Electrical Machines
Interpoles in a DC motor must be

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

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

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

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