Electrical Machines
Which of the following methods of speed control is not affected through stator side?

  Change of rotor resistance.
  Change of number of pole.
  Change of supply frequency.
  Change of supply voltage.

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

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