Electrical Machines
Cogging in induction motor occurs when

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

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Electrical Machines
In speed control of DC Series motor, speed lower than the rated speed can be obtained by

 Connecting an additional resistance in the field circuit.
 Connecting an additional resistance in the armature circuit.
 Connecting an additional resistance in stator circuit.
 Any of the above.

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Electrical Machines
Armature reaction in a DC machine

 is in the same direction as the main poles.
  makes an angle of 90° with the main pole axis.
 is in direct opposition to the main poles.
 makes an angle with the main pole axis which is load dependent.

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