A low cost high performance SPWM inverter

Sine wave pulse width modulation is most widely used in three-phase motor drives, UPS systems, static VAr compensators, etc. Variable frequency sine wave outputs with high stability can be achieved with digital circuits employing EPROMs and DACs. The conventional schemes normally use three EPROMs and three DACs along-with a triangular waveform generator. For better performance, the sine wave should be synchronized with the triangular wave. The performance needs restrict the selection of the frequency ratio. Two schemes are presented in this paper which use only one EPROM to generate the necessary signals for the SPWM. The schemes generate carrier and reference signals using the same EPROM maintaining exact synchronization, keeping the frequency ratio as an odd multiple of three for the entire speed range and ensure that the modulating signal and carrier signal have opposite slopes at coincident zero crossing. The application of these schemes reduces the cost and space for the generation of PWM signals and thus help the growing FHP drive market where cost is of paramount importance to induce a change from fixed speed to variable speed operation.