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
 dc machines are comparatively low rating machines.
 it reduces field structure iron losses
 commutator action is not possible otherwise

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Electrical Machines
In a DC generator, the armature reaction results in

 demagnetization of centers of poles.
 magnetization of leading and demagnetization of trailing pole tip.
  magnetization of inter poles.
  demagnetization of leading & magnetization of trailing pole tip.

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