Electrical Machines
A 3-φ alternator has negligible stator resistance. A short circuit test is conducted on this alternator. At a particular speed, a field current of If1 is required to drive the rated armature current. If the speed of the alternator is reduced to half, the field current required to maintain rated armature current

 Woud be equal to 2If1.
 Would be equal to If1/2.
  Cannot be predicted due to insufficient data.
 Would be equal to If1.

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Electrical Machines
Armature reaction in a DC machine

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

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