DC Generators
The rapid wear of brushes takes place due to

Excessive spring pressure
All of these
Rough commutator bars
Abrasion from dust

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

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

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