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USING SPACE-VECTOR PWM WITH VARIABLE MODULATION FREQUENCY IN MULTILEVEL INVERTERS OF PRECISION ELECTRIC DRIVE SYSTEMS

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Pulsations of load current vector are investigated for various inverter control algorithms with the use of a mathematical model of five-level inverter. It is shown that space-vector modulation has a number of features that need to be taken into account when developing control algorithms aimed at minimization of current pulsations in a wide range of output voltage control. A disadvantage inherent in this modulation method is noted, which consists in an abrupt change in the magnitude of the current pulsations when switching from one modulation level to another, which can lead to resonance phenomena in the instrument drive. While the changes in the pulsations level of are inherent in all modulation algorithms, for sinusoidal PWM and PWM with third harmonic injection, the changes are shown to be significantly smaller and not so abrupt. Current pulsations as a function of the output voltage index are also investigated, and variable frequency modulation is shown to significantly reduce the range of variation of current vector pulsations and provide almost the same average value throughout the entire output voltage control range for all modulation algorithms. The result allows to consider the space-vector modulation as an alternative to sinusoidal PWM and PWM with third harmonic injection in a wide voltage control range. The effect of dead time on the dead zone of the inverter regulation characteristic and, as a result, on the control range of rotation velocity of the electric drive is investigated. An increase in the number of levels is shown to reduce almost proportionally the dead zone and thereby extend the control range.

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