Armature reaction

Armature reaction means various side effects of the armature current on motor performance.
One is the increase of sparks between the commutator and brushes in a brush-type DC motor.
Another is the decrease of the back-emf constant when the armature current exceeds a certain level, which is seen in both a brush-type and a brushless type of DC motor. This takes place due to a strong magnetic saturation at either of magnet’s two edges, and this appears in the two characteristic curves as shown in the figure.