The most commonly used DC motor for traction is

 351. The torque of a DC motor is proportional to

A) Armature current
B) Armature voltage
C) Product of flux and armature current
D) Field current
Ans: C

352. If the polarity of supply to a DC motor is reversed, it will
A) Continue to run in same direction
B) Stop
C) Reverse its direction
D) Burn out
Ans: A

353. For a DC motor to reverse its direction,
A) Either field or armature current must be reversed
B) Both must be reversed
C) Supply polarity must be changed
D) Supply frequency must be changed
Ans: A

354. The flux in a DC motor is proportional to
A) Field current
B) Armature current
C) Voltage
D) Speed
Ans: A

355. The torque developed by a DC motor is zero when
A) Back emf = Supply voltage
B) Armature current = 0
C) Flux = 0
D) All of these
Ans: D

356. The armature resistance of a DC machine is
A) Very small
B) Very large
C) Moderate
D) Variable
Ans: A

357. The main reason for using laminated armature core is to
A) Reduce eddy current losses
B) Increase mechanical strength
C) Increase inductance
D) Increase weight
Ans: A

358. A DC series motor should never be started without load because
A) It will run at dangerously high speed
B) It will draw very high current
C) It will overheat
D) It will reverse its direction
Ans: A

359. In DC shunt motor, field current is
A) Constant
B) Proportional to load
C) Inversely proportional to speed
D) Variable
Ans: A

360. In DC series motor, torque is proportional to
A) Armature current squared
B) Field current
C) Speed
D) Flux
Ans: A

361. The copper losses in DC machine vary as
A) Current squared
B) Voltage
C) Speed
D) Flux
Ans: A

362. Hysteresis loss in DC machine depends upon
A) Frequency of magnetic reversals
B) Flux density
C) Volume of iron
D) All of these
Ans: D

363. In a DC motor, mechanical losses consist of
A) Friction and windage losses
B) Hysteresis and eddy losses
C) Copper losses
D) Armature reaction losses
Ans: A

364. In DC generator, armature reaction causes
A) Flux distortion and reduction
B) Increase in generated voltage
C) Constant flux
D) Improvement in commutation
Ans: A

365. If brushes are shifted in a DC generator, it affects
A) Commutation
B) Generated voltage
C) Both A and B
D) Field current
Ans: C

366. The armature reaction in DC generator is mainly due to
A) Armature current
B) Field current
C) Speed
D) Voltage
Ans: A

367. Which of the following DC generator is used for lighting purpose?
A) Shunt generator
B) Series generator
C) Compound generator
D) Separately excited generator
Ans: A

368. The residual magnetism in a DC generator can be restored by
A) Flashing the field
B) Reversing armature connections
C) Changing brushes
D) Running at higher speed
Ans: A

369. In DC machine, armature coils are short-circuited by brushes during
A) Commutation
B) Armature reaction
C) Sparking
D) Saturation
Ans: A

370. The field windings in a DC machine are placed on
A) Poles
B) Armature
C) Yoke
D) Commutator
Ans: A

371. The ratio of mechanical power developed to electrical power input in a DC motor is
A) Efficiency
B) Electrical efficiency
C) Mechanical efficiency
D) None of these
Ans: C

372. The function of commutator is to
A) Convert AC to DC
B) Convert DC to AC
C) Reverse current in armature coils
D) Rectify voltage
Ans: C

373. The equalizer ring is used in
A) Lap wound armature
B) Wave wound armature
C) Both A and B
D) None
Ans: A

374. The direction of induced emf in a DC generator is determined by
A) Fleming’s right-hand rule
B) Fleming’s left-hand rule
C) Lenz’s law
D) Faraday’s law
Ans: A

375. In a DC generator, the brushes are located
A) Along the neutral axis
B) Along the field axis
C) Along the pole axis
D) Randomly
Ans: A

376. The load on a DC generator is connected to
A) Armature terminals
B) Field terminals
C) Yoke
D) Shaft
Ans: A

377. The current in armature conductors is
A) DC
B) AC
C) Pulsating DC
D) Alternating AC
Ans: B

378. The commutator converts armature
A) AC into DC
B) DC into AC
C) Current into voltage
D) Flux into current
Ans: A

379. The efficiency of a DC generator improves when
A) Copper losses decrease
B) Iron losses increase
C) Load increases indefinitely
D) Speed decreases
Ans: A

380. In DC generator, maximum efficiency occurs when
A) Variable loss = Constant loss
B) Copper loss = Mechanical loss
C) Iron loss = Copper loss
D) Field loss = Armature loss
Ans: A

381. The purpose of compensating winding is to
A) Neutralize cross magnetizing effect
B) Increase voltage
C) Reduce field losses
D) Increase torque
Ans: A

382. A cumulatively compounded DC generator gives
A) Higher voltage under load
B) Lower voltage under load
C) Constant voltage
D) Variable voltage
Ans: A

383. The differential compounded generator is used for
A) Welding
B) Electroplating
C) Variable load
D) Not used normally
Ans: D

384. The mechanical power developed in DC motor =
A) Eb × Ia
B) V × Ia
C) Ia²Ra
D) Eb × If
Ans: A

385. The starting current in a DC motor is high because
A) Back emf is zero at start
B) Armature resistance is high
C) Field flux is zero
D) Load is high
Ans: A

386. The back emf of a DC motor is given by
A) Eb = (ΦZN/60) × (P/A)
B) Eb = (ΦZNP/A)
C) Eb = V – IaRa
D) Both A and C
Ans: D

387. When the load on a DC motor increases,
A) Speed decreases
B) Speed increases
C) Speed remains constant
D) Torque decreases
Ans: A

388. In speed control of DC shunt motor, field control method is used for
A) Above rated speed
B) Below rated speed
C) Constant torque region
D) Low speed operation
Ans: A

389. In armature voltage control, speed is varied
A) Below rated speed
B) Above rated speed
C) Constant power region
D) None
Ans: A

390. The torque-speed characteristic of DC series motor is
A) Hyperbolic
B) Linear
C) Constant
D) Exponential
Ans: A

391. The main disadvantage of DC series motor is
A) High starting torque
B) Poor speed regulation
C) High cost
D) Low efficiency
Ans: B

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

393. The speed of DC shunt motor can be increased by
A) Decreasing field current
B) Increasing field current
C) Increasing load
D) Decreasing voltage
Ans: A

394. In DC generator, the emf equation is
A) E = (ΦZN/60)(P/A)
B) E = (ΦZNP/A)
C) E = (V – IaRa)
D) E = (PΦN/A)
Ans: A

395. The output of DC generator depends on
A) Flux and speed
B) Speed only
C) Field current only
D) Armature resistance
Ans: A

396. The main function of armature core is to
A) Provide magnetic path
B) Provide mechanical strength
C) Reduce hysteresis loss
D) Insulate windings
Ans: A

397. For good commutation, brushes should be placed
A) Along neutral axis
B) Along field axis
C) At 90° to field
D) Along pole axis
Ans: A

398. The voltage regulation of DC generator is
A) (E – V) / V × 100%
B) (V – E) / E × 100%
C) (V / E) × 100%
D) (E / V) × 100%
Ans: A

399. The brushes in DC machine are made of
A) Carbon
B) Copper
C) Aluminium
D) Steel
Ans: A

400. The most commonly used DC motor for traction is
A) DC series motor
B) DC shunt motor
C) Compound motor
D) Separately excited motor
Ans: A