For the operation of any d.c. machine, presence of magnetic flux is essential. Whenever current flows through a field coil, it produces a flux which is called as field flux. Now suppose the d.c. machine is functioning as a generator, an e.m.f. will be induced in the armature when it is driven by a prime mover. If the generator is driving a load, the induced e.m.f. in the armature will cause current to flow through the load. Thus current will start flowing through the armature conductors. Now every current carrying conductor will set up its own magnetic field. All these armature conductors combine together to produce a flux which can be called armature flux.
Note : Thus the armature current will set up its own magnetic field. The effect of this armature flux on the distribution of main flux is called armature reaction.
The armature reaction will try to make the main field flux to be weakened. Also it will distort the main flux.
Note : These two effects of armature reaction will reduce the generated voltage and may cause sparking at the brushes.