基于锯齿载波的最少开关次数调制波的PWM数学分析

Mathematical analysis on least switching PWM based on sawtooth carrier wave

文章分析了最少开关次数调制波的输出线电压波形,并根据三相逆变器实现软开关PWM动作需要,用双重傅里叶级数展开法解析了基于锯齿载波的最少开关次数调制波的PWM频谱特性。分析结果指出,该调制波虽是个不对称波,且含有直流分量,但其实质还是在正弦波基础上叠加了零序谐波分量和一个直流分量。这些零序谐波分量和直流分量在三相逆变器中相互抵消,其输出线电压还是正弦波。正是这种畸形波的调制作用,使得逆变器直流母线电压利用率得以提高,且正负序谐波幅值相差较小,能有效减小谐波损耗,提高波形质量,抑制谐波转矩脉动。通过与SAPWM比较后指出,最少开关次数调制波PWM只能使用单极性调制,其有效开关次数是后者的一半,电流波形质量不如后者。

This paper analyzes the output line voltage waveform of least switching modulation,and in some cases,soft switching three-phase PWM inverters are required,using double Fourier series expansion method to resolve the PWM spectrum characteristics of the least switching modulation based on sawtooth wave carrier. The results indicate that although it is an asymmetric modulation wave,and contains a DC component,but its essence is superimposing zero sequence harmonic components and a DC component on a sine wave. These zero-sequence harmonics and DC components cancel each other out in a three-phase inverter,and the output line voltage is still a sine wave. Owing to the distorted modulation wave,the DC bus voltage utilization of inverter can be improved,and the difference of harmonic amplitudes between the positive and negative sequence is smaller,moreover,it can effectively reduce harmonic losses,improve waveform quality,and suppress harmonics torque ripples. After comparing the minimum switching frequency modulation with SAPWM,it may be noted that the minimum switching frequency modulation can only use unipolar modulation,the effective switching frequency is only half of the latter,so the current waveform quality is not as good as latter.