DEVELOPMENT AND PERFORMANCE OF PWM CONVERTOR FED D.C. DRIVE

Abstract

when the corresponding AC Voltage is negative (or positive for 1 inverter operation). In general converter operation can be divided into four categories. 1. Rectification 0 < a < n/6 auxiliary thyristors do not conduct and operation is similar to conventional six pulse bridge.. 2. Rectification rz/6 < a < 5n/6, S =0 . The auxiliary thyristor conduct alternately and phase voltage and current •are of same polarity. 3. Inversion a = 150 0 < 6 < 2n/3 auxiliary thyristors are fired alternately. Phase voltage and current have opposite . polarity. 4. Inversion Sir/6 < a < ►r auxiliary thyristors are not triggered and converter operate as a conventional six pulse synchronous converter. Thus instaneous power flow is always positive for rectification and always negative for inversion and thus power factor improves and also harmonic contents are lower on DC side. The disadvantages of this technique are additional cost of two auxiliary thyristor, and complex firing angle control.In all the schemes given above if the firing angle is increased to get reduced voltage, it will become practically impossible to get improved fundamental power factor. Also these schemes do not much reduce harmonic content in AC line. As the voltage is reduced, these !iarmonics further increase. Although higher order harmonics can be filtered out by using a filter without considerabl..