The polarity of a DC generator is determined by

 201. The direction of rotation of a DC motor can be reversed by changing

A) Supply voltage
B) Armature connections
C) Field connections
D) Either B or C
Ans: D

202. The magnetic neutral axis (MNA) is the axis along which
A) No emf is induced in armature conductors
B) Maximum emf is induced
C) Brushes are placed
D) Both A and C
Ans: D

203. In a DC machine, the commutation is said to be ideal if
A) No sparking occurs
B) Current reversal is complete
C) Armature reaction is neutralized
D) All of these
Ans: D

204. The speed of a DC shunt motor is approximately proportional to
A) Armature voltage
B) 1 / Flux
C) Both A and B
D) Supply frequency
Ans: C

205. The armature reaction under load in a DC motor
A) Distorts the main field
B) Weakens the main field
C) Causes commutation problems
D) All of these
Ans: D

206. The commutation process can be improved by
A) Using interpoles
B) Increasing brush contact resistance
C) Decreasing brush width
D) All of these
Ans: D

207. A differential compound DC motor is used where
A) Constant speed is required
B) Speed should decrease with load
C) Speed should increase with load
D) None of these
Ans: B

208. The lap winding is generally used in
A) High current, low voltage machines
B) Low current, high voltage machines
C) Small machines
D) Fractional horsepower motors
Ans: A

209. The wave winding is used in
A) High voltage, low current machines
B) Low voltage, high current machines
C) Fractional horsepower motors
D) Compound machines
Ans: A

210. The compensating winding is placed in
A) Pole faces
B) Armature slots
C) Yoke
D) Brush holders
Ans: A

211. Armature reaction in a DC generator results in
A) Demagnetizing and cross-magnetizing effects
B) Increased field flux
C) Improved commutation
D) Constant voltage
Ans: A

212. The terminal voltage of a DC shunt generator decreases with load due to
A) Armature reaction
B) Voltage drop in armature circuit
C) Both A and B
D) Field weakening
Ans: C

213. The critical speed of a DC shunt generator depends on
A) Field resistance
B) Armature resistance
C) Load current
D) Supply voltage
Ans: A

214. The brush contact drop is usually about
A) 1–2 V per brush
B) 10–12 V per brush
C) 0.1–0.2 V per brush
D) Negligible
Ans: A

215. In a DC motor, if the back emf becomes equal to supply voltage,
A) Armature current becomes zero
B) Motor stops
C) Torque increases
D) Speed decreases
Ans: A

216. The speed of a DC motor is controlled by
A) Armature voltage and field current
B) Armature resistance only
C) Flux alone
D) Supply frequency
Ans: A

217. The shunt field current in a DC shunt motor is
A) Nearly constant
B) Varies with load
C) Zero at start
D) Same as armature current
Ans: A

218. The series field winding in a compound motor is connected
A) In series with armature
B) In parallel with armature
C) Across brushes
D) In opposition to armature
Ans: A

219. The differential compound motor has
A) Series field opposing shunt field
B) Series field aiding shunt field
C) No series field
D) No shunt field
Ans: A

220. The long shunt compound generator has
A) Series field in series with both armature and shunt field
B) Series field in series with armature only
C) Shunt field across armature only
D) None of these
Ans: A

221. The short shunt compound generator has
A) Shunt field connected across armature only
B) Shunt field connected across both armature and series field
C) Series field connected in parallel with armature
D) None of these
Ans: A

222. The efficiency of a DC generator is maximum when
A) Variable losses = Constant losses
B) Iron losses = Copper losses
C) Armature losses = Mechanical losses
D) Field losses = Copper losses
Ans: A

223. The function of brushes in DC machines is to
A) Collect current from commutator
B) Conduct current to external circuit
C) Maintain mechanical contact
D) All of these
Ans: D

224. The speed of a DC series motor varies with
A) Load current
B) Flux
C) Supply voltage
D) All of these
Ans: D

225. The armature copper loss is proportional to
A) Ia²Ra
B) V×Ia
C) Eb×Ia
D) Flux×Speed
Ans: A

226. The main disadvantage of DC series motor is
A) Cannot be started without load
B) High speed variation
C) Sparking at brushes
D) Low starting torque
Ans: A

227. In a DC machine, iron losses occur due to
A) Hysteresis and eddy currents
B) Flux leakage
C) Armature reaction
D) Copper losses
Ans: A

228. For the same speed and field, the emf generated depends on
A) Armature conductors
B) Armature turns
C) Type of winding
D) All of these
Ans: D

229. The insulation between commutator segments is
A) Mica
B) Paper
C) Rubber
D) Cotton
Ans: A

230. The brushes are shifted from MNA to GNA to
A) Avoid sparking
B) Improve commutation
C) Reduce copper loss
D) None
Ans: B

231. The speed–torque curve of a DC shunt motor is
A) Nearly straight
B) Exponential
C) Inverse
D) Constant torque
Ans: A

232. The constant losses in a DC motor include
A) Iron and mechanical losses
B) Copper losses
C) Armature losses
D) None of these
Ans: A

233. In a DC generator, the load current is equal to
A) Armature current minus shunt current
B) Armature current plus shunt current
C) Shunt current only
D) Field current only
Ans: A

234. The copper losses are directly proportional to
A) Square of armature current
B) Speed
C) Voltage
D) Flux
Ans: A

235. The residual magnetism in the poles of a DC generator is due to
A) Retentivity
B) Hysteresis
C) Coercivity
D) Reluctance
Ans: A

236. The commutator in a DC generator converts
A) Alternating emf into direct emf
B) Direct emf into alternating emf
C) Mechanical into electrical energy
D) None
Ans: A

237. The voltage equation of a DC generator is
A) V = Eg − IaRa
B) V = Eg + IaRa
C) Eg = V + IaRa
D) Both A and C
Ans: D

238. The torque equation of a DC motor is
A) T = 0.159 × Φ × Ia × Z / P
B) T = kΦIa
C) Both A and B
D) None
Ans: C

239. The braking of a DC motor can be done by
A) Regenerative braking
B) Plugging
C) Dynamic braking
D) All of these
Ans: D

240. In dynamic braking,
A) Motor acts as generator
B) Energy is dissipated in resistors
C) Both A and B
D) None
Ans: C

241. Plugging in DC motors means
A) Reversal of supply polarity
B) Reversal of armature terminals
C) Applying reverse voltage to stop motor
D) All of these
Ans: D

242. The regenerative braking is possible when
A) Back emf > Supply voltage
B) Back emf < Supply voltage
C) Back emf = Supply voltage
D) None
Ans: A

243. The function of the diverter in a DC compound machine is
A) Control current through field winding
B) Improve commutation
C) Limit starting current
D) Reduce armature losses
Ans: A

244. If a shunt generator fails to build up voltage, it may be due to
A) Reversed field connection
B) No residual magnetism
C) High field resistance
D) All of these
Ans: D

245. The polarity of a DC generator is determined by
A) Right-hand rule
B) Left-hand rule
C) Fleming’s right-hand rule
D) Fleming’s left-hand rule
Ans: C

246. In a DC motor, the developed torque is directly proportional to
A) Armature current × Flux
B) Speed
C) Voltage
D) Power factor
Ans: A

247. The load characteristic of a DC shunt generator is
A) Slightly drooping
B) Flat
C) Rising
D) Non-linear
Ans: A

248. The field winding of a DC shunt motor has
A) High resistance and many turns
B) Low resistance and few turns
C) Medium resistance
D) None
Ans: A

249. The field winding of a DC series motor has
A) Few turns and thick wire
B) Many turns and thin wire
C) Medium turns
D) High resistance
Ans: A

250. The voltage drop in brushes affects
A) Terminal voltage
B) Efficiency
C) Commutation
D) All of these
Ans: D