Protection system for immunizing an inverter system against a-c line

Abstract

When a-c power line voltage is rectified by a phase-controlled SCR rectifier bridge to develop an adjustable d-c bus voltage which is then converted by an inverter back to a-c for driving a load, such as a motor, line voltage disturbances, resulting from momentary power outages or major power reductions, will have a deleterious effect on the operation of the inverter system. Such disturbances will cause improper firing of the SCR's and the d-c bus voltage may increase considerably, disrupting the normal operation of the inverter system and possibly destroying the switching devices in the inverter. Immunization against the effects of the line voltage disturbances is obtained by shutting down or disabling the phase-controlled SCR rectifier bridge anytime such a disturbance occurs. In this way, while the output voltage of the rectifier bridge is effectively interrupted during the occurrence of each line voltage disturbance, the inverter will continue to operate in near normal manner in response to the d-c voltage provided by the filter capacitor. At the conclusion of each disturbance, the operation of the rectifier bridge will be slowly restored to normal to avoid overshoot of the d-c bus voltage.