To understand the concept of armature reaction, consider a two pole d.c.generator. For simplicity we will assume that the brushes are touching the armature conductors directly, although, they commutator segments in actual practice.
The brushes are always kept along MNA.
Note : In practice the two m.m.f.s exist simultaneously in the generator under load conditions.
The resultant m.m.f. OFR can be found by vectorially combining OFf and OFA.