Electrical Machines
In a synchronous machine, if the field flux axis is ahead of the armature field axis in the direction of rotation, the machine operating is

  synchronous motor.
  asynchronous motor.
  synchronous generator.
  asynchronous generator.

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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

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

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