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Objective questions and answers or MCQ of Electrical Machines page-11


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176. The wide-open type slots in the design of stator of alternator will present the following disadvantage?

a) The size of the machine will increase
b) The size of the stator will increase
c) Air gap flux is distributed in the tufts and produce ripples in the wave of generated e.m.f.
d) None of the above

Ans: (c)

177. What kind of rotor is most suitable for turbo-alternators which are designed to run at high speed?

a) Salient pole type
b) None-salient pole type
c) (a) or (b)
d) None of the above

Ans: (b)

178. The power developed in the alternator having salient type rotor is less than of having non-salient pole.

a) True
b) False

Ans: (b)

179. The damping winding in the synchronous motors is often used

a) To prevent hunting only
b) To prevent hunting and provide the starting torque
c) To maintain proper synchronism
d) To provide the starting torque only

Ans: (b)

180. The turbo-alternators are seldom characterized by small diameters and very long axial or rotor length.

a) True
b) False

Ans: (b)

181. The maximum speed of an alternator at power frequency of 50 Hz will be

a) 3600 rpm
b) 3300 rpm
c) 3000 rpm
d) 1500 rpm

Ans: (c)

182. Which of following statement is correct?

a) Short pitched coils in the alternators are used to reduce the size of the machines.
b) Short pitched coils are used to reduce the harmonics or to eliminate the harmonics from the generated e.m.f.
c) Short pitched coils are used to provide accurate phase difference of 120 degree to one phase to other.
d) Short pitched coils are used to reduce copper losses.

Ans: (b)

183. The disadvantage of using short pitched coils in the alternator is that

a) Total induced e.m.f. is increased
b) Total induced voltage is reduced
c) Total induced voltage is distorted
d) Total induced voltage is more and hence more insulation is required

Ans: (b)

184. The coil span factor is defined as

a) (vector sum of induced e.m.fs. per coil)/(terminal voltage of the generator)
b) (arithmetic sum of induced e.m.fs. per coil)/(vector sum of induced e.m.fs. per coil)
c) (vector sum of induced e.m.fs. per coil)/(arithmetic sum of induced e.m.fs. per coil)
d) (arithmetic sum of induced e.m.fs. per coil)/(terminal voltage of the generator)

Ans: (c)

185. The induced e.m.f. in an alternator with distributed winding is always more than of with concentrating winding.

a) True
b) False

Ans: (b)

186. A three phase, 4 pole, 24- slots alternator has its armature coil short pitched by one slot. The distribution factor of alternator will be

a) 0.96
b) 0.9
c) 0.933
d) 0.966

Ans: (d)

187. In case of an alternator, the power factor of the load has a considerable effect on the armature reaction unlike d.c. generators.

a) True
b) False

Ans: (a)

188. When the power factor of load is unity, the armature flux of an alternator will be

a) Demagnetizing
b) Cross-magnetizing
c) Square waveform
d) N phase with current

Ans: (b)

189. When the load has power factor zero lagging, the main flux of the alternator will decrease.

a) True
b) False

Ans: (a)

190. The armature reaction of an alternator will be completely magnetizing when

a) Load power factor is unity
b) Load power factor is zero lagging
c) Load power factor is zero leading
d) Load power factor is 7.0 lagging

Ans: (c)


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