A novel linear electromagnetic-drive oil-free refrigeration compressor
Keywords: electromagnetic
linear compressor
oil-free compressor

There is a need to improve the capabilities of cooling technology to dissipate increasingly high heat fluxes (approaching 200 W/cm2) from electronic components in order to maintain acceptable operating temperatures. The linear compressor with clearance seal and flexure spring which has been used for many years for space application, is an attractive proposition for vapour compression refrigeration (VCR) systems in electronics cooling applications because it offers several advantages over traditional compressor technology. This thesis introduces a novel design of oil-free linear compressor and investigates the performance of the linear compressor system and the key issues with respect to the operations of the compressor.
A comprehensive system analysis has been carried out for the linear compressor regarding the moving magnet motor characteristics, spring stiffness, damping, imbalance of current in coils, DC offset and leakage across the clearance seal. To measure the performance of the linear compressor, an experimental apparatus and a data acquistion system have been built. A harmonic fitting approach has been proposed to reconstruct current, voltage and displacement signals using fast Fourier transform (FFT) and minimisation algorithms. The piston dynamics have been modelled to infer pressure-volume (P-V) diagrams.
Preliminary measurements of the linear compressor at resonance using nitrogen show that an adiabatic efficiency of 42% - 60% and a motor efficiency of 71% - 89% are maintained over a wide range of operating parameters, indicating that systems using this type of compressor could have good part load efficiency. At the design point (a pressure ratio of 3.0 and a stroke of 13 mm), pressure drop loss is responsible for 27% of the total losses and the most significant loss is the copper loss (34%). A commercial crank-drive compressor with comparable properties has also been evaluated for comparison, showing that the prototype linear motor has a much higher efficiency than the rotary induction motor, particularly at low electrical power inputs while the volumetric efficiencies in the crank-drive compressor are about 20% higher.
The linear compressor in a refrigeration system using R134a has been measured 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. A model of the resonant frequency using discharge and suction pressures and the stroke has been validated. A coefficient of performance (COP) of 3.2 has been achieved at a cooling capacity of 384 W and an evaporator temperature of 20°C. This is assumed to be a typical CPU cooling condition.
A Proportional-Integral-Derivative (PID)/Pulse-Width-Modulation (PWM) control system using a solenoid operated valve has been developed for DC offset and clearance control. Operation of the linear compressor with a fixed clearance of 0.8 mm using nitrogen has been carried out in comparison with the fixed zero DC offset operation. When operated with a fixed clearance, the volumetric efficiency increases with a decreasing stroke but the fixed clearance operation requires a higher power input. This provides evidence that the piston control in a refrigeration system with capacity control should be for a zero DC offset.