Moving Magnet Linear Compressor Research Articles

Moving magnet linear compressor: Operating characteristics under resonance and off-resonance frequencies

ObjectivesThis study compared the accuracy of a Linear Equivalent model and a Fourier Transform model in approximating the resonant frequency of a moving magnet oil-free linear compressor. Furthermore, moving magnet linear compressor performance under resonance and off-resonance frequencies was also examined via an experimental approach. MethodsA Linear Equivalent model and a Fourier Transform model were developed and compared with experimental results at low-pressure ratios of 2–2.5. By varying the operating frequency and compressor piston stroke, the power consumption, compressor losses, efficiency, and cooling capacity are assessed experimentally. ResultsThis study showed that the disparities between resonance frequencies estimated by theoretical models and experimental data were below 10 %. However, the Linear Equivalent model was more accurate than the Fourier Transform model in forecasting the resonance frequency of the linear compressor at a low-pressure ratio of 2–2.5. Both experimental and modelling results showed that the resonance frequency of a linear compressor declined with the increasing compressor stroke but increased with increasing pressure ratio. Experiments were also carried out to compare the performance of linear compressors in resonance and off-resonance frequencies. Results showed that the lowest compressor input power of 91.96 W and the highest motor efficiency of 81.98 % was achieved when the linear compressor was operated at 38 Hz resonance frequency. Moreover, the cooling capacity has been found to increase by 270 W approximately when the linear compressor piston operating stoke extends from 10 mm to 13 mm. ConclusionsIn all, this study showed that the linear compressor motor efficiency is the highest when operating at resonance frequency. However, the cooling capacity of the linear compressor system does not vary significantly with operating frequency. A higher cooling capacity can be achieved by increasing compressor piston stroke.

Static and dynamic characteristics of a novel moving magnet linear compressor

Static and dynamic characteristics of a novel moving magnet linear compressor

A numerical model of a linear compressor for household refrigerator

Linear compressor is more efficient than conventional reciprocating compressor due to the absence of a crank mechanism. It can also realize capacity modulation, compactness and oil-free operation. A comprehensive numerical model of a linear compressor consisting of a thermodynamic model describing the in-cylinder pressure and temperature, a mechanical dynamics model describing movement the piston and valves and an electrical model describing the interaction of the voltage, current and back electromotive force of the linear motor was established to predict the compressor performance and to help achieve inherent capacity modulation. Experiments were conducted to validate the model using a prototype moving magnet linear compressor and remarkable accuracy is obtained. The measured and predicted piston displacement, current and shaft force agree well with the maximum error less than 10%. The predicted power factor, power consumption, stroke, mass flow rate and CoP show mean absolute percentage errors of 0.8%, 13.3%, 3.9%, 9.2% and 9.0%, respectively. As predicted, the resonant frequency can lead to lowest power consumption as well as the highest power factor and motor efficiency. At pressure ratio of 3.0, when the piston stroke increases by 1 mm, the mass flow rate increases by 0.66 g/s while the cooling capacity increases by 77.2 W. This can indicate the possibility of inherent capacity modulation.

Development of a spaceborne pulse tube cooler operating at 170K

The Infrared Focal Plane Array (IRFPA) detector needs quite low dark noise for image detection. It can be cooled to demanding low temperature by pulse tube cooler (PTC) because of low vibration and low electromagnetic interference (EMI) at cold end. A high-capacity pulse tube cooler driven by a moving-magnet linear compressor is presented in this paper. The regenerator and pulse tube are arranged in coaxial. The inertance tube and reservoir are used as passive phase shifter of the PTC. A numerical thermodynamic model is established to design and optimize overall performance of the pulse tube cold finger aimed for the best efficiency. Based on the principle of electric-mechanical-acoustic coupling field, a transient co-simulation of the PTC is proposed. The mass of the PTC is less than 12 kg without electronic controller. The oscillating linear compressor has a pair of opposite pistons to eliminate vibration and the input electric input power is 400 W at maximum. A typical cooling performance of 50 W at 170 K has been achieved with 228 W input power at reject temperature of 293 K, provided by water cooling. The specified Carnot efficiency is 15.8%. Additionally, overall cooling performances of the PTC at 150K-200 K are investigated by experiment, Not only could this PTC be used for space mission but also a promising alternative to the domestic low temperature applications.

Development of a new moving magnet linear compressor. Part A: Design and modeling

Abstract This article provides a detailed design and a CAD model of a new moving magnet linear compressor for the household refrigerator. A comprehensive electromechanical analytical model is provided to quantify the dynamic parameters necessary for the estimation of a compressor's performance. Additionally, the frequency response functions of the prototype at different input power conditions are investigated in order to evaluate the effect of excitation frequency on the dynamic parameters. The developed compressor is rigorously evaluated on a test-rig to validate all the presented analytical models. This article also provides a methodology to calculate the actuator's back-emf and motor constant which performs a vital role in the analytical model developed for any linear compressor. Furthermore, performance of the proposed compressor is evaluated with R600a to validate the pressure–volume curves and frequency response functions.

Development of a new moving magnet linear compressor. Part B: Performance analysis

Abstract This article is a continuation of the preceding article and focuses on the performance evaluation of an oil-free linear compressor prototype. The article introduces performance analysis parameters for a linear compressor along with their analytical attributes. Initially, the kinetic and kinematic analysis at resonance excitation frequency is performed with the help of a specially designed experimental setup. Furthermore, the frequency response functions of output to input parameters is presented to particularly monitor the shift in resonance frequency in the presence of refrigerant. Four different gases are used to quantify the shift and relating them with the physical properties of the refrigerant. Additionally, the pressure-volume curves for all these gases are presented and analysed. In the end, the motor and overall isentropic efficiencies of the prototype are measured at different input parameters. Additionally, the connection between the pressure output and volumetric efficiency is also discussed.

Design of a Two-stage High-capacity Stirling Cryocooler Operating below 30K

The high capacity cryocooler working below 30K can find many applications such as superconducting motors, superconducting cables and cryopump. Compared to the GM cryocooler, the Stirling cryocooler can achieve higher efficiency and more compact structure. Because of these obvious advantages, we have designed a two stage free piston Stirling cryocooler system, which is driven by a moving magnet linear compressor with an operating frequency of 40Hz and a maximum 5kW input electric power. The first stage of the cryocooler is designed to operate in the liquid nitrogen temperature and output a cooling power of 100W. And the second stage is expected to simultaneously provide a cooling power of 50W below the temperature of 30K. In order to achieve the best system efficiency, a numerical model based on the thermoacoustic model was developed to optimize the system operating and structure parameters.

Modelling and measurement of a moving magnet linear compressor performance

A novel moving magnet linear compressor with clearance seals and flexure bearings has been designed and constructed. It is suitable for a refrigeration system with a compact heat exchanger, such as would be needed for CPU cooling. The performance of the compressor has been experimentally evaluated with nitrogen and a mathematical model has been developed to evaluate the performance of the linear compressor. The results from the compressor model and the measurements have been compared in terms of cylinder pressure, the ‘P–V’ loop, stroke, mass flow rate and shaft power. The cylinder pressure was not measured directly but was derived from the compressor dynamics and the motor magnetic force characteristics. The comparisons indicate that the compressor model is well validated and can be used to study the performance of this type of compressor, to help with design optimization and the identification of key parameters affecting the system transients. The electrical and thermodynamic losses were also investigated, particularly for the design point (stroke of 13 mm and pressure ratio of 3.0), since a full understanding of these can lead to an increase in compressor efficiency.

A novel linear electromagnetic-drive oil-free refrigeration compressor using R134a

A new type of oil-free moving magnet linear compressor with clearance seals and flexure springs has been designed for incorporation into a vapour compression refrigeration system with compact heat exchangers for applications such as electronics cooling. A linear compressor prototype was built with a maximum stroke of 14 mm and a piston diameter of 19 mm. An experimental apparatus was built to measure the compressor efficiencies and coefficient of performance (COP) of a refrigeration system with the linear compressor, using R134a. The resonant frequency for each operating condition was predicted using the discharge pressure, suction pressure and stroke. Refrigeration measurements were conducted for different strokes under each pressure ratio with a fixed condenser outlet temperature of 50 °C and evaporator temperature ranging from 6 °C to 27 °C. The results show that the COPs are around 3.0 for tests with a pressure ratio of 2.5 (evaporator temperature of 20 °C).

Self-Sencing Control Research on a Linear Compressor

Compared to conventional compressors, the linear compressor with a free piston can improve efficiency and save energy, but the displacement varies due to different load conditions, which seriously influences its performance. In this paper, based on the theoretical analysis, a self-sensing control system with a SPWM inverter and a PC software interface for a moving-magnet linear compressor is developed, and the displacement of the linear compressor which is under control is tested. The result shows the error of the control system is under 0.2mm, and the accuracy fulfils the requirements.

Analysis of resonant frequency of moving magnet linear compressor of stirling cryocooler

This paper analyzes resonant frequency of the moving magnet linear compressor of Stirling cryocooler. The CFD (Computational Fluid Dynamics) and FEM (Finite Element Method) are used for the analysis of resonant frequency with FLUENT 6.2 and ANSYS 11.0 and an experiment is designed for testing the resonant frequency of moving magnet linear compressor. Results from simulations and experiments showed that the resonant frequency of the moving magnet linear compressors is affected by the machine spring, the gas spring, the magnet spring, and the mass of moving assembly, while the resonant frequency of the moving coil linear compressors is only affected by the machine spring, the gas spring, and the mass of moving assembly.

Study on static and dynamic characteristics of moving magnet linear compressors

With the development of high-strength NdFeB magnetic material, moving magnet linear compressors have been gradually introduced in the fields of refrigeration and cryogenic engineering, especially in Stirling and pulse tube cryocoolers. This paper presents simulation and experimental investigations on the static and dynamic characteristics of a moving magnet linear motor and a moving magnet linear compressor. Both equivalent magnetic circuits and finite element approaches have been used to model the moving magnet linear motor. Subsequently, the force and equilibrium characteristics of the linear motor have been predicted and verified by detailed static experimental analyses. In combination with a harmonic analysis, experimental investigations were conducted on a prototype of a moving magnet linear compressor. A voltage–stroke relationship, the effect of charging pressure on the performance and dynamic frequency response characteristics are investigated. Finally, the method to identify optimal points of the linear compressor has been described, which is indispensable to the design and operation of moving magnet linear compressors.