Electrical Machines
Short circuiting is caused in the armature winding due to

  grounding of two or more turns of the same coil.
  any of above.
  failure of insulation between two turns of coil.
 failure of insulation between two commutator bars.

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Electrical Machines
Cogging of induction motor occurs at

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

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