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Two-Phase Pulse-Width Modulation in the Voltage-Source Inverters


© Authors, 2009 N.V. Donskoy, K.A. Chubukov

In three-phase bridge voltage-source inverters (VSI) the most perspective method of control two-phase space-vector pulse-width modulation (PWM) is considered since it provides the minimum number commutation in VSI, allows to use completely voltage of a link of a direct current and is convenient for realization of microprocessor control. However, there are also the alternative variants of PWM providing the same characteristics, for example, two-phase piecewise sinusoidal modulation, being modernization classical three-phase sinusoidal PWM. In article on the basis of allocation of smooth components of output voltage VSI the comparative analysis of mentioned variants PWM is carried out. Two-phase space-vector PWM is based on that at invariable combination of the states of the keys of VSI phase voltage on loading is represented the motionless vectors named «base». Change of a position of keys leads to step-wise transition from one base vector to another. The real vector of phase voltage is a linear combination of two adjacent motionless (from 6 nonzero) base vectors and one or two zero base vectors. Two-phase piecewise sinusoidal PWM is based on that the output rectangular impulses of phase voltage VSI only in two phases are simultaneously modulated, and through the one sixth part of the period of the reference frequency modulation passes on two other phases etc., and modulation is carried out by 60-degree pieces of a sinusoidal signal and when modulation is absent, level of modulating (controlling) of a signal has maximum value. Controlling signals of phases on each site in the corresponding scale repeat one of desirable linear voltage or are formed of pieces of the straight lines which are passing at level of amplitude of basing sawtooth voltage. Such linear voltage for each site which would provide the sinusoidal form of demanded linear voltage of loading thus get out. At change of amplitude of controlling signal amplitudes of sinusoidal components change only. Computer modeling of VSI for both variants of PWM shows that smooth components of phase voltage on output VSI have especially not sinusoidal form. And the smooth components of linear voltage representing a difference of phase voltage are sinusoidal. Thus in phase voltage are available 60-degree learning pieces of the maximum voltage, argumentative that on these sites keys of the given phase do not switch. At both variants of PWM always switch keys only in two phases, and it turns out only 4 switching of keys on the period of a basing signal. Therefore it is possible to name these two-phase PWM. Lack of both methods of PWM is the increased amplitude of pulsations of a phase current at the moment of absence of switching. PWM such usually apply when for the purpose of reduction of dynamic losses on switching, wish to reduce number commutation keys to four instead of six during a sawtooth basing signal. Comparison of spectra of real linear voltage VSI at two-phase piecewise sinusoidal and two-phase space-vector PWM shows their identity. Thus, though ways of realization of space-vector and piecewise sinusoidal two-phase PWM are various, resultants target characteristics of VSI with both variants of PWM are almost identical.

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