Electrical Machines
To have sparkless commutation, the armature reaction effect in a DC machine is neutralized by

 increasing field excitation.
 fixing the brush axis inline with the pole axis.
  compensating winding and commutating poles.
 shifting the brush axis from GNA to MNA.

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Electrical Machines
Cogging in induction motor occurs when

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

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