红外线风扇外文原文.doc

Sinusoidal Pulse width modulation The switches in the voltage source inverter(See Fig.1)can be turned on and off as required.In the simplest approach,the top switch is turned on If turned on and off only once in each cycle,a square wave waveform results.However,if turned on several times in a cycle an improved harmonic profile may be achieved. Fig 1:Simple Voltage Sourced In the most straightforward implementation,generation of the desired output voltage is achieved by comparing the desired reference waveform(modulating signal)with a high-frequency triangu-lar‘carrier’wave as depicted schematically in Fig.2.Depending 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 output.Note 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 components,with the higher frequency components being at frequencies of an close to the carrier frequency.Notice 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 components 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 index.Note that controlling the modulation index therefor controls the amplitude of the applied output voltage.With a sufficiently high carrier frequency(see Fig.3 drawn for fc/fm=21 and t=L/R=T/3;T=period of funda-mental),the high frequency components do not propagate significantly in the ac network(or load) due the presence of the inductive elements.However,a higher