Electrical Machines
If the number of rotor slots is equal to number of stator slots, then motor refuses to start. Hence it is known as

  cogging.
  relative speed.
  air gap.
  crawling.

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

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