Electrical Machines
The residual magnetism of a self-excited dc generator is lost. To build up its emf again

 the field winding connection must be reversed
 the armature connection must be reversed
 the field winding must be replaced
 field winding must be excited by low voltage dc supply

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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

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

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