Electrical Machines
In induction motor the 7th harmonic torque may make the motor rotate at (1 / 7)th synchronous speed. This is called

  cogging.
  resonance.
  harmonics.
  crawling.

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Electrical Machines
The mmf produced by rotor currents of a 3 - Φ induction motor

  is at standstill with respect to stator mmf.
  rotates at slip speed with respect to stator mmf.
  rotates at the speed of rotor in the air gap.
  rotates at synchronous speed with respect to rotor.

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