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
 dc machines are comparatively low rating machines.
 commutator action is not possible otherwise

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Electrical Machines
Armature reaction in a DC machine

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

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