Wave Factors for Rectifiers with Capacitor Input Filters, and Other High Crest-Factor Loads

Abstract

Conventional, capacitor-input-filtered, full-wave rectifier power supplies draw much higher peak and rms line currents than the supply power drains would imply if the loads were resistive. Effective multiplication factors-termed wave factors-range from 5 to 15 for peak, and from 2 to 3 for rms, while remaining between 0.7 and 1 for average. Experimental data verify theoretical calculations for the wave factors involved. Ordinary clip-on and other conventional ammeters, measuring average but calibrated to read in rms for an equivalent sine wave, are thus unable to account for the difficulties sometimes encountered in the field. These include apparently undersized fuses, breakers and lines, plus severe clipping of the line voltage wave at the load. Solutions include use of auxiliary line inductors, plus wider recognition of the need for true rms metering with adequate crest factor capability in making all line current measurements. Implicacations for UPS (Uninterruptible Power Supply) manufacturers and users are also discussed. Wave factor WF is defined as the ratio of actual current drawn, to that which would be drawn by a resistive load with the same power consumption. Wave factors are derived for peak, rms and average input currents to full-wave rectifiers with capacitor input filters.