501 MCQ Questions on DC Machine

251. In a DC motor the windage losses are proportional to

(a) supply voltage.
(b) square of supply voltage.
(c) square of flux density.
(d) square of the armature speed. [UPSSSC-JE-2016]

SHOW ANSWER

252. The function of equalising connections in a lap wound d.c. generator is

(a) to neutralise the armature reaction effect .
(b) to avoid unequal distribution of currents at brushes.
(c) to avoid short circuit current.
(d) none of these. [MPJE – 2016]

SHOW ANSWER

 

253. In which type of compound generator the terminal voltage rises with application of load so that its full load voltage exceeds its no load voltage? [MP JE 2016]

(a) flat compound generator.
(b) under compound generator.
(c) series compound generator.
(d) over compound generator.

SHOW ANSWER

254. In a DC machine which statement states the purposes of compensating winding and interpoles? [LMRC – 2015]

(a) Compensating winding to improve commutation. Interpoles to neutralize armature reaction.
(b) Compensating winding to neutralize armature reaction. Interpoles to improve commutation.
(c) Compensating winding to improve commutation. Interpoles to produce residual flux.
(d) Compensating winding to neutralize armature reaction. Interpoles to produce residual flux.

SHOW ANSWER

255. An electric motor with constant output power will have a torque-speed characteristic in the form of a

(a) straight line through the origin.
(b) straight line parallel to the speed axis.
(c) circle about the origin.
(d) rectangular hyperbola. [DMRC 2015]

SHOW ANSWER

 

256. If number of poles in lap wound d.c. generator are doubled, the generated e.m.f. will be

(a) increased by a factor of 2.
(b) decreased by a factor of 2.
(c) increased by a factor of 4.
(d) unchanged. [Uttarakhand – 2013]

SHOW ANSWER

257. A 4-pole d.c. generator runs at 1500 rpm. The frequency of current in armature winding is

(a) zero.
(b) 25 Hz.
(c) 50 Hz.
(d) 100 Hz. [Uttarakhand – 2013]

SHOW ANSWER

258. In order to reverse the polarity of a DC generator,

  1. field current should reversed.
  2. field current should be increased.

Which of the following is/are correct?
(a) Only 2
(b) Only 1
(c) Both 1 and 2
(d) Neither 1 nor 2 [UPPCL – 2016]

SHOW ANSWER

259. Self-excitation of a series generator

(a) is possible only when load current is zero.
(b) is independent of the load current.
(c) is not possible.
(d) is possible only when load current is not zero. [UPPCL – 2016]

SHOW ANSWER

260. Harmonics in a generator are developed due to

(a) saturation of core.
(b) vibration in the core.
(c) fluctuating load.
(d) poor insulation. [UPPCL – 2016]

SHOW ANSWER

261. What will be the approximate value of the open circuit voltage, if a 220V generator is run at full speed without any excitation?

(a) 220 V
(c) 2 V
(b) 110 V
(d) 0 V [UPPCL – 2016]

SHOW ANSWER

262. Critical resistance in a DC generator increased by

(a) increasing speed.
(b) decreasing speed.
(c) keeping field resistance constant.
(d) continuously fluctuating field resistance. [UPPCL – 2016]

SHOW ANSWER

263. Two DC series generators are running in parallel. An equalizer bar will be employed in order to

(a) increase the speed.
(b) increase the generated emf.
(c) pass equal currents to the load by two similar machines.
(d) increase the series flux. [UPPCL – 2016]

SHOW ANSWER

264. I have to get my automobile’s battery charged. Out of the following, which generator should I prefer? [UPPCL – 2016]

(a) Series generator
(b) Shunt or Series motor
(c) Compound Generator
(d) Shunt generator

SHOW ANSWER

265. Emf generated in a DC generator is directly proportional to

  1. flux.
  2. number of poles.
  3. speed of armature.

(a) Only 2 and 3
(b) 1, 2 and 3
(c) Only 1 and 3
(d) Only 1 [UPPCL – 2016]

SHOW ANSWER

266. If once destroyed, the residual magnetism of a shunt generator can be restored by connecting its shunt field

(a) to a battery.
(b) in reverse.
(c) to earth.
(d) to transformer. [UPPCL – 2016]

SHOW ANSWER

267. Full-load terminal voltage in an over-compounded generator with respect to no-load terminal voltage is

(a) equal.
(b) zero.
(c) less.
(d) more. [UPPCL-2016]

SHOW ANSWER

268. Iron losses in DC machines are independent of variation in

(a) speed.
(b) load.
(c) voltage.
(d) speed and voltage. [LMRC – 2015]

SHOW ANSWER

269. Pole shoe of a D.C. machine is laminated for the purpose of

(a) decreasing hysteresis loss.
(b) decreasing eddy current loss.
(c) decreasing both hysteresis and eddy current.
(d) manufacturing ease. [UPSSC JE – 2016]

SHOW ANSWER

270. What is the unit of torque of a DC Motor in SI unit? [DMRC – 2015]

(a) Kilogram-meter
(b) Newton-meter
(c) Joule
(d) Newton per meter

SHOW ANSWER

271. The armature current in a D.C. shunt generator is given as

(a) (Eg + V)/Ra
(b) Eg/Ra
(c) (Eg – V)/Ra
(d) IL + Ish [UPSSC JE 2016]

SHOW ANSWER

272. In the case of lap winding resultant pitch is

(a) multiplication of front and back pitches.
(b) division of front pitch by back pitch.
(c) sum of front and back pitches .
(d) difference of front and back pitches. [DSSB – 2015]

SHOW ANSWER

273. If an excited D.C. generator does not produce voltage after installing. What does first at all?

(a) Increase speed of prime mover.
(b) Increase the field resistance.
(c) Check armature winding.
(d) Reverse field connection. [LMRC – 2015]

SHOW ANSWER

274. In a D.C. machine, on no load the magnetic neutral axis

(a) moves from geometrical neutral axis in the direction of rotation.
(b) moves from geometrical neutral axis in the opposite direction of rotation.
(c) concides with the geometrical neutral axis.
(d) none of the above. [UPSSSC – 2016]

SHOW ANSWER

275. Why interpoles are used in dc machines?

(a) To counteract the reactance voltage.
(b) To counteract the demagnetizing effect of armature mmf in the commutating zone.
(c) To counteract the magnetizing effect of armature mmf in the commutating zone.
(d) To convert dc emf to ac emf. [UPRVUNL AE-2014]

SHOW ANSWER

276. What is the effect of armature flux on the main field?

(a) Cross magnetizing only.
(b) Demagnetizing only.
(c) Cross magnetizing as well as demagnetizing.
(d) Neither cross nor demagnetizing. [UPPCL AE – 2016]

SHOW ANSWER

277. If in a DC machine, the ratio of constant losses to armature resistance is 2, then at maximum efficiency, the magnitude of current flowing would be

(a) 6.667 A
(b) 2 A
(c) 4 A
(d) 1.414 A [UPPCL AE]

SHOW ANSWER

279. Choose the INCORRECT statement amongst the given statements.

(a) Magnetic flux usually flows through a minimum reluctance path.
(b) Field winding produces the working flux.
(c) Working emf is induced by working flux in armature winding.
(d) Commutator serves as mechanical inverter for DC generator. [UPPCL AE – 2016]

SHOW ANSWER

280. A 6-pole DC machine having wave winding has ………. number of parallel paths. [UPPCL AE – 2016]

(a) 3
(b) 1
(c) 2
(d) 6

SHOW ANSWER

281. Which of the following speed control method of a DC machine is not a constant torque drive? [UPPCL AE – 2016]

(a) Armature voltage control.
(b) Armature resistance control.
(c) Field flux control.
(d) Armature voltage, Armature resistance and Field flux control.

SHOW ANSWER

282. Constant current region I external characteristics (V-I) of a series generator is best suited for

(a) line drop compensation.
(b) welding.
(c) rectification.
(d) drilling. [UPPCL AE – 2016]

SHOW ANSWER

283. The detrimental effects of Armature reaction CANNOT be controlled by

(a) providing the machine with a compensating winding.
(b) using commutation poles.
(c) reducing the cross section of poles pieces.
(d) decreasing the length of air gap. [UPPCL AE – 2016]

SHOW ANSWER

284. If δ is the torque angle, then interaction torque in a DC machine is proportional to

(a) sin δ
(b) tan δ
(c) cos2δ
(d) sin2δ [UPPCL AE – 2016]

SHOW ANSWER

285. In a 6 pole D.C. machine, 100° mechanical corresponds to how many electrical degrees?

(a) 600°
(b) 300°
(c) 200°
(d) 150° [UPRVUNL AE – 2016]

SHOW ANSWER

286. Armature reaction in a D.C. shunt generator runing at full load with brushes not shifted from the geometrical neutral plane and saturation neglected is

(a) non-existent.
(b) cross magnetizing.
(c) demagnetizing.
(d) magnetizing. [UPRVUNL AE – 2016]

SHOW ANSWER

287. What do we call a winding when the ends of a coil are joined to a commutator bar at a distance of one pole pitch? [UPPCL JE – 2016]

(a) Lap winding
(b) Wave winding
(c) Ring winding
(d) Rectangular winding

SHOW ANSWER

288. For a Lap wound generator, the number of parallel paths in armature is taken to be

(a) 2
(b) 4
(c) number of poles/2
(d) number of poles [UPPCL JE – 2016]

SHOW ANSWER

289. The Eb/V ratio of a d.c. motor is an indication of its

(a) efficiency.
(b) speed regulation.
(c) starting torque.
(d) running torque.

SHOW ANSWER

290. In DC generator, which type of electricity is produced in the armature? [UPPCL JE – 2016]

(a) DC
(b) AC
(c) AC and DC
(d) No electricity is produced in the armature

SHOW ANSWER

291. Tension in brushes of DC generator is quite important for its working. With excess tension in brushes

(a) velocity of machine decreases.
(b) output voltage decreases.
(c) output voltage increases.
(d) commutator gets over heated and surface wears out. [UPPCL JE – 2016]

SHOW ANSWER

292. In which one of the following, the armature brushes are shorted together rather than connected in series with the field? [UPPCL JE – 2016]

(a) Universal motor
(b) Repulsion motor
(c) Exterior motor
(d) Sliding rotor motor

SHOW ANSWER

293. Whenever machine is operating as generator the emf is called as….. whereas when machine is operated as motor the emf is called as……. [UPPCL JE – 2016]

(a) generated emf, generated emf
(b) back emf, back emf
(c) generated emf, back emf
(d) back emf, generated emf

SHOW ANSWER

294. Power output in a motor is 140 W. Input to the motor is 176.84 W including losses. Then, the efficiency is

(a) 76%
(b) 79%
(c) 81%
(d) 74% [LMRC – 2015]

SHOW ANSWER

295. The yoke of small DC machine is made up of

(a) cast iron
(b) aluminium
(c) stainless steel
(d) copper [RRB JE 2015]

SHOW ANSWER

296. The field of self-exited generator is excited by

(a) AC.
(b) DC.
(c) by its own current.
(d) either AC or DC. [RRB JE – 2015]

SHOW ANSWER

297. For constant speed line shafting …….. type of DC motor is used. [UPPCL – 2016]

(a) under compounded
 (b) shunt
(c) over compounded
(d) series

SHOW ANSWER

298. The terminal voltage of a DC shunt generator ……. on loading.

(a) increases slightly.
(b) increases sharply.
(c) remains constant.
(d) decreases slightly. [RRB JE – 2015]

SHOW ANSWER

299. A D.C. series motor is best suited for driving

(a) lathes.
(b) cranes and hoists.
(c) shears and punches.
(d) machine tools. [RRB JE – 2015]

SHOW ANSWER

300. The counter e.m.f. of a d.c. motor

(a) often exceeds the supply voltage.
(b) aids the applied voltage
(c) helps in energy conversion.
(d) regulates its armature voltage. [RRB SSE 2015]

SHOW ANSWER

Leave a Comment

Your email address will not be published. Required fields are marked *