DC Motors MCQs Quiz Multiple Choice Questions & Answers Page 2

51. Which of the following motors one will choose to drive the rotary compressor ?

1. D.C. shunt motor
2. D.C. series motor
3. Universal motor
4. Synchronous motor

52. Which of the following steps is likely to result in reduction of hysteresis loss in a D.C. generator ?

1. Providing laminations in armature core
2. Providing laminations in stator
3. Using non-magnetic material for frame
4. Using material of low hysteresis co-efficient for armature core material

53. Which of the following tests will be suitable for testing two similar D.C. series motorsof large capacity ?

1. Swinburnes test
2. Hopkinsons test
3. Field test
4. Brake test

54. Which one of the following is not necessarily the advantage of D.C. motors over A.C. motors ?

1. Low cost
2. Wide speed range
3. Stability
4. High starting torque.

55. Which one of the following is not the function of pole shoes in a D.C. machine ?

1. To reduce eddy current loss
2. To support the field coils
3. To spread out flux for better unifor-mity
4. To reduce the reluctance of the mag-netic path

56. Why are the D.C. motors preferred for traction applications ?

1. Torque and speed are inversely proportional to armature current
2. Torque is proportional to armature current
3. Torque is proportional to square root of armature current
4. The speed is inversely proportional to the torque and the torque is proportional to square of armature current

57. A D.C. series motor is that which

1. has its field winding consisting of thick wire and less turns
2. has a poor torque
3. can be started easily without load
4. has almost constant speed

58. A direct on line starter is used: for starting motors

1. iip to 5 H.P.
2. up to 10 H.P.
3. up to 15 H.P.
4. up to 20 H.P.

59. A three point starter is considered suitable for

1. shunt motors
2. shunt as well as compound motors
3. shunt, compound and series motors
4. all D.C. motors

60. According to Flemings left-hand rule, when the forefinger points in the direction of the field or flux, the middle finger will point in the direction of

1. current in the conductor aovtaat of conductor
2. resultant force on conductor
3. none of the above

61. As -the load is increased the speed of D.C. shunt motor will

1. reduce slightly
2. increase slightly
3. increase proportionately
4. remains unchanged

62. As there is no back e.m.f. at the instant of starting a D.C. motor, in order to prevent a heavy current from flowing though the armature circuit

1. a resistance is connected in series with armature
2. a resistance is connected parallel to the armature
3. armature is temporarily open circuited
4. a high value resistor is connected across the field winding

63. At the instant of starting when a D.C. motor is put on supply, it behaves like

1. a highly resistive circuit
2. a low resistance circuit
3. a capacitive circuit
4. none of the above

64. Buses, trains, trolleys, hoists, cranes require high starting torque and therefore make use of

1. D.C. series motor
2. D.C. shunt motor
3. induction motor
4. all of above motors

65. By looking at which part of the motor, it can be easily confirmed that a particular motor is D.C. motor?

1. Frame
2. Shaft
3. Commutator
4. Stator

66. D.C. generators are installed near the load centres to reduce

1. iron losses
2. line losses
3. sparking
4. corona losses

67. D.C. generators are normally designed for maximum efficiency around

1. full-load
2. rated r.p.m.
3. rated voltage
4. all of the above

68. D.C. motor is to a drive a load which is almost nil for certain part of the load cycle and peak value for short duration. We will select this

1. series motor
2. shunt motor
3. compound motor
4. any of the above

69. D.C. motor is to drive a load which has certain minimum value for most of the time and some peak value for short duration. We will select the

1. series motor
2. shunt motor
3. compound motor
4. any of the above

70. D.C. series motors are used

1. where load is constant
2. where load changes frequently
3. where constant operating speed is needed
4. in none of the above situations.

71. D.C. shunt motors are used for driving

1. trains
2. cranes
3. hoists
4. machine tools

72. Differentially compound D.C. motors can find applications requiring

1. high starting torque
2. low starting torque
3. variable speed
4. frequent on-off cycles

73. During rheostat braking of D.C. series motors

1. motor is run as a generator
2. motor is reversed in direction
3. motor is run at reduced speed

74. Flywheel is used with D.C. compound motor to reduce the peak demand by the motor, compound motor will have to be

1. level compounded
2. under compounded
3. cumulatively compounded
4. differentially compounded

75. Following motor is used where high starting torque and wide speed range control is required.

1. Single phase capacitor start
2. Induction motor
3. Synchronous motor
4. D.C. motor

76. For a D.C. shunt motor if the excitation is changed

1. torque will remain constant
2. torque will change but power will remain constant
3. torque and power both will change
4. torque, power and speed, all will change

77. For constant torque drive which speed control method is preferred ?

1. Field control
2. Armature voltage control
3. Shunt armature control
4. Mechanical loading system

78. For starting a D.C. motor a starter is required because

1. it limits the speed of the motor
2. it limits the starting current to a safe value
3. it starts the motor
4. none of the above

79. For the same H.P. rating and full load speed, following motor has poor starting torque

1. shunt
2. series
3. differentially compounded
4. cumulativelycompounded

80. For which of the following applications a D.C. motor is preferred over an A.C. motor ?

1. Low speed operation
2. High speed operation
3. Variable speed operation
4. Fixed speed operation

81. For which types of D.C. motor, dynamic braking is generally used ?

1. Shunt motors
2. Series motors
3. Compound motors
4. All of the above

82. Hopkinsons test on D.C. machines is conducted at

1. no-load
2. part load
3. full-load
4. overload

83. If a D.C. motor designed for 40°C ambient temperature is to be used for 50°C ambient temperature, then the motor

1. of lower H.P. should be selected
2. of higher H.P. should be selected
3. can be used for 50°C ambient temperature also
4. is to be derated by a factor recom-mended by manufacturer and select the next higher H.P. motor

84. If a D.C. motor is connected across the A.C. supply it will

1. run at normal speed
2. not run
3. run at lower speed
4. burn due to heat produced in the field winding by .eddy currents

85. If a D.C. motor is to be selected for conveyors, which rriotor would be preferred ?

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

86. If a D.C. shunt motor is working at full load and if shunt field circuit suddenly opens

1. this will make armature to take heavy current, possibly burning it
2. this will result in excessive speed, possibly destroying armature due to excessive centrifugal stresses
3. nothing will happen to motor
4. motor will come to stop

87. If a D.C. shunt motor is working at no load and if shunt field circuit suddenly opens

1. nothing will happen to th£ motor
2. this will make armature to take heavy current, possibly burning it
3. this will result in excessive speed, possibly destroying armature due to excessive centrifugal stresses
4. motor will run at very slow speed

88. If I2 be the armature current, then speed of a D.C. shunt motor is

1. independent of Ia
2. proportional to la
3. varies as (Ia)
4. varies as la

89. If the field of a D.C. shunt motor gets opened while motor is running

1. the speed of motor will be reduced %
2. the armature current will reduce
3. the motor will attain dangerously high speed 1
4. the motor will continue to nuvat constant speed

90. If the speed of a D.C. shunt motor is increased, the back e.m.f. of the motor will

1. increase
2. decrease
3. remain same
4. become zero

91. If the supply voltage for a D.C. motor is increased, which of the following will decrease ?

1. Starting torque
2. Operating speed
3. Full-load current
4. All of the above

92. If the terminals of armature of D.C. motor are interchanged, this action will offer following kind of braking

1. regenerative
2. plugging
3. dynamic braking
4. none of the above

93. In a D.C. generator all of the following could be the effects of iron losses except

1. Loss of efficiency
2. Excessive heating of core
3. Increase in terminal voltage
4. Rise in temperature of ventilating air

94. In a D.C. generator, the iron losses mainly take place in

1. yoke
2. commutator
3. armature conductors
4. armature rotor

95. In a D.C. series motor, if the armature current is reduced by 50%, the torque of the motor will be equal to

1. 100% of the previous value
2. 50% of the previous value
3. 25% of the previous value
4. 10% of the previous value

96. In a D.C. shunt motor, speed is

1. independent of armature current
2. directly proportional to the armature current
3. proportional to the square of the current
4. inversely proportional to the armature current

97. In a D.C. shunt motor, under the conditions of maximum power, the current in the armature will be

1. almost negligible
2. rated full-load current
3. less than full-load current
4. more than full-load current

98. In a differentially compounded D.C. motor, if shunt field suddenly opens

1. the motor will first stop and then run in opposite direction as series motor
2. the motor will work as series motor and run at slow speed in the same direction
3. the motor will work as series motor and run at high speed in the same direction
4. the motor will not work and come to stop

99. In a manual shunt motor starter

1. over load relay is connected in series and no volt relay in parallel with the load
2. over load relay is connected in paral¬lel and no volt relay in series with the load
3. over load relay and no volt relay are both connected in series with the load
4. over load relay and no volt relay are both connected in parallel with the load

100. In case of conductively compensated D.C. series motors, the compensating winding is provided

1. as separately wound unit
2. in parallel with armature winding
3. in series with armature winding
4. in parallel with field winding