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

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

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DC Generators
Inter-pole flux should be sufficient to

Neutralize the armature reaction flux
Neutralize both the armature reaction flux as well as commutating e.m.f. induced in the coil
None of these
Neutralize the commutating self induced e.m.f.

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