Small hermetic reciprocating compressors driven by linear oscillating motors have been studied through the years owing to their potential for cost reduction and efficiency enhancement. Their basic structure is very simple in comparison to that of conventional reciprocating compressors, that is, pist...
Small hermetic reciprocating compressors driven by linear oscillating motors have been studied through the years owing to their potential for cost reduction and efficiency enhancement. Their basic structure is very simple in comparison to that of conventional reciprocating compressors, that is, piston is directly connected to the oscillating part of the linear oscillating motor in cylinder. It is to be noted, however, that motion of the piston is not kinematically restricted as in conventional reciprocating compressors that employ rotary motors and motion changing mechanisms such as Scotch-Yoke. Inherently, therefore, stroke of the piston and center position of the piston movement can change sensitively with supply voltage and load. In other words, dynamics of the piston is linked to the electrodynamics and thermodynamics in a complicated manner. The major difficulty in the modeling and analysis of oscillating compressors lies in the characterization of the thermodynamic part, i.e. representation of a gas force acting on the piston. The gas force is inherently a nonlinear function of the top clearance and stroke of the piston. In this thesis, which is concerning a single-cylinder electrodynamic oscillating compressor only, the difficulty has been circumvented by applying the describing function approach to a gas compression cycle and by representing the gas force effects as an equivalent spring and a hysteretic damper. First, for a predefined compression cycle, 4 different methods have been applied to represent the gas force effects as equivalent parameters; (1)'to use the slope of two extreme points in the gas force-piston displacement diagram', (2) 'to apply piecewise equivalency of the potential energy', (3) 'to apply the describing function approach to the compression cycle only', and (4) 'to apply the describing function approach to a single degree of freedom system'. To verify the usefulness of the equivalent modeling in design, system parameters of the compressor which has the capacity of a heat lift of 150 Watts from -23°C to 32°C have been determined using the equivalent model. The frequency response characteristics of the 4 different equivalent models and full nonlinear model of the designed compressor have been compared. Next, a generalized equivalent model has been developed for an undefined compression cycle, where the top clearance and stroke are not predefined, by extending the method of 'to apply the describing function approach to the compression cycle only' and employing non-dimensional piston amplitude. Using this model, the supply voltage and coil current have been derived in the function of the non-dimensional piston amplitude. The supply voltage response characteristics of the designed compressor has been analyzed using this equivalent model and compared with those of the full nonlinear model that were obtained by employing the 4th order Runge-Kutta method. Jump phenomena were observed in the analysis process, which are abrupt increases in the piston amplitude in spite of the smooth increases of supply voltage under certain conditions. The hardening tendency and softening tendency of gas spring effect cause these jump phenomena. The necessary conditions for such jumps to occur have been derived and the effects of the moving mass and tuning springs on the jumps have been discussed. Finally, the voltage response characteristics of 4 different compressors in piston mass were obtained by experiment and they have been compared with those of the proposed generalized equivalent models. The effect of piston mass and pressure ratio have been analyzed experimentally and compared with those analyzed using the generalized equivalent models.
Small hermetic reciprocating compressors driven by linear oscillating motors have been studied through the years owing to their potential for cost reduction and efficiency enhancement. Their basic structure is very simple in comparison to that of conventional reciprocating compressors, that is, piston is directly connected to the oscillating part of the linear oscillating motor in cylinder. It is to be noted, however, that motion of the piston is not kinematically restricted as in conventional reciprocating compressors that employ rotary motors and motion changing mechanisms such as Scotch-Yoke. Inherently, therefore, stroke of the piston and center position of the piston movement can change sensitively with supply voltage and load. In other words, dynamics of the piston is linked to the electrodynamics and thermodynamics in a complicated manner. The major difficulty in the modeling and analysis of oscillating compressors lies in the characterization of the thermodynamic part, i.e. representation of a gas force acting on the piston. The gas force is inherently a nonlinear function of the top clearance and stroke of the piston. In this thesis, which is concerning a single-cylinder electrodynamic oscillating compressor only, the difficulty has been circumvented by applying the describing function approach to a gas compression cycle and by representing the gas force effects as an equivalent spring and a hysteretic damper. First, for a predefined compression cycle, 4 different methods have been applied to represent the gas force effects as equivalent parameters; (1)'to use the slope of two extreme points in the gas force-piston displacement diagram', (2) 'to apply piecewise equivalency of the potential energy', (3) 'to apply the describing function approach to the compression cycle only', and (4) 'to apply the describing function approach to a single degree of freedom system'. To verify the usefulness of the equivalent modeling in design, system parameters of the compressor which has the capacity of a heat lift of 150 Watts from -23°C to 32°C have been determined using the equivalent model. The frequency response characteristics of the 4 different equivalent models and full nonlinear model of the designed compressor have been compared. Next, a generalized equivalent model has been developed for an undefined compression cycle, where the top clearance and stroke are not predefined, by extending the method of 'to apply the describing function approach to the compression cycle only' and employing non-dimensional piston amplitude. Using this model, the supply voltage and coil current have been derived in the function of the non-dimensional piston amplitude. The supply voltage response characteristics of the designed compressor has been analyzed using this equivalent model and compared with those of the full nonlinear model that were obtained by employing the 4th order Runge-Kutta method. Jump phenomena were observed in the analysis process, which are abrupt increases in the piston amplitude in spite of the smooth increases of supply voltage under certain conditions. The hardening tendency and softening tendency of gas spring effect cause these jump phenomena. The necessary conditions for such jumps to occur have been derived and the effects of the moving mass and tuning springs on the jumps have been discussed. Finally, the voltage response characteristics of 4 different compressors in piston mass were obtained by experiment and they have been compared with those of the proposed generalized equivalent models. The effect of piston mass and pressure ratio have been analyzed experimentally and compared with those analyzed using the generalized equivalent models.
주제어
#Oscillating compressor Jump phenomena Describing function Nonlinear Dynamics 진동식 압축기 점프현상 묘사함수 비선형 동특성
※ AI-Helper는 부적절한 답변을 할 수 있습니다.