MCQ on DC motor 61-80

61. Which D.C. motor will be suitable along with flywheel for intermittent light and heavy loads?
(a) Series motor
(b) Shunt motor
(c) Cumulatively compounded motor✔
(d) Differentially compounded motor

62. If a D.C. shunt motor is working at no load and if shunt field circuit suddenly opens
(a) nothing will happen to the motor
(b) this will make armature to take heavy current, possibly burning it
(c) this will result in excessive speed, possibly destroying armature due to✔ excessive centrifugal stresses
(d) motor will run at very slow speed

63. D.C. series motors are used
(a) where load is constant
(b) where load changes frequently
(c) where constant operating speed is needed
(d) in none of the above situations.✔

64. For the same H.P. rating and full load speed, following motor has poor starting torque
(a) shunt
(b) series
(c) differentially compounded✔
(d) cumulativelyc’ompounded

65. In case of conductively compensated D.C. series motors, the compensating winding is provided
(a) as separately wound unit
(6) in parallel with armature winding
(c) in series with armature winding✔
(d) in parallel with field winding

66. Sparking at the commutator of a D.C. motor may result in
(a) damage to commutator segments
(b) damage to commutator insulation
(c) increased power consumption
(d) all of the above✔

67. Which of the following motor is preferred for operation in highly explosive atmosphere ?
(a) Series motor
(b) Shunt motor
(c) Air motor✔
(d) Battery operated motor

68. If the supply voltage for a D.C. motor is increased, which of the following will decrease ?
(a) Starting torque
(b) Operating speed
(c) Full-load current✔
(d) All of the above

69. Which one of the following is not the function of pole shoes in a D.C. machine ?
(a) To reduce eddy current loss✔
(b) To support the field coils
(c) To spread out flux for better uniformity
(d) To reduce the reluctance of the magnetic path

70. The mechanical power developed by a shunt motor will be maximum when the ratio of back e.m.f. to applied voltage is
(a) 4.0
(b) 2.0
(c) 1.0
(d) 0.5✔

71. In D.C. motor which of the following can sustain the maximum temperature rise?
(a) Slip rings
(b) Commutator
(c) Field winding✔
(d) Armature winding

72. Which of the following law/rule can he used to determine the direction of rotation of D.C. motor ?
(a) Lenz’s law
(b) Faraday’s law
(c) Coloumb’s law
(d) Fleming’s left-hand rule✔

73. Which of the following load normally needs starting torque more than the rated torque?
(a) Blowers
(b) Conveyors✔
(c) Air compressors
(d) Centrifugal pumps

74. The starting resistance of a D.C. motor is generally
(a) low✔
(b) around 500 Q
(c) 1000 Q
(d) infinitely large

75. The speed of a D.C. series motor is
(a) proportional to the armature current
(b) proportional to the square of the armature current
(c) proportional to field current
(d) inversely proportional to the armature current✔

76. In a D.C. series motor, if the armature current is reduced by 50%, the torque of the motor will be equal
to
(a) 100% of the previous value
(b) 50% of the previous value
(c) 25% of the previous value✔
(d) 10% of the previous value

77. The current drawn by the armature of D.C. motor is directly proportional to
(a) the torque required✔
(b) the speed of the motor
(c) the voltage across the terminals
(d) none of the above

78. The power mentioned on the name plate of an electric motor indicates
(a) the power drawn in kW
(b) the power drawn in kVA
(c) the gross power
(d) the output power available at the shaft✔

79. Which D.C. motor has got maximum self loading property?
(a) Series motor
(b) Shunt motor
(c) Cumulatively compounded ‘motor
(d) Differentially compounded motor✔

80. The condition for maximum power in case of D.C. motor is
(a) back e.m.f. = 2 x supply voltage
(b) back e.m.f. = | x supply voltage✔
(c) supply voltage = | x back e.m.f.
(d) supply voltage = back e.m.f.