ADJUSTABLE SPEED A-C MOTOR DRIVE WITH SMOOTH TRANSITION BETWEEN OPERATIONAL MODES AND WITH REDUCED HARMONIC DISTORTION

摘要

ABSTRACT OF THE DISCLOSUREAlternating voltage for energizing an adjustablespeed a-c motor is derived from unipolarity input voltage bymeans of electric power inverting apparatus comprising atleast one pair of alternately conducting controllable electricvalves. The conducting states of these valves are periodicallyswitched by a control which is responsive to frequency andamplitude command signals, whereby the frequency and theamplitude of the fundamental component of the alternating voltageare varied as functions of the respective command signals. Thecontrol provides a triangular wave for obtaining interceptionpoints for deriving pulse width modulation (PWM) of thealternating voltage so long as the amplitude command signaldoes not exceed a predetermined reference value (which is lessthan 1.0 per unit) and the speed of the motor does not exceeda predetermined reference speed. The control additionally pro-vides a "transition" PWM mode of operation whenever theamplitude command signal exceeds its reference value or themotor speed exceeds said reference speed. As disclosed hereinthe transition PWM mode is provided by a dual a-c level setscheme in which the lower level is varied as a function of thehigher level so as to minimize selected harmonics of thealternating voltage and the higher level is varied as a functionof the amplitude command signal thereby to vary the amplitudeof the fundamental voltage component. A smooth transitioningto a square wave mode of operation is made as the amplitudecommand signal approaches 1.0 per unit.