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Objective questions and answers or MCQ of Electrical Machines page-14


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221. A 2 kW single phase rotary convertor operates at full load from 230 voltage a.c. source. Assuming unity power factor and 100% efficiency the d.c. current will be

a) 6 A
b) 6.15 A
c) 8.7 A
d) 5 A

Ans: (b)

222. A rotary convertor can be run as a

a) Induction motor
b) Synchronous motor
c) D.c. series motor
d) D.c. compound motor

Ans: (a)

223. In the operation of the poly-phase rotary convertors, which of the following should be introduced between the a.c. sources and slip rings?

a) Amplifier
b) Rectifier
c) Transformer
d) Diode

Ans: (a)

224. Which of the following statements is true?

a) A rotary convertor can be called a Synchronous invertor.
b) A rotary convertor can be called a Synchronous convertor.
c) A rotary convertor can be called a Synchronous divertor.
d) A rotary convertor can be called a Synchronous capacitor.

Ans: (b)

225. Which of the following rotary convertors is used in the standard practice?

a) Single phase rotary convertor.
b) Three phase rotary convertor.
c) Six phase rotary convertor.
d) Twelve phase rotary convertor.

Ans: (c)

226. For a given temperature rise and hence output, a six-phase convertor is smaller than three or two phase convertor.

a) True
b) False

Ans: (a)

227. Given the four methods to control d.c. voltage of rotary convertor :

a) By using tap changing transformer.
b) Series reactance control method.
c) Induction regular.
d) Synchronous booster control method.

Ans: ()

228. The mercury arc rectifier is used as

a) Convertor only
b) Inventor only
c) (a) and (b) both
d) Transducer

Ans: (c)

229. The arc between the anode and cathode of mercury arc rectifier will persist

a) When anode is as positive potential with respect to cathode
b) When anode is at negative potential with respect to cathode
c) When anode will repel the electrons
d) When anode is at very low temperature

Ans: (a)

230. The purpose of introducing reactor in the ignition circuit of mercury arc rectifier is

a) To limit the rate of change of flux in the circuit
b) To limit the voltage of the circuit
c) To limit the rate of change of the current in the circuit
d) To limit the current in the circuit

Ans: (d)

231. The advantage of using larger phase mercury arc rectifier is that

a) They are easily available
b) They work noiselessly
c) They are very cheap
d) Their output is much smoother

Ans: (d)

232. The utility factor of mercury-arc rectifier is defined as

a) (actual power)/(full load rating)
b) (wattless power component)/(full load rating)
c) (actual power)/(half load rating)
d) (total a.c. input)/(total d.c. output)
e) (total d.c output)/(total d.c. input)

Ans: (a)

233. Angle of overlap is related to

a) Diode rectifier
b) Silicon controlled rectifier
c) Mercury-arc rectifier
d) Metal rectifier

Ans: (c)

234. Give the three types of voltage drop when mercury-arc rectifier is on load :

a) Reactance drop
b) Mean resistance drop
c) Arc voltage drop

Ans: ()

235. The utilization factor of the six-phase mercury arc rectifier is more than that of three-phase mercury-arc rectifier.

a) True
b) False

Ans: (b)


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