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

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

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Electrical Machines
Cogging of induction motor occurs at

 high voltage and when stator and rotor poles are not equal.
 low voltage and when stator and rotor poles are equal.
 high voltage and when stator and rotor poles are equal.
 low voltage and when stator and rotor poles are not equal.

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