Electrical Machines
The rotor of a 3 - Φ wound induction motor is provided with

  heavy copper or aluminium bars placed in rotor slots.
  single phase winding.
  heavy short-circuited end rings.
  three phase winding.

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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.
 commutator action is not possible otherwise
 it gives more uniform air-gap flux distribution

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