Electrical Machines
In a DC generator sparking between brushes and commutator surface may be due to

  too rapid reversal of current.
  under commutation.
  any of the above.
 over commutation.

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Electrical Machines
In a DC shunt generator working on load, the brushes are moved forward in the direction of rotation, as a result of this, commutation will

  worsen and terminal voltage will rise.
 improve but terminal voltage will fall.
  worsen and terminal voltage will fall
  improve and terminal voltage will rise .

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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
 it reduces field structure iron losses
 it gives more uniform air-gap flux distribution
 dc machines are comparatively low rating machines.

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