B)

 
 

48. The power scale of circle diagram of an induction motor can be found from
A. stator resistance test
B. no-load test only
C. short-circuit test only
D. none of the above

... Answer is (C)

 
 

49. The shape of the torque/slip curve of induction motor is
A. parabola
B. hyperabola
C. rectangular parabola
D. straight line

 
  ... Answer is (C)

 
 

50. A change of 4% of supply voltage to an induction motor will produce a change of approximately
A. 4% in the rotor torque
B. 8% in the rotor torque
C. 12% in the rotor torque
D. 16% in the rotor torque

  ... Answer is (D)

 

 

Page 5 of  17

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Chapter 8: Induction Motor-Page 3

  Chapter 8: Induction Motor

31. 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


... Answer is (C)

 

 
 

32. 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

... Answer is (C)

 
 
 

33. Slip ring motor is recommended where
A. speed control is required
B. frequent starting, stopping and reversing is required
C. high starting torque is needed
D. all above features are required

... Answer is (D)

 

 
 

34. As load on an induction motor goes on increasing
A. its power factor goes on decreasing
B. its power factor remain constant
C. its power factor goes on increasing even after full load
D. its power factor goes on increasing upto full load and then it falls again

  ... Answer is (D)

 

 
 
35. 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
D. with synchronous speed
  ...
Answer is (B)

 
 
 

36. 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

  ... Answer is (C)

 

 
 

27. The speed characteristic of an induction motor closely resemble the speedload characteristic of which of the following machines
A. D.C. series motor
B. D.C. shunt motor
C. universal motor
D. none of the above

... Answer is (B)

 
 

38. 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

... Answer is (A)

 
 

39. A pump induction motor is switched on to a supply 30% lower than its rated voltage. What will eventually happen? It will
A. start after sometime
B. start immediately
C. continue to run at lower speed without damage
D. get heated and subsequently get damaged

 
  ... Answer is (D)

 
 

40. 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.

  ... Answer is (A)

 

 

Page 4 of  17

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Chapter 8: Induction Motor-Page 3

  Chapter 8: Induction Motor-

21. 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


... Answer is (D)

 

 
 

22. 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

... Answer is (C)

 
 
 

23. The 'crawling' in an induction motor is caused by
A. high loads
B. low voltage supply
C. improper design of machine
D. harmonics developed in the motor

... Answer is (D)

 

 
 

24. 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

  ... Answer is (A)

 

 
 
25. 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)
  ...
Answer is (D)

 
 
 

26. If an induction motor with certain ratio of rotor to stator slots, runs at of the normal speed, the phenomenon will be termed as
A. humming
B. hunting
C. crawling
D. cogging

  ... Answer is (C)

 

 
 

27. Slip of a 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

... Answer is (C)

 
 

28. 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

... Answer is (B)

 
 

29. 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

 
  ... Answer is (A)

 
 

30.  A 3-phase induction motor delta connected in 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 windig
D. will continue running without any harm to the winding

  ... Answer is (C)

 

 

Page 3 of  17

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Chapter 8: Induction Motor-Page 2

  Chapter 8: Induction Motor-

11. In some induction motors, copper bars are placed deep in the rotor. It is done so that
A. slip under normal running conditions is reduced
B. p.f. is improve
C. starting torque is increased
D. output is increased


... Answer is (C)

 

 
 

12. The fractional slip of an induction motor is the ratio
A. rotor Cu loss/rotor input
B. stator Cu loss/stator input
C. rotor Cu loss/rotor output
D. rotor Cu loss/stator Cu loss

... Answer is (A)

 
 
 

13. Which method is the least used for 3-phase motor starting?
A. Y-Δ
B. auto-transformer
C. across-the-line
D. primary resistors

... Answer is (A)

 

 
 

14. In case of the induction motors the torque is
A. inversely proportional to (slip)²
B. directly proportional to (slip)²
C. inversely proportional to slip
D. directly proportional to slip

  ... Answer is (D)

 

 
 
15. The torque developed in the cage induction motor with autostarter is
A. K/torque with direct switching
B. K x torque with direct switching
C. K2 x torque with direct switching
D. K2/torque with direct switching
  ...
Answer is (C)

 
 
 

16. 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

  ... Answer is (B)

 

 
 

17. 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

... Answer is (A)

 
 

18. 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

... Answer is (C)

 
 

19. 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

 
  ... Answer is (D)

 
 

20.  In three-phase squirrel-cage induction motors
A. rotor conductor ends are shortcircuited 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

  ... Answer is (B)

 

 

Page 2 of  17

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