Electrical Machines
Cogging in induction motor occurs when

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

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Electrical Machines
Flashing the field of DC generator means

 increasing flux density by adding extra turns of winding.
  creating residual magnetism by a DC source.
  making the magnetic losses forces parallel.
  neutralising residual magnetism.

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