Electrical Machines
The armature shaft must be able to withstand

All of these
 any unbalanced magnetic pull on the armature core.
 bending moment due to weight of the armature.
 twisting strains due to transmission of torque.

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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.
 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.
 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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