INFLUENCE OF THE MODULATION METHOD ON THE CONDUCTION AND SWITCHING LOSSES OF A PWM CONVERTER SYSTEM

The optimization of the modulation method of a three-phase pulse-width modulation (PWM) converter system generally does not lead to purely sinusoidal phase modulation functions. In connection with the output currents and the forward characteristics of the electric valves, these phase modulation functions directly define the conduction losses of the power electronic devices. This paper explores the dependency of the conduction losses of a bridge leg of a PWM converter system with a high pulse rate on the shape of the phase modulation functions. This is done for modulation methods that are optimized with respect to minimum harmonic current rms values. The results are compared with the results gained for simple sinusoidal modulation. Besides conduction losses, the switching losses of the electric valves are calculated. Deviations from the classical sinusoidal modulation here are obtained only for modulation methods for which the output voltage is formed by a cyclic change via only two active bridge legs and a third, unswitching bridge leg. As the calculations show. these modulation methods allow a significant increase of the effective switching frequency, This effect is dependent on the phase angle between the fundamental of the converter output phase voltage and the converter output phase current; for this comparison, equal switching losses as for the simple sinusoidal modulation are assumed. The optimal modulation of the pulse frequency of a PWM converter system is treated. A side condition that has to be observed is that the switching power loss has to correspond to the power loss occurring for operation with constant pulse frequency. The optimal modulation as calculated leads to a reduction of the harmonic power loss in the upper modulation region. Furthermore, due to the frequency modulation, the spectrum is spread out to a wider frequency band as compared with the operation with constant pulse frequency; there, the spectrum is concentrated to harmonics in the vicinity of multiples of the pulse frequency. This effect can influence the noise generation of a motor supplied by a converter, for example.