Electrical Machines
The oscillation in a synchronous motor can be damped out by

  providing damper bars in the rotor pole faces.
  running the motor on leading power factors.
  maintaining constant excitation.
  oscillation cannot be damped.

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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 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.
 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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