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 pulse width 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 3-phase PWM inverter feeding a 3-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.