Electrical Machines
For satisfactory parallel operation of two transformers which of the following conditions is most essential,

  polarities of all transformers must be same.
 percentage impedance of all transformers must be same.
 MVA rating of all transformer inversely proportional to impedance.
 voltage ratio of all transformers must be same.

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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 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.
 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.

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