Spectral Design of Randomized Pulse Width Modulation in DC to AC Converters

This paper presents several new results and ideas. First the harmonic spectrum and power spectral density (noise density) of random width modulation for minimum loss vector PWM is analyzed and plots for one case of modulation and randomization parameters are given. As a result of the analysis of this modulation scheme, a novel time-domain formulation (autocorrelation) of the spectral information is presented. In this form we find all of the necessary details for understanding the mechanisms of general randomization schemes for suppressing harmonics and converting harmonic power to the noise spectral density. The insights found in the autocorrelation expressions allow at least one optimum design of the randomization parameters of any PWM method. The proposed optimization is but one of many indicated and implied by our methods. Introduction Research on random pulse width modulation (RPWM) techniques for static power converters, mainly three -phase inverters, has recently gained momentum. Initiated by our paper [l] in 1987, in 1992 alone the studies on various WWM issues were reported in over a dozen publications [2]. The RPWM techniques have been found to significantly improve the noise and vibration characteristics of converter-fed motors in adjustable speed drive systems [3,4]. Figure 1 shows both deterministic and random switching rate or random width P W M signals for producing ac from a dc source. The deterministic pattern is often calculated to maximize fundamental power while sometimes nulling or minimizing some selected harmonics. However all deterministic modulations have exactly the same switching patterns in all periods of the fundamental. This naturally leads to harmonics. The basic principle of RPWM consists in introduction of a random factor to the switching patterns of the controlled converter. With regard to three-phase inverters, each cycle of the output voltage is generated by a different randomized combination of pulses of the a(t) deterministic PWM T I