Electrical Machines
In DC generators, the cause of rapid brush wear may be

 any of above.
 rough commutator surface.
 severe sparking.
 imperfect contact.

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Electrical Machines
Cogging of induction motor occurs at

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

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