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

  cogging.
  harmonics.
  crawling.
  resonance.

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Electrical Machines
What is/are the necessary conditions for voltage build up in self excited generator?

All of these
 Poles should contain residual flux.
 Field winding resistance should be less than critical resistance.
 Field winding should be properly connected to armature winding.

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