Analysis of the Dynamic Characteristics of a Variable Speed Compressor

A dynamic simulation model of a rolling piston type rotary compressor is developed to predict the dynamic characteristics of a variable speed compressor. The model is based on the principles of conservation, real gas equations, kinematics of the crankshaft and roller, mass flow loss due to leakage, and heat transfer. For the computer simulation of the compressor, the experimental data were first obtained with the motor performance tests at various operating frequencies. Using the model developed, the compression performance, friction loss, mass flow loss and heat transfer loss etc. were estimated as a function of the crankshaft speed in a variable speed compressor. Since the transient state of the compressor strongly depends on the system, the transient model developed for a variable speed compressor is combined with a transient system simulation program to get transient variations of the compression process in the system. INTRODUTION The variable speed compressor is designed to change the motor speed compared to a constant speed compressor. The efficiency of a constant speed motor is dependent on the load of the motor. However, the efficiency of variable speed motor is changed with not only the load of the motor but also the driving frequency. Therefore, the efficiency of the compressor with a variable speed motor in the compression and suction process will be continuously changed with driving frequency and load. This study is concerned with developing a dynamic program to predict the effect of the frequency on compressor performance, fiiction loss, mass flow loss, and heat transfer loss etc. Several computational models for the compressor are based on the polytropic process. In this study, the model is developed with conservation equations and a detailed simulation model of heat transfer, friction and mass losses. The present study characterizes like as follows. I) The model in this study is a dynamic analysis model based on the principles of the mass and energy conservation equations. Therefore, the variations of temperature and pressure of the refiigerant can be calculated with the rotation of the crankshaft. 2) The refiigerant is considered as a real gas rather than an ideal gas. 3) All of the efficiencies of the compressor are calculated or measured. There is no assumption about the efficiencies. 4) With analysis of the dynamic equation for the moving part, it is possible to calculate accurate friction loss although there is angular velocity variation in the crankshaft and roller. 5) This model considered variation of the heat transfer coefficient and area with rotation of crankshaft. 6) The variable motor efficiency was measured experimentally. Therefore, the error of the motor efficiency is reduced and it is possible to predict the motor efficiency with the variation of driving frequency. MATHEMATICAL MODELLING The rolling piston type rotary compressor in this model is widely used for controlling the speed of the compressor due to the simple mechanism. The calculations are performed by increasing the angle of the crankshaft with small time intervals. The refiigerant temperature, pressure, heat transfer, and mass loss can be calculated by analyses of the compression and suction process in the cylinder at the same time. The mechanical loss, indicated work, volumetric efficiency etc. also can be calculated at the end of the one cycle. In order to more accurately calculate the losses in the compression process, dynamic analysis of the compressor is necessary. In this model, the dynamic equations of the mechanical part ofthe compressor are used to predict accurate fiiction loss with calculated velocity of the moving parts.