Control Strategies for PWM Drives

The majority of pulsewidth modulation (PWM) drives operate in three distinct modes: pure PWM at the lower speeds, pure six-step at the higher speeds, and quasi-PWM at the intermediate speeds. The pure PWM stage can be operated with a constant frequency, unsynchronized carrier. The transitional quasi-PWM stage, because of the operation of the minimum dwell time protection system, must be synchronized. The phase of the carrier relative to the signal then becomes important. Under these circumstances it is advisable that the carrier to signal frequency ratio be an odd multiple of three and the phase of the carrier be such that, at a signal zero, the carrier and signal have slopes of opposite sign.