Stepless control of the modulation index of a three-phase drive inverter in the flat-top’ mode

Abstract

The article discusses one of the ways of stepless overmodulation in a semiconductor three-phase voltage inverter implemented according to the B6 scheme. In the review part of the article, the features of the PWM and overmodulation (“flat-top”) modes are briefly considered. The formation of the maximum possible values of the output voltage of the converter is possible due to the maximum values of the modulation index and the voltage inverter coefficient. It is shown that a variable-modulation index in the "flat-top" mode has a number of significant advantages over a fixed one, formed, for example, by "mixing" the PWM reference signal with additional harmonic signals. The geometric interpretation of the positions of the inverter output voltage vectors on the plane and their connection with the PWM reference signal made it possible to synthesize a high-precision (basic) non-linear characteristic in the inverter reference voltage path and to form a linear stepless control law for the inverter output voltage when the “flat-top” of the specified voltage coefficient is activated. The results of comparison of the basic variant of converter output voltage control with a simpler one obtained by approximating the basic equations using numerical methods of mathematics and computer simulation are presented. The absolute error of the modulation index in the case of approximation is no more than one percent in the entire "flat-top" range. The hodographs of the output voltage of the inverter in the X-Y axes are given for three different coefficients of the converter during the transition from PWM to "flat-top". The advantages of the method include the simplicity of its implementation in a new or existing space-vector PWM, the implementation of which is carried out exclusively by a software product. This aspect makes this method particularly attractive in low budget drives.