Electrical Machines
The sparking at the the brushes, in a DC machines is due to

 armature reaction.
 presence of commutator.
  reactance voltage.
 high resistance of carbon brushes.

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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

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

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