DC Generators
The main drawback of a dc shunt generator is that

Generated voltage is small
It is expensive
Terminal voltage drops considerably
Shunt field circuit has high resistance

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

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

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