Performance of a Permanent-Magnet Axial-Field d.c. Machine

The performance of permanent-magnet d.c. disc-armature motors is discussed. Calculation of the field distribution allows this to be accurately predicted, and it also permits study of power losses associated with commutation. It is found that, when several pairs of brushes are used with a wave-wound armarure, and each is located opposite a different pole, then every brush will carry terminal current only intermittently. This effect severely degrades motor performance, and indicates that, whenever possible, only a single brush pair should be used with this winding. If additional current capacity is required, then extra brushes should be added opposite the same two poles.