Testing and modelling of a variable speed scroll compressor

Abstract

Variable speed compressors offer continuous control, low noise level, reduced vibration, low-start current, rapid temperature control, by operating the compressor at higher speeds initially, and better COPs than the conventional on/off control. However, there exist some drawbacks concerning the inverter efficiency, the effect of the inverter on the induction motor and the effect of variable speed on the compressor isentropic and volumetric efficiencies. This study gives some experimental results as to inverter and compressor performances: it can be observed that the inverter efficiency varies between 95% and 98% for compressor electrical power varying between 1.5 and 6.5 kW ; and that compressor efficiencies are not enormously influenced by compressor supply frequency, but depend mainly on compressor pressure ratio, except the tests developed at 35 Hz and one test at 40 Hz, for which the difference is attributed to the compressor internal leakages due to a lack of lubrication at low speeds. At 75 Hz there was also observed a slight degradation that can be attributed to the electromechanical losses that increase with compressor speed. A maximal isentropic efficiency of 0.65 for a pressure ratio of the order of 2.2 is obtained. The volumetric efficiency decreases linearly from 0.98 for a pressure ratio of 1.5 to 0.83 for a pressure ratio of 5.6. In spite of the test conditions (condensing and evaporating pressures up to 40 and 20 bar, respectively), the compressor performance stays unchanged. The experimental results obtained at 50 Hz are used to identify six parameters of a semi-empirical model which is then used to simulate the different tests developed at different compressor speeds. The simulated results are in very good agreement with those measured with averages errors of −0.5 K; +3 g s −1 and −24 W for the exhaust temperature, the refrigerant flow rate and the compressor electrical power, respectively. The results show that motor losses induced by the inverter are negligible.