Calculation of inductance in permanent-magnet DC motors

The classical literature on DC motor design tends to focus on commutating i nductance rather than total armature inductance, and does not provide a com plete procedure for all the inductance calculations that are needed in perm anent-magnet motors. The armature inductance is important, partly because o f its effect on the electrical time constant, and partly because of its eff ect on the current waveform and torque when the motor is supplied from a ph ase-controlled converter or chopper. The inductance formulas needed in PM m otors differ appreciably from those used with wound-field machines. The pap er presents a comprehensive but concise account of the theory and computati on of both the armature inductance and the commutating inductance in a form suitable for rapid computer calculation. In the armature inductance, new f ormulas for the airgap component are introduced and its importance is under lined by the fact that it accounts for about half the total inductance in s everal of the motors tested. This is an unexpected result, since the airgap component appears to have been overlooked in previous literature. Results are verified by experiment and finite-element analysis, and the paper descr ibes the theoretical basis for the necessary connections to achieve the cor rect inductance by measurement.