Inverter control system

Abstract

AC motor drives which include static inverters typically are operated by inverter controls that are designed according to certain criteria so that the inverter is operated in an efficient manner. Generally, it is desirable to design such a control to reduce as much as possible losses in the motor windings and losses associated with the inverter switches. However, inverters switching losses cannot be reduced without increasing motor losses, and vice versa, since motor losses decrease with increasing PWM frequency while inverter losses decrease with decreasing PWM frequency. In order to overcome this problem, a control system according to the present invention for controlling an inverter which converts DC power into variable frequency AC power for energizing a motor in accordance with an input command includes circuitry coupled to the input command for generating a plurality of waveforms at different frequencies, each frequency being an integer multiple of the frequency of the AC output. PWM circuitry is included for modulating the AC output with a pulse width modulating signal derived from one of the waveforms and switching circuitry for selecting one of the waveforms in dependence upon the frequency of the AC output to maintain a particular voltage to frequency relationship therein. The instant control strikes a balance between reduction of switching losses and reduction of motor losses so that the overall losses are minimized.