The development and performance of a digital subharmonic modulator for AC drives

The subharmonic modulation process has been in use for more than twenty-five years with analogue circuitry and more recently using digital circuits which are not sensitive to offsets or drift to impair the fidelity of the output PWM pulse trains. The modulation process compares a triangular carrier waveform, quantised, with a similarly quantised modulating sine wave producing an output pulse when the two are equal, and repeating this to produce three output phases. The integrated PWM modulator, together with a microcontroller and a minimum of external components can provide a high quality motor current waveform. In simple form with linked voltage and frequency it is an economic solution to many industrial drives: with a control microprocessor it can also be used in flux vector controllers and complex closed loop systems