Measurement Problems in Determining the Efficiency of Thyristor-Supplied Motor Drives

The reasons why simple ammeter and voltmeter measurements cannot be used to determine accurately the power flow into the armature of a dc motor supplied from a thyristor circuit are reviewed. Ratios of average to effective value of voltage are considered as an aid to determining the conditions under which approximate methods may give reasonable results. Ratios of fundamental to effective value in the case of the ac part of a motor drive are used to determine the appropriate frequency range for the ac measurements on a motor drive system. The implications on the problem of measuring power and losses are considered, and some criteria for the necessary instrumentation are proposed.