Algebraic and trigonometric algorithms of synchronized PWM for variable speed drives

Control of the widespread voltage source inverters is based on the principle of pulsewidth modulation (PWM). A fast development of digital PWM methods for three-phase inverters has occurred including investigation of randomized (nondeterministic) techniques of modulation, which can give some advantages compared with deterministic control. Both continuous and discontinuous switching strategies are now widely used in the drive systems fed by the PWM inverters. At the same time a majority of existing PWM methods and techniques has asynchronous character, that results in sub-harmonics (combined harmonics) in the spectrum of the output voltage of inverter, which are very undesirable in adjustable speed drives, especially in high power systems. In order to avoid asynchronism of typical PWM schemes, a novel method of direct digital pulsewidth modulation has been proposed. This paper presents results of analysis and comparison of both algebraic and trigonometric versions of a new method of direct synchronous PWM for three-phase inverters, based on a vector approach and applied to basic continuous and discontinuous schemes of PWM.