501 MCQ Questions on DC Machine

201. Critical speed of d.c. shunt generator is the speed below which

(a) the generator fails to build up voltage.
(b) the generator build up voltage.
(c) the generator can not take load.
(d) the generator can take load. [FCI – 2015]

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202. The current drawn by a 460 V dc motor of armature resistance 0.5 ohm and back emf 420 V is

(a) 8 Amp
(b) 0.8 Amp
(c) 80 Amp
(d) 800 Amp [UPRVUNL AE – 2014]

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203. When the series field is so connected that its ampere turns act in the same direction as those of shunt field, the generator is said to be

(a) shunt generator.
(b) differentially compound generator.
(c) series generator.
(d) cumulatively compound generator.

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204. Which of the following theorem uses current generator

(a) Superposition theorem.
(b) Maximum power transfer theorem.
(c) Thevenin’s theorem.
(d) Norton’s theorem.

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205. The mechanical power developed by a d.c. motor is maximum when the ratio of back emf and applied voltage is

(a) 1.5
(b) 0.7
(c) 0.5
 (d) 1.0 [LMRC – 2015]

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206. A 6 pole, wave connected DC armature has 250 conductors and runs at 1200 rpm. The emf generated is 600 V. The useful Flux/pole is

(a) 0.04 Wb
(b) 0.4 Wb
(c) 4.0 Wb
(d) 0.44 Wb [Uttarakhand AE 2013]

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207. The commercial efficiency of a shunt generator is maximum when its variable loss equals the ….. loss.

(a) constant
(c) iron
(b) stray
(d) friction and windage [BSNL TTA – 2016]

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208. The basic requirement of a d.c. armature winding is that it must be

(a) a lap winding
(b) a closed one
(c) a wave winding
(d) either ‘a’ or ‘c’ [BSNL TTA – 2016]

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209. Ward-Leonard system of speed control is NOT recommended for

(a) frequent motor reversals.
(b) very low speeds.
(c) constant speed operation.
(d) wide speed range. [LMRC – 2015]

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210. The critical resistance of the D.C. generator is the resistance of

(a) armature.
(c) load.
(b) field.
(d) brushed. [BSNL TTA – 2016]

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211. When there is a variation of load resistance which of the following DC generator can not deliver power at constant voltage

(a) separately excited generator.
(b) shunt generator.
(c) series generator.
(d) compound generator. [BSNL TTA 2016]

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212. Which loss in a DC generator significantly varies with the load current? [BSNL TTA 2013]

(a) Armature copper loss
(b) Field copper loss
(c) Windage loss
(d) Hysteresis loss

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213. In a DC machine if ‘P’ is the number of poles, ‘N’ is the Armature speed in r.p.m., then the frequency of magnetic reversals will be

(a) PN/60
(b) PN/100
(c) PN/120
(d) PN/180 [BSNL TTA – 2013]

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214. Load saturation characteristics of a DC generator gives relation between

(a) V and Ia
(b) E and Ia
(c) Eo and If
(d) V and If [BSNL TTA – 2016]

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215. In a d.c. generator the generated e.m.f. is directly proportional to the

(a) field current.
(b) number of armature parallel paths.
(c) number of dummy coils.
(d) pole flux. [BSNL TTA – 2016]

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216. The brushes in d.c. machines are made of carbon

(a) to reduce the size of brushes.
(b) to reduce the wear and tear of commutator.
(c) to increase the efficiency of the commutator.
(d) none of these. [BSNL TTA – 2016]

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217. In d.c. generators, supply to external load is tapped through

(a) brush and springs.
(b) brush and commutator.
(c) solid connection.
(d) helical springs. [BSNL TTA – 2016]

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218. An AC generator running at 1000 rpm produced emf of 50 Hertz. The number of poles on the generator is

(a) 2
(b) 4
(c) 6
 (d) 8 [BSNL TTA 2016]

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219. What would be the value of pole pitch if there are 80 conductors and 8 poles?

(a) 640
(c) 80
(b) 64
(d) 10 [UPRVUNL – 2014]

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220. Match List-I (types of DC machines) with List- II (applications) and select the correct answer using given the codes. [UPRVUNL – 2015]

List-I

  1. Series Generator
  2. Shunt Generator
  3. Cumulative Compounded
  4. Differentially Compounded

List-II

  1. Arc welding
  2. Supply of distance loads
  3. Booster
  4. Battery Charging

 (a) A-1, B-4, C-3, D-2
(b) A-3, B-4, C-2, D-1
(c) A-3, B-4, C-1, D-2
(d) A-4, B-3, C-2, D-1

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221. In a shunt generator the voltage build up is generally restricted by

(a) speed limitation.
(b) armature heating.
(c) insulation restrictions.
(d) saturation of iron. [UPRVUNL – 2015]

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222. The emf induced in the armature of a shunt generator is 600 V. The armature resistance is 0.1 ohm. If the armature current is 200 A, the terminal voltage will be

(a) 640 V
(b) 620 V
(c) 600 V
(d) 580 V [UPRVUNL – 2015]

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223. In DC generator, pole shoes are fastened to pole core by

(a) rivets.
(b) brazing.
(c) welding.
(d) counter shunk screws. [UPRVUNL – 2015]

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224. In DC generators, current to the external circuit from the armature is given through

(a) commutator.
(b) slip rings.
(c) solid connection.
(d) yoke. [UPRVUNL-2015]

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225. The bearing used to support the rotor shaft are generally

(a) ball bearing.
(b) bush bearing.
(c) magnetic bearing.
(d) needle bearing. [UPRVUNL – 2015]

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226. An exciter is nothing but a

(a) DC shunt motor.
(b) DC series motor.
(c) DC shunt generator.
(d) DC series generator. [DMRC – 2015]

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227. The generator which gives d.c. supply to the rotor is called:

(a) converter.
(b) exciter.
(c) inverter.
(d) rectifier. [DMRC – 2014]

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228. The field coils of DC generator are generally made up of

(a) mica.
(b) copper.
(c) cast iron.
(d) carbon. [LMRC – 2015]

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229. Which type of winding is generally preferred for generating large currents on DC generators? [LMRC 2015]

(a) progressive wave winding.
(b) lap winding.
(c) retrogressive wave winding.
(d) current depends on design.

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230. If residual magnetism is present in a DC generator, the induced e.m.f. at zero speed will be

(a) zero.
(b) small.
(c) the same as rated voltage.
(d) high. [LMRC – 2015]

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231. Variation in speed by Ward-Leonard control method of D.C. motors occurs due to variation in

(a) field excitation.
(b) armature current.
(c) armature voltage.
(d) supply voltage. [UPSSSC-JE-2016]

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232. For a generator, Shaft Power Input – Rotational Losses = ? [UPPCL AE – 2016]

(a) Zero
(b) Electrical Power Output
(c) All copper losses
(d) Armature power developed

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233. What is proportional to the critical resistance of dc generator? [JMRC – 2012]

(a) Speed
(b) Speed x Speed
(c) 1/Speed
(d) Independent of Speed

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234. If a d.c. series motor is operated on a.c. supply, it will show

(a) poor effeciency.
(b) poor p.f.
(c) spark excessively.
(d) all of the above. [Uttarakhand AE 2013]

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235. If an AC voltage is applied on the field of a DC generator, then what will be the output of the DC generator? [JMRC – 2012]

(a) AC voltage
(b) DC voltage
(c) Zero
(d) Machine will burn out

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236. Which is the following dc generators is most suited for parallel operation?

(a) Series generator
(b) Shunt generator
(c) Compound Generator
(d) Differential Generator

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237. In D.C. generators, the cause of rapid brush wear may be

(a) severe sparking.
(b) rough commutator surface.
(c) imperfect contact.
(d) any of the above. [DSSB – 2015]

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238. In lap winding, the number of brushes is always

(a) double the number of poles.
(b) same as the number of poles.
(c) half the number of poles.
(d) two. [UPSSC – 2016]

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239. For a D.C. generator when the number of poles and the number of armature conductors is fixed, then which winding will give the higher emf?

(a) Lap winding
(b) Wave winding
(c) Both (a) and (b)
(d) Depends on other features of design

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240. Welding generator will have

(a) Lap winding
(b) Wave winding
(c) Delta winding
(d) Duplex wave winding

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241. The torque-speed characteristic of a D.C. shunt motor is

(a) a rectangular hyperbola.
(b) a drooping straight line.
(c) a parabola.
(d) None of the above [UPSSC-JE-2016]

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242. For both lap and wave windings, there are as many commutator bars as the number of

(a) slots.
(b) armature conductors.
(c) winding elements.
(d) poles. [DSSB – 2015]

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243. In a D.C. generator all of the following could be the effects of iron losses except

(a) loss of efficiency.
(b) excessive heating of core.
(c) increase in terminal voltage.
(d) rise in temperature of ventilating air.

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244. The losses occuring in a D.C. generator are given below. Which loss is likely to have highest proportion at rated load of the generator? [DSSB – 2015]

(a) Hysteresis loss
(b) Field copper loss
(c) Armature copper loss
(d) Eddy current loss

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245. In a rotary convertor I2R losses as compared to a D.C. generator of the same size will be

(a) same
(b) less
(c) double
(d) three times [DSSB – 2015]

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246. With the increase in speed of a DC motor

(a) both back emf as well as line current increase.
(b) both back emf as well as line current fall.
(c) back emf increases but line current fall.
(d) back emf fall and line current increases. [UPSSSC-JE-2016]

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247. The DC generator runs at a speed of 1000 rpm and flux per pole is 0.2 Wb, number of poles are 2, the winding is wave wound and having 480 conductors. The critical speed is 1200 rpm. Then the generated emf (E) will be

(a) 0 V
(b) 1500 V
(c) 160 V
(d) 2000 V [MPJE – 2015]

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248. A D.C. Generator has 6 poles, a brush shift of 6° actual means a brush shift of

(a) 6° electrical
(b) 18° electrical
(c) 30° electrical
(d) 2° electrical

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249. A shunt generator has a full load voltage regulation of 10%. If the generator is separately excited and delivers rated load the regulation will be

(a) 10%
(b) More than 10%
(c) Less than 10%
(d) 0

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250. A 130 MW generator is usually cooled by

(a) air.
(b) oxygen.
(c) nitrogen.
(d) hydrogen.

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