A NEW METHOD TO EXTEND LINEARITY OF A SINUSOIDAL PWM IN THE OVERMODULATION REGION

A new method to extend the linearity of sinusoidal pulsewidth modulati on (PWM) is presented. A square wave is added to each of the phase vol tage commands in the pulse dropping region. Addition of the square wav e maintains the fundamental component gain of the PWR in the pulse dro pping region and extends the PWM linearity all the way to the six-step mode of operation. The method eliminates the need to overmodulate the phase voltage commands to extend linearity, resulting in increased dy namic range for the associated control. The magnitude of the square wa ve is governed by a function of the modulation index. Analytical expre ssions are derived for the required function. A simple approximation o f this function is suggested and its effectiveness demonstrated for ac tual implementation. All analytical results are experimentally verifie d on a 100-kW three-phase ac-dc converter.