A Study on PWM Bypass Capacity Control of Scroll Compressors

A pulse width modulation (PWM) bypass capacity control technique for air conditioners has been developed to achieve high scroll compressor efficiency over a wide capacity range, and we investigated the technique’s fundamental characteristics in this study. With this capacity control technique, the refrigerant flow rate is controlled by periodically switching a full load mode, in which the refrigerant is compressed and discharged out of the compressor, and an unload mode, in which the refrigerant is not compressed so that the power needed to compress is unnecessary. In this study, we measured the pressure and input power of a compressor using the control technique to clarify the technique’s dynamic characteristics. We also developed a numerical simulation model to predict the dynamic behavior of the refrigerant. The calculated pressures were found to be in good agreement with the measured pressures and were used to estimate the relation between the PWM period and the efficiency or the discharge flow rate. Since the PWM period could not be determined by the demanded load capacity, the estimations were used to determine it. Using this period, we carried out performance tests of a compressor with PWM bypass capacity control. The results showed the capacity reached 30% of the lower limit of rotational speed and that the calculated efficiencies agreed well with the experimental findings.