Electrical Machines
In 3 - Φ induction motor some times copper bars are placed deep in the rotor to

  improve efficiency.
  improve power factor.
  improve starting torque.
  reduce copper loss.

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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

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

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