DC MACHINES 13

121. A dc series motor is driving a load with a diverter connected across its armature. If the diverter resistance is decreased , the speed of the motor
a) increases
b) decreases
c) remains unchanged
d) becomes zero
ans. b

122. A dc series motor is running with a diverter connected across its field winding. If the diverter resistance is increased then the speed of the motor.
a) decreases
b) increases
c) remains unchanged
d) becomes excessively high
ans. a

123. Two dc series motors connected in series are driving the same mechanical load. If the motors are now connected in parallel the speed becomes
a) slightly less than double
b) slightly less than half
c) slightly more than double
d) slightly more than half
ans. c

124. A dc series motor is running at rated speed with rated excitation. The motor has two resistance R1 and R2 connected across the armature and the field respectively. Speeds above the rated speed can be achieved by
a) decreasing R1 only
b) increasing R2 only
c) decreasing R1 and increasing R2
d) increasing R1 and decreasing R2
ans. d

125. A dc series motor is running at rated speed without any additional resistance in series. If an additional resistance is placed in series, the speed of the motor
a) increases
b) decreases
c) remains unchanged
d) oscillates around the rated speed.
ans. b

126. The most economic method of electrical braking is
a) regenerative braking
b) dynamic braking with self excitation
c) dynamic braking with separate excitation
d) plugging
ans. a

127. Plugging of dc motors is normally executed by
a) reversing the field polarity
b) reversing the armature polarity
c) reversing both armature and field polarity
d) connecting a resistance across the armature
ans. b

128. For non-reversing dc drives it is preferable to employ
a) regenerative braking
b) dynamic braking with separate excitation
c) dynamic braking with self excitation
d) plugging
ans. b

129. An amplitude has split poles to
a) provide space for Interpoles
b) increase amplification factor
c) increase efficiency
d) damp out mechanical oscillations.
ans. a

130. If the number of poles in a cross-field generator is 2, the number of Interpoles will be
a) 2
b) 4
c) 6
d) 8
ans. b