Electrical Machines
In a DC generator sparking between brushes and commutator surface may be due to

  any of the above.
  too rapid reversal of current.
 over commutation.
  under commutation.

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

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