Incremental Method of Pulse Width Modulation for Three-Phase Inverters

Existing microprocessor-based PWM techniques rely on linear approximation of the sinusoidal reference waveform or on large look-up tables, as contemporary microprocessors are not fast enough to handle in real time all the PWM-related computations involving sine functions. This paper presents a novel approach to on-line stepless frequency, magnitude, and phase-angle controls of output voltages in three-phase PWM inverters. The idea on which the new, so called incremental PWM method is based consists in treating the inverter as an integral logic system. It is shown that certain class of PWM strategies yield invariant sequences of logic states of the inverter, while timing of those states displays a linear dependence on the magnitude control ratio. These features allow the controlling microprocessor to complete in real time all the computations required for adjustable frequency, magnitude, and phase angle control of inverter output voltages. An experimental system, based on the popular 8085A-2 microprocessor, was built and investigated. The experiments confirmed feasibility of the incremental PWM method. Simplicity of the system, operating completely in real time, should foster its application in low-cost AC drives.