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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Electrical Machines
In case of 3 - Φ IM, plugging means

  pulling the motor directly on line without a rotor.
  interchanging two supply phases for quick stopping.
  starting the motor on load which is more than the rated load.
  locking of rotor due to harmonics.

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Electrical Machines
In dc machines, the field system has to be provided on stator unlike synchronous machines Where in it could be on any member, because

 it gives more uniform air-gap flux distribution
 dc machines are comparatively low rating machines.
 it reduces field structure iron losses
 commutator action is not possible otherwise

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