Electrical Machines
The residual magnetism of a self-excited dc generator is lost. To build up its emf again

 the field winding must be replaced
 the field winding connection must be reversed
 the armature connection must be reversed
 field winding must be excited by low voltage dc supply

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

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