Electrical Machines
In dc machines, the field system has to be provided on stator unlike synchronous machines Where in it could be on any member, because

 it reduces field structure iron losses
 it gives more uniform air-gap flux distribution
 commutator action is not possible otherwise
 dc machines are comparatively low rating machines.

ANSWER DOWNLOAD EXAMIANS APP

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

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

ANSWER DOWNLOAD EXAMIANS APP