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【正文】 Power Electronics Sinusoidal Pulse width modulation The switches in the voltage source inverter(See )can be turned on and off as the simplest approach,the top switch is turned on If turned on and off only once in each cycle,a square wave waveform ,if turned on several times in a cycle an improved har monic profile may be achieved. Fig 1:Simple Voltage Sourced In the most straightforward implementation,generation of the desired output voltage is achieved by paring the desired reference waveform(modulating signal)with a highfrequency triangu lar?carrier?wave as depicted schematically in on whether the signal voltage is larger or smaller than the carrier waveform,either the positive or negative dc bus voltage is applied at the that over the period of one triangle wave,the average voltage applied to the load is proportional to the amplitude of the signal(assumed constant)during this period. The resulting chopped square waveform contains a replica of the desired waveform in its low fre quency ponents,with the higher frequency ponents being at frequencies of an close to the carrier that the root mean square value of the ac voltage waveform is still equal to the dc bus voltage,and hence the total harmonic distortion is not affected by the PWM process. The harmonic ponents are merely shifted into the higher frequency range and are automati cally filtered due to inductances in the ac system. When the modulating signal is a sinusoid of amplitude Am,and the amplitude of the triangular carrier is Ac,the ratio m=Am/Ac is known as the modulation that controlling the modulation index therefor controls the amplitude of the applied output a sufficiently high carrier frequency(see drawn for
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