Investigation of the Effect of Inverters on the Power Supply and the Performance of Variable Speed Refrigeration Systems

This paper reports the results of an experimental investigation into the characteristics and effects of a PWM inverter on the power supply and overall performance of a capacity controlled refrigeration system employing a positive displacement compressor. The results indicate that the efficiency of variable speed refrigeration systems at part load is severely affected by the poor performance of induction motors. The efficiency of the inverter was found to be high, approximately 95%, and to remain approximately constant over the speed and load range tested. The harmonic analysis showed that the total current harmonic distortion on the supply side increased with a decrease in speed and load. This can cause power quality problems and can contribute to a reduction in the power factor, resulting in high circulating current and lower efficiency. It has been found that the net efficiency gains from variable speed operation at part load conditions are not significant enough to outweigh the higher capital cost of the system and overcome the concerns about the effects of the inverter on the supply and the motor. The attractiveness of positive displacement variable speed refrigeration systems will increase considerably if motors are developed whose efficiency remains fairly constant at part load conditions. Nomenclature AHD VHD THD IDC MDC PWM rms OF "" Arms Vrms Vf Af Current Harmonic Distortion, % Voltage Harmonic Distortion, % Total Harmonic Distortion, % Inverter Driven Compressor Mains Driven Compressor Pulse Width Modulation Root Mean Square Operating Frequency, Hz rms Current, amps rms Voltage, volts Fundamental Voltage, volts Fundamental Current, volts INTRODUCTION Traditionally, the capacity of single compressor refrigeration and air conditioning systems was matched to the load by one of a number of control methods such as on/off control, cylinder unloading and hot-gas by pass. The main disadvantage of these methods is that reduction in capacity is not accompanied by a corresponding reduction in the power consumption of the compressor. Capacity variation through the control of the compressor motor speed offers the potential for significant energy savings compared to the other methods of capacity control but its application so far to commercial refrigeration systems has produced only a variable degree of success. The main problems associated with inverter based variable speed capacity control include: • injection of voltage harmonics to the motor which tend to heat up the motor windings and reduce its efficiency