DC Generators
The main factor which leads to unstable parallel operation of flat and over compounded generators is

unequal speed regulation of their primemovers
unequal series field resistances
unequal number of turns in their series field windings
their rising voltage characteristics

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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 reduce the combined effect of armature reaction of both machines
To increase the series flux
To increase the speed and hence generated e.m.f.

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