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Objective questions and answers or MCQ of Electrical Machines page-13


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206. The working of synchronous motor is, in many ways, similar to

a) The power transformer
b) The hydraulic motor
c) The gear train arrangement
d) The transmission of mechanical power by a shaft

Ans: (d)

207. The coupling angle or load angle of synchronous motor is defined as

a) The angle between the rotor and the stator poles of same polarity
b) The angle between the rotor and the stator poles of opposite polarity
c) The angle between the rotor and the stator teeth
d) None of the above

Ans: (b)

208. The coupling angle of synchronous motor is independent of load.

a) True
b) False

Ans: (b)

209. The torque developed by the synchronous motor is independent of coupling angle.

a) True
b) False

Ans: (b)

210. The back e.m.f. set up in the stator of synchronous motor will depend on

a) The rotor excitation only
b) The rotor excitation and speed both
c) The rotor speed only
d) The coupling angle only

Ans: (a)

211. If the synchronous motor (properly synchronized to the supply) is running on no load and is having negligible loss then.

a) The stator current will be very high
b) The stator current will be zero
c) The stator current will be very small
d) The back e.m.f. will be more than the supply voltage

Ans: (b)

212. The maximum power developed in the synchronous motor will depend on

a) The supply voltage only
b) The rotor excitation only
c)The rotor excitation and supply voltage both
d) The rotor excitation, supply voltage and maximum value of coupling angle (90 degree)

Ans: (d)

213. The armature current of the synchronous motor

a) Has large values for low excitation only
b) Has large values for high excitation only
c) Has large values for low and high excitation
d) Has large current for low lagging excitation

Ans: (c)

214. The minimum armature current of the synchronous-motor

a) Corresponds to 0.8 power factor
b) Corresponds to zero power factor
c) Corresponds to 0.866 power factor
d) Corresponds to unity power factor

Ans: (d)

215. The synchronous motor can be used as phase advancer.

a) True
b) False

Ans: (a)

216. A synchronous capacitor is nothing but a synchronous motor running on no load with over excitation.

a) True
b) False

Ans: (a)

217. If the field of the synchronous motor is left short circuited and connected to supply through auto-transformer

a) The motor will run at its normal speed
b) The motor will just crawl
c) The motor will run as induction motor
d) The armature will burn because there is no back e.m.f. in the armature

Ans: (c)

218. The synchronous motor can be operated at desired power factor by varying the excitation to the motor.

a) True
b) False

Ans: (a)

219. The rotary convertor can be used to

a) Convert a.c. to d.c.
b) Convert d.c. to a.c.
c) Convert liner motion to rotary motion
d) (a) and (b) both

Ans: (d)

220. A rotary convertor is used to convert a.c. to d.c. but cannot be used to convert d.c. to a.c.

a) True
b) false

Ans: (a)


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