The number of brushes in a DC machine depends on

 251. The energy conversion in a DC machine takes place in the

A) Commutator
B) Armature
C) Field winding
D) Shaft
Ans: B

252. The armature reaction mainly affects the
A) Main field flux distribution
B) Speed of the machine
C) Brush pressure
D) Shaft torque
Ans: A

253. The demagnetizing effect of armature reaction can be minimized by
A) Using interpoles
B) Increasing brush shift
C) Increasing field current
D) Using carbon brushes
Ans: A

254. The function of interpoles in a DC generator is to
A) Neutralize armature reaction
B) Improve commutation
C) Reduce sparking
D) All of these
Ans: D

255. The generated emf in a DC generator is given by
A) Eg = (ΦZN/60) × (P/A)
B) Eg = (ΦZN/120) × (A/P)
C) Eg = (ΦZN/30) × (P/A)
D) Eg = (ΦZN/60) × (A/P)
Ans: A

256. The back emf in a DC motor is expressed as
A) Eb = (ΦZN/60) × (P/A)
B) Eb = V − IaRa
C) Both A and B
D) None
Ans: C

257. The losses that remain constant irrespective of load in a DC machine are
A) Iron and mechanical losses
B) Armature copper losses
C) Field copper losses
D) Eddy current losses only
Ans: A

258. The field copper losses vary as
A) (If)²Rf
B) Ia²Ra
C) Speed²
D) Flux²
Ans: A

259. The load test on a DC machine directly determines
A) Efficiency
B) Voltage regulation
C) Losses
D) Temperature rise
Ans: A

260. A differential compound generator is used where
A) Constant voltage is required
B) Voltage decreases sharply with load
C) Load is fluctuating
D) None
Ans: B

261. The speed of a DC shunt motor slightly falls on load due to
A) Armature reaction
B) Voltage drop in armature
C) Both A and B
D) None
Ans: C

262. The efficiency of a DC motor is maximum when
A) Variable losses = Constant losses
B) Copper losses = Iron losses
C) Load current is maximum
D) Supply voltage is maximum
Ans: A

263. The brushes are always placed along the
A) Magnetic neutral axis (MNA)
B) Geometric neutral axis (GNA)
C) Between MNA and GNA
D) Flux axis
Ans: A

264. The field poles in a DC machine are made of
A) Laminated steel
B) Solid cast steel
C) Iron blocks
D) Aluminium
Ans: B

265. The purpose of using a starter in DC motor is to
A) Limit starting current
B) Protect motor
C) Control speed at start
D) All of these
Ans: D

266. The three-point starter is used for
A) DC shunt motor
B) DC series motor
C) DC compound motor
D) Both A and C
Ans: D

267. The four-point starter is preferred when
A) Field current is controlled by a variable resistor
B) Motor runs at variable speed
C) Motor has a field control circuit
D) All of these
Ans: D

268. The most suitable speed control method for constant torque loads is
A) Armature voltage control
B) Flux control
C) Rheostatic control
D) Series resistance control
Ans: A

269. The method of speed control below rated speed is
A) Armature voltage control
B) Field control
C) Armature resistance control
D) None
Ans: A

270. The method of speed control above rated speed is
A) Field control
B) Armature voltage control
C) Rheostatic control
D) None
Ans: A

271. The total loss in a DC machine is the sum of
A) Copper, iron, and mechanical losses
B) Copper and stray losses
C) Only copper losses
D) Iron and friction losses
Ans: A

272. The stray losses in DC machines are approximately
A) 1% of output
B) 2% of output
C) 3% of output
D) 5% of output
Ans: B

273. The magnetizing characteristic of a DC machine is
A) Non-linear due to saturation
B) Linear
C) Exponential
D) Constant
Ans: A

274. The equalizer rings are used in
A) Lap winding
B) Wave winding
C) Both A and B
D) None
Ans: A

275. The number of parallel paths in lap winding is
A) Equal to number of poles
B) Two
C) Half of poles
D) Four
Ans: A

276. The number of parallel paths in wave winding is
A) Two
B) Equal to poles
C) Half of poles
D) None
Ans: A

277. The torque developed in a DC motor is given by
A) T = 0.159 × Φ × Ia × Z / P
B) T = kΦIa
C) Both A and B
D) None
Ans: C

278. The back emf acts in
A) Opposite direction to supply voltage
B) Same direction as supply voltage
C) Neutral direction
D) None
Ans: A

279. The induced emf in a DC machine is directly proportional to
A) Flux and speed
B) Armature current
C) Torque
D) Supply voltage
Ans: A

280. The torque of a DC motor is proportional to
A) Armature current
B) Field current
C) Product of flux and armature current
D) Voltage
Ans: C

281. The speed regulation of a DC shunt motor is typically
A) 5–10%
B) 15–20%
C) 25–30%
D) 50%
Ans: A

282. The load on a DC shunt generator increases, the terminal voltage
A) Decreases
B) Increases
C) Remains same
D) None
Ans: A

283. In a DC motor, if field flux increases, the speed
A) Decreases
B) Increases
C) Remains constant
D) Becomes zero
Ans: A

284. The armature resistance of a DC motor is generally
A) Very small
B) Very high
C) Moderate
D) None
Ans: A

285. The brush contact resistance in DC machines is
A) Very small
B) Moderate
C) High
D) None
Ans: A

286. The losses that depend on load current are
A) Copper losses
B) Iron losses
C) Mechanical losses
D) Stray losses
Ans: A

287. The function of the yoke in a DC machine is
A) To carry magnetic flux
B) To provide mechanical support
C) To hold the poles
D) All of these
Ans: D

288. The DC generator which gives almost constant voltage at all loads is
A) Cumulative compound generator
B) Differential compound generator
C) Series generator
D) Shunt generator
Ans: A

289. The direction of rotation of a DC motor can be changed by reversing
A) Armature current or field current
B) Supply voltage
C) Load
D) Speed
Ans: A

290. The eddy current losses in DC machine are minimized by
A) Laminating the core
B) Using solid steel
C) Using cast iron
D) Cooling fan
Ans: A

291. The efficiency of a DC machine is given by
A) Output / Input × 100
B) (Input − Losses)/Input × 100
C) Both A and B
D) None
Ans: C

292. The speed of a DC series motor is
A) High at no load
B) Low at heavy load
C) Varies inversely with flux
D) All of these
Ans: D

293. In Hopkinson’s test, two identical machines are coupled
A) Mechanically
B) Electrically
C) Both A and B
D) None
Ans: C

294. The Swinburne’s test is a
A) No-load test
B) Full-load test
C) Load test
D) None
Ans: A

295. The brake test on DC motor directly measures
A) Output power
B) Input power
C) Losses
D) Efficiency
Ans: A

296. The regenerative braking improves
A) Efficiency
B) Energy recovery
C) Both A and B
D) None
Ans: C

297. The insulation resistance of a DC machine decreases with
A) Temperature
B) Speed
C) Voltage
D) Flux
Ans: A

298. The commutation in a DC machine is best when
A) Current reversal is smooth
B) No sparking occurs
C) Brush contact resistance is high
D) Both A and B
Ans: D

299. The number of brushes in a DC machine depends on
A) Number of poles
B) Current rating
C) Type of winding
D) All of these
Ans: D

300. The maximum efficiency of a DC generator occurs when
A) Variable losses = Constant losses
B) Armature current = Field current
C) Voltage = Current
D) Output = Input
Ans: A