An on-line, real-time harmonics elimination PWM (HEPWM) for inverters

Abstract

Harmonics elimination Pulse-width modulation (HEPWM) originally proposed by Patel and Hoft [1], is a method to eliminate (specific) selected harmonics from the PWM waveform spectra. It has been seen as a viable alternative to the more widely used sinusoidal PWM (SPWM) method. The technique has been recognized to reduce harmonic losses, radio frequency interference, torque pulsations, speed ripples, etc. The main impetus for using HEPWM is that for a given inverter switching frequency, the incidence of the first non eliminated harmonic is almost double that for SPWM scheme. This results in a far superior pole switching waveform harmonic spectrum. As a result, about 50% reduction in the inverter switching frequency is achievable. The reduction in the switching frequency contributes to the reduction in the switching losses which is advantageous for high power converter applications. Furthermore, it is possible to obtain a much higher pole switching waveform fundamental amplitude before the minimum pulse-width limit of the inverter is reached. This fact, coupled with the ability to operate in the overmodulation region allows for higher utilization of the DC link voltage. Due to the high quality of the output voltage and current, the ripple in the dc link current is also small. Thus, a reduction in the size of the dc link filter components is achieved. Despite the above-mentioned prominent advantages there is one particular problem that has prevented the use of HEPWM in many drive and power converter industries, namely the equations to calculate switching angles in a HEPWM scheme are non-linear and transcendental. As a result, they cannot be solved online by a microprocessor. Normally, an off-line numerical technique had to be sought. However such operation requires large computational power and with a large number of possibilities of modulation index, ratio, and the interpolation involved, the memory requirement can be very large. Besides the off-line method suggested in [1], there exist several methods to generate on-line HEPWM waveforms. The most prominent of such work was published by Taufiq et.al. [2]. They derived a set of nontranscendental equations for near-optimal solution using sine-wave approximation approach. Using this scheme, the transcendental equations are “reduced” to a simpler form which permits on-line HEPWM computation using digital methods. Another scheme, based on regular sampled PWM technique was suggested by Bowes [3]. Other works are mostly based on pre-calculated angles which are stored in memory. These are referred to as preprogrammed harmonic elimination method. Reference [4] provides excellent review of this technique. This Chapter proposes a new method of near-optimal harmonics elimination based on quadratic-approximated equations. It is envisaged that the proposed method permits even faster and more efficient on-line, real-time computations using a microprocessor due to the simplicity of the algorithm.