ADJUSTABLE SPEED AC DRIVE SYSTEM CONTROL FOR OPERATION IN PULSE WIDTH MODULATION AND QUASI-SQUARE WAVE MODES

摘要

A polyphase AC motor drive system comprises anelectric power inverter and control means which enablesoperation in, and allows smooth transition between, a currentcontrolled PWM mode and a quasi-square wave mode ofoperation. At low motor speeds, the system operates in PWMmode. At higher speeds, above the corner point speed of themotor, the system operates in a quasi-square wave mode.Smooth transitions between modes of operation are controlledby a microcomputer which counts the number of current chopsmade by a switching device in a preselected phase leg of theinverter over a time interval that varies with the desiredfundamental period of the motor current. As speed increasesand approaches the corner point, the number of chopsdecreases below a first predetermined number, and the systemtransitions from PWM to quasi-square wave mode. As speeddecreases, the number of chops increases above a secondpredetermined number (larger than the first), and the systemtransitions back to PWM mode. A motor speed command islimited by an adjustable clamp circuit, depending upon the DClink voltage, resulting in a robust drive that is notaffected by utility brownouts or line transients.