A novel resonant frequency tracking technique for linear compressors

Linear compressors are more efficient than conventional reciprocating com- pressors due to the absence of a crank mechanism. One of the main challenges for linear compressors is to keep operating at resonant frequency to maximise the efficiency. In this study, a novel resonant frequency tracking method that keeps the current and piston velocity in phase is proposed using a numerical linear compressor model with two Luenberger observers for the current and piston velocity, a zero-crossing detector for the phase difference and a PI (proportional-integral) controller updating the operating frequency. Experiments are conducted using R134a for model verification. The observers can give good prediction of the current and piston velocity while the phase detector and the PI controller are capable of adjusting the operating frequency to resonance with an uncertainty less than 1% compared to the experiments. Resonant operation can achieve the smallest current amplitude thus a lower power consumption.