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
 dc machines are comparatively low rating machines.
 it gives more uniform air-gap flux distribution
 it reduces field structure iron losses

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Electrical Machines
In speed control of DC Series motor, speed lower than the rated speed can be obtained by

 Connecting an additional resistance in the armature circuit.
 Any of the above.
 Connecting an additional resistance in stator circuit.
 Connecting an additional resistance in the field circuit.

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