In DC machines, neutral plane is the plane where

 301. The commutator in a DC machine converts

A) AC to DC
B) DC to AC
C) AC to pulsating DC
D) DC to pulsating AC
Ans: C

302. The function of brushes in a DC generator is to
A) collect current from commutator
B) convert AC to DC
C) reduce armature reaction
D) insulate the field winding
Ans: A

303. Armature reaction in DC generator causes
A) Flux strengthening
B) Flux weakening
C) Constant flux
D) No effect on flux
Ans: B

304. Interpoles are used in DC machines to
A) Improve commutation
B) Reduce eddy current losses
C) Reduce hysteresis losses
D) Reduce mechanical losses
Ans: A

305. In a DC generator, the brushes are shifted from the geometrical neutral axis due to
A) Armature reaction
B) Commutation
C) Speed variation
D) Voltage drop
Ans: A

306. The speed of a DC shunt motor is
A) Almost constant
B) Directly proportional to armature current
C) Inversely proportional to field current
D) Very variable
Ans: A

307. In a DC motor, back emf is proportional to
A) Speed × Flux
B) Armature resistance
C) Supply voltage
D) Load torque
Ans: A

308. The back emf in a DC motor opposes
A) Supply voltage
B) Armature current
C) Magnetic field
D) Load torque
Ans: A

309. In a DC generator, the emf is directly proportional to
A) Flux × Speed
B) Speed only
C) Flux only
D) Armature resistance
Ans: A

310. The function of equalizer rings in DC machines is to
A) Prevent circulating currents
B) Increase armature voltage
C) Improve commutation
D) Reduce field current
Ans: A

311. The efficiency of a DC machine is maximum when
A) Variable loss = Constant loss
B) Armature loss = Field loss
C) Iron loss = Copper loss
D) Mechanical loss = Magnetic loss
Ans: A

312. The open circuit characteristic of a DC generator is
A) Nonlinear initially
B) Linear throughout
C) Parabolic
D) Exponential
Ans: A

313. Which DC motor is suitable for traction work?
A) DC series motor
B) DC shunt motor
C) DC compound motor
D) DC separately excited motor
Ans: A

314. In a DC machine, eddy current losses are reduced by
A) Laminating the core
B) Increasing flux density
C) Using solid core
D) Reducing resistance
Ans: A

315. In DC motors, speed control by armature voltage is
A) Below rated speed
B) Above rated speed
C) Constant torque region
D) Both A and C
Ans: D

316. If the field of a DC shunt motor opens during running, it will
A) Run at dangerously high speed
B) Stop immediately
C) Run at constant speed
D) Burn the armature
Ans: A

317. DC compound motor has
A) Both shunt and series field
B) Only series field
C) Only shunt field
D) Permanent magnet
Ans: A

318. The polarity of interpoles in a DC generator is
A) Same as main pole ahead
B) Opposite to main pole ahead
C) Same as main pole behind
D) Opposite to both poles
Ans: A

319. The commutation in DC machines is improved by
A) Using interpoles
B) Reducing brush contact area
C) Reducing load current
D) Increasing armature speed
Ans: A

320. Sparking at brushes in a DC machine is caused by
A) Poor commutation
B) Weak field
C) High armature reaction
D) All of these
Ans: D

321. The main losses in a DC machine are
A) Copper and iron losses
B) Iron and mechanical losses
C) Copper and friction losses
D) Copper, iron, and mechanical losses
Ans: D

322. The load characteristic of a DC shunt generator is
A) Drooping
B) Flat
C) Rising
D) Constant
Ans: A

323. In DC series motor, speed varies as
A) 1 / Armature current
B) Armature current
C) Armature voltage
D) Flux
Ans: A

324. The direction of rotation of a DC motor can be reversed by
A) Reversing armature current
B) Reversing field current
C) Reversing both
D) Reversing either A or B
Ans: D

325. The residual magnetism in DC generator is used to
A) Initiate emf build-up
B) Reduce losses
C) Maintain commutation
D) Improve cooling
Ans: A

326. In self-excited generator, the voltage builds up due to
A) Residual magnetism
B) External supply
C) Separate excitation
D) Constant flux
Ans: A

327. The torque produced in DC motor is proportional to
A) Φ × Ia
B) Ia²
C) V × Ia
D) Φ / Ia
Ans: A

328. A cumulatively compounded DC motor has
A) Stronger field at higher load
B) Weaker field at higher load
C) Constant field
D) No effect on load
Ans: A

329. For regenerative braking,
A) Back emf > Supply voltage
B) Back emf < Supply voltage
C) Back emf = Supply voltage
D) Field current zero
Ans: A

330. In a DC generator, brush contact drop is about
A) 1–2 V
B) 5–10 V
C) 0.5 V
D) 10–15 V
Ans: A

331. DC shunt motor is not suitable for
A) Variable load
B) Constant speed
C) Heavy starting torque
D) Light load
Ans: C

332. Armature winding type in DC machine depends on
A) Voltage and current rating
B) Speed
C) Core size
D) Brush material
Ans: A

333. In a lap winding, number of parallel paths =
A) Number of poles
B) 2 × poles
C) 1
D) Number of armature conductors
Ans: A

334. In a wave winding, number of parallel paths =
A) 2
B) 4
C) Equal to poles
D) Equal to conductors
Ans: A

335. For a given current, lap winding gives
A) Higher current capacity
B) Higher voltage
C) Less efficiency
D) More speed
Ans: A

336. In a DC generator, voltage regulation depends on
A) Load current
B) Speed
C) Field current
D) All of these
Ans: D

337. Shunt field resistance should be
A) High
B) Low
C) Moderate
D) Variable
Ans: A

338. Series field resistance should be
A) Low
B) High
C) Moderate
D) Infinite
Ans: A

339. The function of yoke in DC machine is to
A) Provide mechanical support
B) Provide magnetic path
C) Both A and B
D) Reduce losses
Ans: C

340. Armature core in DC machine is laminated to
A) Reduce eddy current losses
B) Reduce hysteresis losses
C) Increase magnetic strength
D) Increase weight
Ans: A

341. The shunt motor speed decreases with
A) Increase in load
B) Decrease in load
C) Increase in field current
D) Decrease in field resistance
Ans: A

342. The condition for maximum efficiency in DC generator is
A) Variable losses = Constant losses
B) Armature loss = Iron loss
C) Copper loss = Mechanical loss
D) Field loss = Iron loss
Ans: A

343. In DC machines, hysteresis loss depends on
A) Frequency
B) Flux density
C) Volume of iron
D) All of these
Ans: D

344. DC generator fails to build up voltage if
A) Field reversed
B) Residual magnetism lost
C) Wrong field connection
D) All of these
Ans: D

345. When load increases on DC shunt generator, its terminal voltage
A) Decreases
B) Increases
C) Remains constant
D) Fluctuates
Ans: A

346. The brush material in DC machines is usually
A) Carbon
B) Copper
C) Silver
D) Aluminium
Ans: A

347. The commutator segments are insulated by
A) Mica
B) Paper
C) Rubber
D) Plastic
Ans: A

348. The purpose of compensating winding is to
A) Neutralize armature reaction
B) Improve speed control
C) Reduce copper loss
D) Increase torque
Ans: A

349. In DC machines, neutral plane is the plane where
A) No emf is induced
B) Maximum emf is induced
C) Field strongest
D) Field weakest
Ans: A

350. If a DC shunt motor runs too fast, the likely cause is
A) Open shunt field circuit
B) Shorted armature
C) Weak brushes
D) Overload
Ans: A