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

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

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Electrical Machines
Interpoles in a DC motor must be

 Series excited and should have the same polarity as that of the ahead main pole in the direction of rotation of armature.
 Shunt excited and should have the same polarity as that of the behind main pole in the direction of rotation of armature.
 Shunt excited and should have the same polarity as that of the ahead main pole in the direction of rotation of armature.
 Series excited and should have the same polarity as that of the behind main pole in the direction of rotation of armature.

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