D.C. Motors

51. In D.C. motor which of the following can sustain the maximum temperature rise?

  1. Slip rings
  2. Commutator
  3. Field winding
  4. Armature winding

Correct answer: (C)
Field winding

52. In D.C. shunt motors as load is reduced

  1. the speed will increase abruptly
  2. the speed will increase in proportion to reduction in load
  3. the speed will remain almost/constant
  4. the speed will reduce

Correct answer: (C)
the speed will remain almost/constant

53. In the D.C. motor the iron losses occur in

  1. the field
  2. the armature
  3. the brushes
  4. the commutator

Correct answer: (B)
the armature

54. In variable speed motor

  1. a stronger commutating field is needed at low speed than at high speed
  2. a weaker commutating field is needed at low speed than at high speed
  3. same commutating field is needed at low speed than at high speed
  4. none of the above is correct

Correct answer: (B)
a weaker commutating field is needed at low speed than at high speed

55. In Ward-Leonard control the lower limit of speed is imposed by

  1. residual magnetism of the generator
  2. core losses of motor
  3. mechanical losses of motor and generator together
  4. all of the above

Correct answer: (A)
residual magnetism of the generator

56. In which of the following applications D.C. series motor is invariably tried?

  1. Starter for a car
  2. Drive for a water pump
  3. Fan motor
  4. Motor operation in A.C. or D.C.

Correct answer: (A)
Starter for a car

57. No-load speed of which of the following motor will be highest?

  1. Shunt motor
  2. Series motor
  3. Cumulative compound motor
  4. Differentiate compound motor

Correct answer: (B)
Series motor

58. One D.C. motor drives another D.C. motor. The second D.C. motor when excited and driven

  1. runs as a generator
  2. does not run as a generator
  3. also runs as a motor comes to stop after sometime
  4. none

Correct answer: (A)
runs as a generator

59. Regenerative method of braking is based on that

  1. back e.m.f. is less than the applied voltage
  2. back e.m.f. is equal to the applied voltage
  3. back e.m.f. of rotor is more than the applied voltage
  4. none of the above

Correct answer: (B)
back e.m.f. is equal to the applied voltage

60. Sparking at the commutator of a D.C. motor may result in

  1. damage to commutator segments
  2. damage to commutator insulation
  3. increased power consumption
  4. all of the above

Correct answer: (D)
all of the above

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