The E.M.F. induced in the end-turns of armature coils during commutation

A method is developed for the calculation of the inductances of armature-coil end-turns of d.c. machines having strap windings. The calculation of the e.m.f.s induced in the end-turns during commutation is described for a prescribed form of current reversal. The calculated inductances may be used as input data for the analysis of commutation performance using electronic computers, as described in a companion paper. Particular attention is given to the calculation of the effective values of inductance, as modified by eddy currents, at the frequencies associated with commutation. The variation of these effective inductances with frequency is measured by means of search coils in endwindings. Inductance and e.m.f. calculations assuming the iron parts to have the same permeability as air show reasonable agreement with measurements at commutation frequencies. Magnetic banding over the end-windings is shown to decrease slightly the effective endturn inductances at commutation frequencies.