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

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Electrical Machines
The armature mmf of a DC machine has

  triangular space distribution and rotates at the speed of armature.
  triangular space distribution and is stationary in space.
 stepped distribution and rotates at the speed of armature.
  trapezoidal space distribution and is stationary in space.

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