Abstract: Stabilized operation of an inverter and an a-c motor, driven by the inverter, is obtained in the presence of sudden undesired load changes by controlling the inverter so that both the amplitude and frequency of the inverter output a-c voltage will be functions of, and will vary directly with, the d-c bus voltage, with the result that anytime an abrupt abnormal load change causes the d-c bus voltage to vary, a frequency change occurs to compensate for the load change and to stabilize the bus voltage. By employing the d-c bus voltage to regulate the inverter frequency, if there is a sudden decrease, for example, in the mechanical load on the motor, the d-c bus voltage, which will tend to increase, causes the frequency of the inverter output voltage to increase, thereby increasing the motor speed so that the electrical load change across the d-c bus is reduced. As a result, the d-c bus voltage tends to remain relatively constant in spite of the abrupt mechanical load change.

Abstract: An a-c supply system includes an input bridge circuit, connected to receive a-c input voltage and produce an output d-c voltage which is passed through a filter to an inverter having a plurality of thyristor switches, which switches are regulated to produce an a-c output voltage for supplying a load. If two of the switches are inadvertently triggered on to produce a fault across the bus, the rapid discharge of the filter capacitor into this fault is sensed, and the protection circuit provides both simultaneous turn-on of all of the inverter power switches, and turn-off of the input bridge. The input bridge turn-off is achieved by simultaneous removal of the gate drive from all the power switches in the input bridge and commutation of all the power switches in the input bridge circuit, or by interrupting power transfer from the input bridge toward the inverter.