Electrical Machines
The slight curvature at the lower end of the OCC of a self excited DC generator is due to

  high field resistance.
  residual flux.
  magnetic inertia.
  high speed.

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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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