Wednesday, January 13, 2021

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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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