246. The hysteresis loss in a D.C. generator varies with the frequency of magnetic reversals as
A. ?f
B.f
C. f 1 6
D.f 2

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247. Which of the following methods of braking is used in rolling mills?
A. Dynamic braking
B. Plugging
C. Regenerative braking
D. Mechanical brakes

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248. Regenerative method of braking is based on that
A. back e.m.f. is less than the applied voltage
B. back e.m.f. voltage
C. back e.m.f. of rotor is applied voltage
D. none of the above

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249. The retardation test is applicable to shunt motors and generators and is used to find
A. the copper losses
B. the stray losses
C. the friction losses
D. the eddy current losses

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250. Magnetic field in a D.C. generator is produced by
A. electromagnets
B. permanent magnets
C. both (a) and (b)
D. none of the above

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251. The number of brushes in a commutator depends on
A. speed of armature
B. type of winding
C. voltage
D. amount of current to be collected

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252. Compansating winding are used in D.C. generators
A. mainly to reduce the eddy currents by providing local short-circuits
B. to provide path for the circulation of cooling air
C. to neutralise the cross-magnetising effect ofthe armature reaction
D. none of the above

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