Pole-entry flux pulsations

The rapid changes in flux distribution, which occur as a slotted armature moves relative to a pole corner, give rise to losses both in the copper and in the iron. These flux pulsations can have a considerable effect on the performance of both a.c. and d.c. machines. The author presents an accurate determination of the field of a slot passing under a rectangular pole corner, and the flux changes under relative motion are calculated over the range of dimensions of practical interest. The flux entering the armature slots is calculated and compared with values measured by search coils; the two sets of results are in close agreement. The effect of the slot width on the induced e.m.f. in the armature conductors is commonly ignored in design work. However, the effect is shown to be appreciable. Finally, the variation in the forces acting on the armature teeth is determined; this quantity is exceedingly difficult to obtain experimentally.