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

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

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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
Torque angle of a synchorous machine operating from a constant voltage bus is usually defined as the space angle between

 Rotor mmf wave and resultant flux density wave.
 Stator mmf wave and resultant flux density.
 Stator mmf wave and resultant mmf wave.
  Rotor mmf wave and stator mmf wave.

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