Resonance analysis of opposed piston linear compressor for refrigerator application

The opposed piston linear compressor is considered to be the future of silent, reliable and efficient compression technologies. For long valves, less linear compressors have been widely used in cryocoolers for space applications. Recently research has established its high-performance characteristics which are useful in household refrigeration application. Unlike reciprocating compressors which are driven by rotary motors and need a crank connecting rod mechanism to convert rotary into linear motion, a linear compressor is driven by a linear motor which reduces the number of moving parts. This feature makes the compressor more reliable, more efficient and has a silent operation. The piston (suspended on spring stiffness) in the linear compressor is a free piston, i.e. motion of the piston is not constrained hence the performance of the linear compressor is highly dependent on system resonance. Free piston motion offers the unique advantage of ease of modulation, i.e. the stroke of the compressor and hence the delivery mass flow rate coming out of the compressor can be easily controlled by changing the supply voltage. Resonance in the linear compressor is a function of moving mass, spring stiffness and operating or electrical frequency. Achieving resonance in the linear compressor results in maximising the cooling performance of the refrigerator and minimising the input power requirement and hence an increase in the COP of the refrigeration system. The present paper discusses the results from resonance testing of the opposed piston linear compressor for household refrigerator using the fast Fourier transformation analysis. There are different parameters that are considered to optimise the natural frequency of the opposed piston linear compressor, consisting of frequency, moving mass, and spring stiffness. The piston performs compression and suction in a similar manner as in the conventional reciprocating compressor. An opposed piston linear compressor exhibits high energy efficiency due to its simple construction and less moving parts, its mechanical losses are much less than the reciprocating compressor.