Electrical Machines
Slip speed is the

  sum of synchronous and rotor speeds.
  half of the sum of synchronous and rotor speeds.
  difference of actual rotor speed and synchronous speed.
  difference of synchronous speed and actual rotor speed.

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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
 it reduces field structure iron losses
 dc machines are comparatively low rating machines.
 commutator action is not possible otherwise

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