Electrical Machines
An IM cannot run at synchronous speed because

  none of above.
  induction motor will become synchronous motor.
  rotor torque will not be produced.
  air friction will prevent it.

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Electrical Machines
Cogging of induction motor occurs at

 high voltage and when stator and rotor poles are not equal.
 low voltage and when stator and rotor poles are equal.
 low voltage and when stator and rotor poles are not equal.
 high voltage and when stator and rotor poles are equal.

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