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

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Electrical Machines
In a DC machine

 current and emf in armature are unidirectional
 emf in armature conductors and at the terminals is alternating.
 current and emf in armature are alternating.
 current and emf at the terminals are alternating.

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