Modulation of the Chopping Frequency in DC Choppers and PWM Inverters Having Current-Hysteresis Controllers

In a dc chopper converter having its load current regulated by a feedback controller of the fixed hysteresis type, the chopping frequency is shown to depend on the square of the counter EMF of the load. In a pulsewidth modulated (PWM) inverter, the counter EMF is modulated at the output frequency, so the chopping frequency is modulated at twice the output frequency. For a three-phase PWM inverter feeding a three-wire load, the chopping frequency modulation is not smooth. This is explained to be a consequence of the fact that the three phase currents must sum to zero, rendering one of the three current controllers redundant.