Optimizing the PWM Waveform of a Thyristor Inverter

An optimization method, which will be of use in selecting the modulation technique and Carrier frequency for a PWM inverter system, is presented. The difficult decision, concerning the tradeoff between efficiency and unfiltered harmonic content, can be made with the help of this method. A relation between second-order filter rating and unfiltered harmonic attentuation, for optimum component values, is also obtained. The optimization method is used for illustration to select the optimum modulation technique and its carrier frequency for the modified McMurray inverter [1] to be used in a rapid transit auxiliary power converter. The design aim is to obtain a constant sine wave output with a specific input supply voltage variation. The inverter efficiency is predicted for various carrier frequencies using two modulation schemes. The Schonung and Stemmler [2] modulation approach, with a carrier frequency of 900 Hz, is found to be optimum as it results in low filter rating and an efficient inverter.