DC Motors Questions and Answers in Quiz Form 7 for Practice

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

101. In case of D.C. shunt motors the speed is dependent on back e.m.f. only because

1. back e.m.f. is equal to armature drop
2. armature drop is negligible
3. flux is proportional to armature current
4. flux is practically constant in D:C. shunt motors

102. In case the back e.m.f. and the speed of a D.C. motor are doubled, the torque developed by the motor will

1. remain unchanged
2. reduce to one-fourth value
3. increase four folds
4. be doubled

103. In case-the conditions for maximum power for a D.C. motor are established, the efficiency of the motor will be

1. 100%
2. around 90%
3. anywhere between 75% and 90%
4. less than 50%

104. In D.C. machines fractional pitch winding is used

1. to improve cooling
2. to reduce copper losses
3. to increase the generated e.m.f.
4. to reduce the sparking

105. In D.C. machines the residual magnetism is of the order of

1. 2 to 3 per cent
2. 10 to 15 per cent
3. 20 to 25 per cent
4. 50 to 75 per cent