Linear control of PWM inverter by avoiding the pulse dropping

In PWM inverter fed ac machine drives, at higher modulation indices, the output pulse-width becomes very narrow. These short pulses do not contribute to produce torque in the machine but introduce additional losses and stress in the power switching devices. This is especially true in large drive systems, where the sine triangle based PWM strategies are being widely used. In this paper, a technique to eliminate these narrow pulses is proposed. This is based on injecting an optimized amount of third harmonic signal to the reference waveform. This optimized amount of the third harmonic is defined off-line for different modulation indices and minimum pulse-width required by the power electronics devices used in the inverter. Finally, use of this method in vector-controlled drives is explained and improvement of the efficiency is demonstrated by experiment.