DC Generators
Carbon brushes are used in a dc machine because _______

Carbon is cheap
Contact resistance is decreased
Carbon lubricates and polishes the commutator
None of the listed here

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DC Generators
When two D.C. series generators are running in parallel, an equalizer bar is used

To increase the series flux
To increase the speed and hence generated e.m.f.
So that two similar machines will pass approximately equal currents to the load
To reduce the combined effect of armature reaction of both machines

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