Electrical Machines
A self-excited shunt generator fails to build up, because 1. the field circuit is open 2. The armature circuit is open 3. the residual magnetism is absent 4. The direction of rotation is wrong From these, the correct answer is

 1, 2, 3 and 4
 1, 2 and 3
 3 only
 1, 3

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Electrical Machines
Interpoles in a DC motor must be

 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.
 Series 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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