Variable time step control with synchronous PWM in low frequency modulation index for AC machine drive

This paper proposes a variable time step controller with synchronous PWM. Since a frequency modulation index is low in an ultra-high speed or high power drive system, the modulated voltage by a fixed time step control includes large harmonic components. The optimal PWM method is one of the solution to minimize the harmonic components, but it needs many offline calculations and its structure is complicated. The proposed variable time step control has small calculation burden and it does not need offline calculations. Furthermore, the harmonic components of the modulated voltage is small because the frequency modulation index is kept as integer value in all speed conditions. Also, the transition strategy between synchronous PWM methods is proposed and it is verified with simulation and experimental results.

I. Introduction

Voltage Source Inverter (VSI) can modulate an accurate phase voltages with small harmonic components where the ratio of the switching frequency to the fundamental frequency of the phase voltage, called as the frequency modulation index , is over than 15 [1]. However, in high-power machine drives, such as a fan, pump, and railway traction, a switching frequency of the switch device is limited to a few hundred hertz [2]. In ultra-high speed drive system, such as an air compressor, spindle drill, and portable power generator, the fundamental frequency of the phase voltage is large [3]. Therefore, in the high-power or ultra-high speed drive systems, is small value witch is not integer and the modulated voltage has large harmonic component.