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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Electrical Machines
The synchronous reactance( Xs) is the

 reactance either due to armature reaction or leakage flux
 combined reactance due to armature reaction and leakage flux
 reactance due to armature reaction of the machine
 reactance due to leakage flux

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