DC MACHINES 12

111. Direct-on-line starters are not suitable for starting large dc motors because
a) the motor may not start.
b) large voltage drop may occur in the supply.
c) the starting torque becomes very low.
d) the motor may run away.
ans. b

112. For dc shunt motor, speed control by armature resistance variations is best suited for
a) constant power drive
b) variable power drive
c) constant torque drive
d) variable torque drive
ans. c

113. For dc shunt motor, speed control by the variation of field flux is best suited for

a) constant power drive
b) variable power drive
c) constant torque drive
d) variable torque drive
ans. a

114. If the field circuit of a dc shunt motor running at rated speed gets open-circuited , then immediately after    this the speed of the motor would tend to
a) decrease
b) increase
c) remain unchanged
d) oscillate around the rated speed
ans. b

115. A dc shunt motor is running at rated speed with rated supply voltage. If the supply voltage is halved , then the speed of the motor becomes
a) half of the rated speed
b) double the rated speed
c) slightly less than the rated speed
d) slightly more than the rated speed
ans. c

116. A dc shunt motor is driving a constant torque load without any additional resistance in the armature circuit . If an additional resistance is placed in the armature circuit then the speed of the motor
a) increases
b) decreases
c) remains unchanged
d) becomes zero
ans. b

117. A dc shunt motor is driving a constant torque load with rated excitation. If the field current is halved then the speed of the motor
a) becomes half
b) becomes slightly more than half
c) becomes double
d) becomes slightly less than double.
ans. d

118. A dc shunt motor is running at rated speed with rated excitation, rated voltage  and with an additional resistance in the armature circuit .Speeds less than the rated speed can be achieved by
a) Reducing the supply voltage and increasing the field excitation.
b) Increasing the supply voltage and decreasing the armature circuit resistance
c) Decreasing the field excitation and increasing the supply voltage.
d) Decreasing the armature circuit resistance and decreasing the field excitation.
ans. a

119. In Ward-Leonard method of speed control the direction of rotation of the motor is reversed usually by
a) reversing the connections of the generator field terminals
b) reversing the connection of the generator armature terminals
c) reversing the connections of the motor armature terminals
d) none of these.
ans. a

120. A dc shunt motor is driving a mechanical load at rated voltage and the rated excitation. If the load torque becomes double then the speed of the motor
a) increase slightly
b) decreases slightly
c) becomes double
d) becomes half
ans. b