501 MCQ Questions on DC Machine

101. In dc machines, the field-flux axis and armature-mmf axis are respectively along

(a) direct axis and indirect axis.
(b) quadrature axis and direct axis.
(c) direct axis and inter-polar axis.
(d) quadrature axis and inter-polar axis. [UPPCL AE – 2016]

102. If the speed of a DC machine is doubled and the flux remains constant, the generated e.m.f.

(a) remains the same.
(b) is doubled.
(c) is halved.
(d) is thrice. [UPPCL AE 2016]

103. If the flux per pole of a shunt-wound DC generator is halved, the generated e.m.f. at constant speed

(a) is doubled.
(b) is halved.
(c) remains the same.
(d) becomes three times. [UPPCL AE – 2016]

104. Assertion (A): A d.c. motor draws high current at the time of starting.

Reason (R) While starting a d.c. motor, it takes some time to develop a non-zero value of back e.m.f.
(a) Both A and R are individually true and R is the correct explanation of A.
(b) Both A and R are individually true but R is not the correct explanation of A.
(c) A is true but R is false.
(d) A is false but R is true. [UPPCL AE – 2016]

105. In DC machines, the main parts where core losses significantly occurs at

(a) the armature only.
(b) both the armature and pole faces.
(c) the yoke only.
(d) the pole faces only. [UPPCL AE 2016]

106. Power transformers are generally designed to have maximum efficiency around.

(a) No load
(b) Half load
(c) Near full load
(d) 10% overload [UPPCL AE – 2016]

107. If the total number of slots in a D.C machine is 25 and the total number of poles is 5, then what is the coil span of the machine?

(a) 5
(c) 20
(b) 30
(d) 125 [UPPCL JE 2018]

108. To save energy during braking, which type of braking is used? [UPPCL JE – 2018]

(a) Regenerative
(c) Dynamic
(b) Plugging
(d) All of these

109. Which of the following DC generators is employed in arc welding? [UPPCL JE – 2018]

(a) Shunt
(b) Cumulative Compound
(c) Series
(d) Differential Compound

110. Which one acts as a mechanical rectifier in the process of converting AC current into DC current where the emf is induced in the armature winding? [UPPCL JE – 2018]

(a) Rheostat
(b) Rotor
(c) Commutator
(d) Stator

111. Shunt motors CANNOT be employed for

1. Machine tools
2. Blowers
3. Elevators
4. Lathes

(a) Only 3
(b) Both 1 and 4
(c) Both 3 and 4
(d) Only 2 [UPPCL AE – 2016]

112. Select the three methods of improving commutation. [UPPCL JE – 2018]

(a) Ring windings, Armature winding and Series commutation
(b) Resistance commutation, Voltage Commutation and Compensating Windings
(c) Capacitance commutation, Voltage Commutation and Series Windings
(d) Capacitance commutation, Inductance Commutation and Compensating Windings

113. How can we find the applications of Differentially compound D.C. motors? [UPPCL JE – 2018]

(a) Zero torque
(b) Armature resistance
(c) Constant speed
(d) Low starting torque

114. A conductor of length L has current I passing through it, when it is placed parallel to a magnetic field. The force experienced by the conductor will be? [UPPCL JE 2018]

(a) BLI
(b) B²LI
(c) Zero
(d) None of these

115. Commutator motors have

(a) absolutely uniform torque.
(b) approximately uniform torque.
(c) non-uniform torque.
(d) none of these. [UPPCL JE – 2018]

116. The speed of a D.C. Motor is directly proportional to

(a) armature current.
(b) field current.
(c) impressed voltage.
(d) number of poles.

117. A shunt generator gives the greatest voltage at

(a) No-load
(b) Full-load
(c) Open field
(d) Drooping

118. Which of the following could be aproximately the thickness of laminations of a D.C. machine?

(a) 0.005 mm
(c) 0.5 mm
(b) 0.05 mm
(d) 5 mm

119. In D.C. generators, the cause of rapid brush wear may be

(a) severe sparking.
(b) rough commutator surface.
(c) imperfect contact.
(d) any of the above.

120. In lap winding, the number of brushes is always

(a) double the number of poles.
(b) same as the number of poles.
(c) half the number of poles.
(d) two.

121. For a D.C. generator when the number of poles and the number of armature conductors is fixed, then which winding will give the higher e.m.f.?  [DSSSB JE – 2015]

(a) Lap winding
(b) Wave winding
(c) Both (a) and (b)
(d) Depends on other features of design

122. In a four-pole D.C. machine

(a) all the four poles are north poles.
(b) alternate poles are north and south.
(c) all the four poles are south poles.
(d) two north poles follow two south poles. [DSSSB JE – 2015]

123. Copper brushes in D.C. machine are used

(a) where low voltage and high currents are involved.
(b) where high voltage and small currents are involved.
(c) both of the above cases.
(d) none of the above cases. [DSSSB JE – 2015]

124. A separately excited generator as compared to a self-excited generator

(a) is enable to better voltage control.
(b) is more stable.
(c) has exciting current independent of load current.
(d) has all above features

125. Iron losses in a D.C. machine are independent of variations in

(a) speed.
(b) load.
(c) voltage.
(d) speed and voltage. [DSSSB JE – 2015]

126. In D.C. generators, current to the external circuit from armature is given through

(a) commutator.
(b) solid connection.
(c) slip rings.
(d) none of the above.

127. Brushes of D.C. machines are made of

(a) carbon.
(b) soft copper.
(c) hard copper.
(d) all of the above.

128. For a DC motor, E is the back emf, I is the armature current and R is the armature resistance, then the terminal voltage equals

(a) E
(b) IR
(c) E + IR
(d) E – IR [UPPCL AE – 2016]

129. The insulating material used between the commutator segments is normally

(a) graphite.
(c) mica.
(b) paper.
(d) insulating varnish.

130. Welding generator will have

(a) lap winding.
(b) wave winding.
(c) delta winding.
(d) duplex wave winding.

131. In case of D.C. machine winding, number of commutator segments are equal to

(a) number of armature coils.
(b) number of armature coil sides.
(c) number of armature conductors.
(d) number of armature turns.

132. For a D.C. machines laboratory following type of D.C. supply will be used.

(a) Rotary converter.
(b) Mercury arc rectifier.
(c) Induction motor D.C. generator set.
(d) Synchronous motor D.C. generator set.

133. The function of pole shoes in the case of D.C. machine is

(a) to reduce the reluctance of the magnetic path.
(b) to spread out the flux to achieve uniform flux density.
(c) to support the field coil.
(d) all the above functions. [DSSSB JE 2015]

134. In the case of lap winding resultant pitch is

(a) multiplication of front and back pitches.
(b) division of front pitch by back pitch.
(c) sum of front and back pitches.
(d) difference of front and back pitches. [DSSSB JE – 2015]

135. In a D.C. generator the critical resistance can be increased by

(a) increasing its field resistance.
(b) decreasing its field resistance.
(c) increasing its speed.
(d) decreasing its speed.

136. The number of armature parallel paths in a two-pole D.C. generator having duplex lap winding is

(a) 2
(c) 6
(b) 4
(d) 8

137. For both lap and wave windings, there are as many commutator bars as the number of

(a) slots.
(b) armature conductors.
(c) winding elements.
(d) poles.

138. The series field of a short-shunt D.C. generator is excited by

(a) external current
(b) armature current
(c) shunt current
(d) load current [DSSSB JE – 2015]

139. As a result of armature reaction, the reduction in the total mutual air gap flux in a D.C. generator is approximately

(a) 40 percent
(b) 25 percent
(c) 10 percent
(d) 5 percent [DSSSB JE 2015]

140. No-load speed of which of the following motor will be highest?

(a) Shunt motor
(b) Series motor
(c) Cumulative compound motor
(d) Differential compound motor [DSSSB JE 2015]

141. The direction of rotation of a D.C. series motor can be changed by

(a) interchanging supply terminals.
(b) interchanging field terminals.
(c) both (a) and (b).
(d) none of the above. [DSSSB JE 2015]

142. Which of the following application requires high starting torque?

(a) Lathe machine
(b) Centrifugal pump
(c) Locomotive
(d) Air blower [DSSSB JE – 2015]

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

(a) Series motor
(b) Shunt motor
(c) Differential compound motor
(d) Cumulative compound motor [DSSSB JE – 2015]

144. Which D.C. motor will be preferred for machine tools?

(a) Series motor
(b) Shunt motor
(c) Cumulative compound motor
(d) Differential compound motor [DSSSB JE 2015]

145. Differential compound D.C. motors can find applications requiring

(a) high starting torque.
(b) low starting torque.
(c) variable speed.
(d) frequent on-off cycles. [DSSSB JE – 2015]

146. Which D.C. motor is preferred for elevators?

(a) Shunt motor
(b) Series motor
(c) Differential compound motor
(d) Cumulative compound motor [DSSSB JE – 2015]

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

(a) reduce slightly.
(b) increase slightly.
(c) increase proportionately.
(d) remains unchanged. [DSSSB JE – 2015]

148. The armature torque of the D.C. shunt motor is proportional to

(a) field flux only.
(b) armature current only.
(c) both (a) and (b).
(d) none of the above.

149. Which of the following methods of speed control of D.C. machine will offer minimum efficiency?

(a) voltage control method.
(b) field control method
(c) armature resistance control method.
(d) all the above methods. [DSSSB JE – 2015]

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

(a) low cost.
(c) stability.
(b) wide speed range.
(d) high starting torque.