DC motor to change the armature end polarity does not change the polarity of the winding will be how?

DC shunt motor changes only in the polarity armature end, changing the direction of the armature current, but excitation winding polarity has not changed, so the electromagnetic torque direction change, thus motor to change. Because the armature current reverse, so the q-axis armature reaction magnetomotive force in the opposite direction. If the original directional well, namely commutating winding magnetomotive force direction and direction of axis armature magnetomotive force and turned out to be the opposite of, so when modifying the armature winding of the motor to change after, because reversing group does not change after that commutating winding magnetomotive force direction unchanged. Therefore commutating winding magnetomotive force and cross axis armature magnetomotive force in the same direction, so reversing the situation worse, cause delay of reversing, brush, side brush may spark.