Optimal Pulse Width Modulation in the Electric Drive Control System

Abstract

The process of pulse width modulation (PWM) in the converter - electric motor system is considered. It is noted that the main task of pulse-width modulation of the voltage generated by the electron-key converter in the converter - electric motor system is to reduce current pulsations during the modulation period. It is proved that the dynamic process of changing the current in an electric machine on the PWM period can be approximated by an R, L-filter. It is established that the most important task of PWM is to minimize the current modulation error in the electric motor. The analysis of possible criteria for assessing the quality of the modulation process in electric drive control systems is carried out. It is shown that as an optimality criterion for assessing the quality of modulation in a converter - electric motor system, it is advisable to use an integral quadratic criterion of relative error, called local current dispersion, as well as the average value of local current dispersion over the period of the modulating function, called integral current dispersion. The process of single-phase modulation on a single-phase load is considered. Analytical estimates of the load current dispersion for this process are obtained. A condition, under which the current dispersion takes a minimum value, is recorded. The efficiency of pulse-width modulation optimization by the criterion of current dispersion is investigated. The average dispersion of currents in the load with a centrally symmetric arrangement of pulses on the modulation interval is taken as the base value of the dispersion. An expression that allows us to evaluate the efficiency of optimizing the current dispersion, taking into account the increment of the center of the modulation interval, is obtained. The dependence illustrating the efficiency of optimization by the criterion of pulse displacement relative to the center of the PWM interval on the relative frequency of modulation at different amplitudes of the modulating voltage is constructed. An expression is obtained for estimating the average value of the local current dispersion over the half-period of the modulating function.

Assessment of filtering properties of asynchronous electric drive with pulse width modulation

“…The important question of providing of qualitative processes of modulation is a choice of criterion estimating this process. Nowadays a sufficiently large number of methods realising PWM is known, for comparison of which quality criteria of modulated voltage [6], [7] and current [8,9] are used. The harmonic coefficient [6], [10] is quite often used as a measure of PWM quality.…”