Electrical Machines
DC generator will not build up voltage

  if it has lost residual magnetism.
 due to wrong direction of rotation.
 any one of above.
 if field resistance is more than critical resistance.

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Electrical Machines
Cogging in induction motor occurs when

 number of stator teeth - number of rotor teeth = even number.
 number of stator teeth - number of rotor teeth = negative number.
 number of stator teeth - number of rotor teeth = 0.
 number of stator teeth - number of rotor teeth = odd number.

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