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

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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 reduces field structure iron losses
 it gives more uniform air-gap flux distribution
 commutator action is not possible otherwise

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