A calculation of the commutating time for machines using thyristor-assisted commutation is as important to these machines as the calculation of current-reversal curves for conventional d.c. machines. New factors enter into the determination of the inductance of a commutating coil, particularly an inductance component attributable to a main-pole-axis flux change. Different procedures also apply to the calculation of armaturereaction m.m.f. and interpole-induced voltage. A logical design procedure is described and applied to a typical machine. The development and the terminal characteristics of such a machine are described in companion papers.