Electrical Machines
Unbalance 3 - Φ stator currents cause

All of these
  vibrations.
  double frequency currents in the rotor.
  heating of rotor.

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

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