Effects of surface film on commutation

COMMUTATION normally is considered from the standpoint of the reactance voltage produced in the machine due to load current and rotation, which voltage is short-circuited by the brush. This voltage can be considered as being produced by a changing and reversing current through the self and mutual inductance of the armature coils being commutated or it may be thought of as being produced by the armature coils being commutated, cutting a resultant armature field, stationary in space, located in the interpolar space between each main pole and produced by load current in the armature coils themselves. In any case, the designer is interested in proportioning his machine so that the magnetic structure and the winding combinations and commutator and brush rigging give a structure which develops the desired voltage, carries the designed current, and will commutate successfully.