Electrical Machines
A dc shunt generator, when commissioned after a major repair of the field system, failed to build Up voltage. It was because

 there was a break in the armature circuit
 field resistance was higher than critical resistance
 field winding was wrongly connected
 field system lost residual magnetism

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Electrical Machines
In large sized DC machines the compensating windings are provided to

None of these
  neutralize voltage induced in coil due to fluctuation of l
 compensate for the decrease in speed due to increase of load.
 reduce eddy current losses by providing local short circuits.

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