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Electrical Engineering Objective Questions { DC Motors }

148. A dc cumulatively compounded generator was operating satisfactorily and supplying power to on infinite bus when the mechaisal power supply from the prime mover failed. The machine will then run as a
A. differentially compounded motor with speed reversed
B. differentially compounded motor with the direction of speed the same as before
C. cumulatively compounded motor with the same direction of speed as before
D. cumulatively compounded motor with speed reversed
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149. A dc shunt generator builds up to a voltage of 220 V at no-load while running at its rates speed. If the speed of the generator is raised by 25% keeping the circuit conditions unaltered, then the voltage to which the machine would build up will
A. not change and remain at 220 V
B. increase to 1.25 times 220 V
C. increase to a value lying between 220 V ad 1.25 times 220 V
D. increase to a value greater than 1.25 times 220 V.
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150. Two identical loss-less series motors connected in series across a dc supply voltage, run at speeds of N1 and N2, The ratio of their output powers will be
A. N12 : N22
B. 1 : 1
C. Ni : N2
D. N2 : N1
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151. In a dc machine, the demagnetising effect of armature reaction is due to
A. component of armature mmf along field axis
B. non-sinusoidal nature of armature mmf
C. magnetic saturation in half of the field pole
D. uneven air gap length
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152. Plugging of dc motors is normally done bY
A. connecting a resistance across the armature
B. reversing simulataneously the armature and field polarity
C. reversing the field polarity
D. reversing the armature polarity
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153. A dc shunt wound motor finds application in
A. electric trains
B. tape recorders
C. blowers
D. steel rolling mills
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154. In dc machine, the field system has to be provided on stator unlike synchronous machines wherein it could be on any member, because
A. it reduces field structure iron losses
B. it gives more uniform air-gap flux distribution
C. commutator action is not possible otherwise
D. dc machines are comparatively low rating machines
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