Electrical Machines
Copper brushes in DC machine are used

None of these
 in both of above cases.
 where high voltage and low current.
 where low voltage and high currents are involved.

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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

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

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