Hopkinson’s test on DC machines is used to determine

 151. The armature winding of a DC machine is placed on

A) Stator
B) Rotor
C) Field poles
D) Yoke
Ans: B

152. The function of the commutator in a DC machine is to
A) Collect current from the armature
B) Convert AC to DC
C) Supply excitation
D) Provide mechanical support
Ans: B

153. In a DC generator, the direction of induced emf is given by
A) Fleming’s left-hand rule
B) Fleming’s right-hand rule
C) Lenz’s law
D) Faraday’s law
Ans: B

154. The armature reaction in a DC generator causes
A) Distortion of main field flux
B) Increase in field strength
C) No effect
D) Decrease in speed
Ans: A

155. Interpoles in a DC machine are used to
A) Reduce armature reaction
B) Improve commutation
C) Reduce eddy current loss
D) Support the brushes
Ans: B

156. The field winding in a shunt generator is connected
A) In series with armature
B) In parallel with armature
C) In opposition
D) In series with load
Ans: B

157. The residual magnetism in a DC generator helps in
A) Starting voltage build-up
B) Reducing losses
C) Commutation
D) Excitation loss
Ans: A

158. The voltage regulation of a DC generator is defined as
A) (No-load V – Full-load V)/Full-load V × 100
B) Full-load V/No-load V × 100
C) Load current × Armature resistance
D) None of these
Ans: A

159. If the field winding of a shunt generator is open-circuited, the generator will
A) Build up voltage
B) Fail to build voltage
C) Generate higher voltage
D) Burn out
Ans: B

160. The back emf in a DC motor is proportional to
A) Speed
B) Flux
C) Product of speed and flux
D) Armature current
Ans: C

161. The torque in a DC motor is directly proportional to
A) Flux only
B) Armature current only
C) Product of flux and armature current
D) Speed
Ans: C

162. In a DC shunt motor, if the field current is reduced, the speed
A) Increases
B) Decreases
C) Remains same
D) Stops
Ans: A

163. The starting current of a DC motor is high because
A) Back emf is zero at start
B) Resistance is large
C) Supply voltage is high
D) Commutator delay
Ans: A

164. The purpose of a starter in a DC motor is to
A) Limit starting current
B) Increase speed
C) Reduce losses
D) Improve commutation
Ans: A

165. The speed control of a DC shunt motor can be done by
A) Armature voltage control
B) Field flux control
C) Both A and B
D) None
Ans: C

166. The mechanical characteristic of a DC shunt motor is
A) Drooping
B) Flat
C) Rising
D) Non-linear
Ans: B

167. The field winding of a series motor is made of
A) Thick wire, few turns
B) Thin wire, many turns
C) Thick wire, many turns
D) Thin wire, few turns
Ans: A

168. The speed–torque characteristic of a DC series motor is
A) Constant speed
B) Inversely proportional
C) Directly proportional
D) Flat
Ans: B

169. A DC series motor should not be started without load because
A) It will overspeed
B) It will stall
C) It will not start
D) It will draw more current
Ans: A

170. The back emf of a DC motor depends on
A) Flux and speed
B) Armature current
C) Torque
D) Supply voltage
Ans: A

171. The efficiency of a DC generator is maximum when
A) Variable losses = Constant losses
B) Armature current is minimum
C) Load current is maximum
D) Speed is minimum
Ans: A

172. The core of a DC machine is laminated to reduce
A) Eddy current losses
B) Hysteresis losses
C) Copper losses
D) Stray losses
Ans: A

173. The yoke of a DC machine serves to
A) Support poles
B) Provide mechanical protection
C) Carry magnetic flux
D) All of these
Ans: D

174. Armature coils are short-pitched to
A) Save copper
B) Reduce eddy currents
C) Improve commutation and reduce harmonics
D) Increase emf
Ans: C

175. Commutation in a DC machine means
A) Reversal of current in armature coil
B) Supply of current to brushes
C) Field excitation
D) Speed control
Ans: A

176. The brushes of a DC machine are made of
A) Carbon
B) Copper
C) Aluminium
D) Iron
Ans: A

177. The losses in a DC machine include
A) Copper losses
B) Iron losses
C) Mechanical losses
D) All of these
Ans: D

178. The hysteresis loss in DC machines depends on
A) Frequency and flux density
B) Current
C) Temperature
D) Speed only
Ans: A

179. The open circuit characteristic (OCC) of a DC generator is
A) Linear at low excitation
B) Non-linear at high excitation
C) Both A and B
D) Constant
Ans: C

180. The voltage drop due to armature reaction is known as
A) Commutation loss
B) Demagnetizing effect
C) Cross magnetizing effect
D) Resistive drop
Ans: B

181. The function of compensating winding is to
A) Neutralize armature reaction
B) Increase voltage
C) Improve efficiency
D) Reduce copper losses
Ans: A

182. In a DC shunt motor, speed varies
A) Inversely with flux
B) Directly with flux
C) With square of current
D) Constant
Ans: A

183. The torque developed by a DC motor is maximum when
A) Back emf = ½ Supply voltage
B) Back emf = Supply voltage
C) Armature current = Zero
D) Field flux = Zero
Ans: A

184. In a DC motor, the power developed =
A) Eb × Ia
B) V × Ia
C) Eb × If
D) V × If
Ans: A

185. In a DC series motor, speed is
A) Proportional to 1/Ia
B) Proportional to Ia
C) Constant
D) Proportional to flux
Ans: A

186. The field control method gives
A) Speed above normal
B) Speed below normal
C) Constant speed
D) Variable torque
Ans: A

187. The armature voltage control gives
A) Speed below normal
B) Speed above normal
C) Constant speed
D) None
Ans: A

188. Hopkinson’s test on DC machines is used to determine
A) Efficiency
B) Losses
C) Temperature rise
D) Both A and B
Ans: D

189. In Swinburne’s test, the machine is
A) Run as motor
B) Run on no-load
C) Tested for constant losses
D) All of these
Ans: D

190. The efficiency of a DC machine is maximum when
A) Copper losses = Iron + Mechanical losses
B) Copper losses are minimum
C) Voltage is maximum
D) Current is zero
Ans: A

191. The speed regulation of a DC shunt motor is
A) Low
B) High
C) Very high
D) Infinite
Ans: A

192. In a DC generator, iron losses occur in
A) Armature core
B) Field poles
C) Yoke
D) Brushes
Ans: A

193. The commutation can be improved by
A) Using interpoles
B) Using compensating winding
C) Reducing brush width
D) All of these
Ans: D

194. The no-load speed of a DC series motor is
A) Very high
B) Zero
C) Constant
D) Very low
Ans: A

195. The armature winding type suitable for high current and low voltage is
A) Lap winding
B) Wave winding
C) Series winding
D) Shunt winding
Ans: A

196. The field winding type suitable for high voltage and low current is
A) Wave winding
B) Lap winding
C) Series winding
D) Compound winding
Ans: B

197. The DC compound motor combines the features of
A) Shunt and Series motors
B) Induction and Synchronous motors
C) Alternator and Motor
D) None
Ans: A

198. The function of the commutator segments is to
A) Connect armature coils to brushes
B) Reduce field current
C) Increase voltage
D) Cool the machine
Ans: A

199. In a DC generator, when the load increases, the terminal voltage
A) Decreases
B) Increases
C) Remains constant
D) Doubles
Ans: A

200. The speed of a DC motor is independent of
A) Armature resistance
B) Load torque
C) Supply voltage
D) Flux
Ans: B