Electrical Machines
Armature reaction in a DC machine

 is in direct opposition to the main poles.
 is in the same direction as the main poles.
  makes an angle of 90° with the main pole axis.
 makes an angle with the main pole axis which is load dependent.

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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.
 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 ahead main pole in the direction of rotation of armature.

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