Research On The Control Technology Of Piston Balance Position Adjustment For Moving Coil Linear Compressor

In the working process,the piston of the linear compressor under the influence of gas force will appear piston offset phenomenon,if the piston offset phenomenon is not adjusted,it will cause a series of chain of malignant reaction.For example,it will make the piston clearance too large for the compressor to achieve effective compression,so it is very urgent for the piston offset problem to be solved.In addition,the resonant frequency and piston stroke are mutually influenced by each other,and the resonant frequency is a changing quantity during operation.To ensure the continuous and efficient operation of the linear compressor,the driving frequency of the linear compressor needs to be adjusted in time.Therefore,this paper conducts theoretical and experimental research on multi-loop moving coil linear compressor for the piston offset phenomenon and resonant frequency variation problem,and the main research contents are as follows:(1)Establish a multi-parameter coupled piston offset prediction model.The mutual relationship between the parameters is established by building a sub-model of the compressor operating process.The electromagnetic force of multi-loop permanent magnets is calculated by the analytical method,and finite element simulations are performed.The results show that the electromagnetic force is greatest when the coil is in the equilibrium position,and it tends to decrease symmetrically as the coil deviates from the equilibrium position.The effect of piston diameter on the offset is studied and quantitatively analyzed.When the piston diameter is increased from 18 mm to 34 mm,the offset is increased by 5 times.In addition,the presence of the offset is found to cause a hysteresis in the current,with the steady state current lagging about 5°behind the voltage.(2)For the piston offset problem,a technique of adding a DC excitation to the AC excitation is proposed to adjust it.To study the effect of DC excitation on the operating parameters of the linear compressor and to analyze the effect of frequency conversion mode,step size and step frequency on the system.The results show that the increase in DC excitation leads to an increase in power loss.At no load the piston stroke is almost unaffected by the DC excitation,while at load the piston stroke has a certain degree of reduction,and the pressure ratio can be increased by up to 27.08%.When the DC excitation accounts for 22.9% of the total voltage,the volumetric efficiency drops abruptly by 16.2%.In the process of frequency conversion,the initial stage should give priority to small step length and high step frequency,when the stroke reaches the set value,change to the small step length and medium step frequency continuous adjustment method,the piston stroke changes more smoothly and can avoid the occurrence of cylinder collision.(3)The piston offset adjustment by asymmetric waveform excitation method is proposed for the first time.This method can target regional adjustment for the direction of piston offset,but low exhaust pressure and high amplitude ratio should be avoided during the adjustment.An offset inhibition model is developed,and the method is validated by experiments.The results show that the power fluctuation remains within 2.1% as the amplitude ratio of the asymmetric waveform increases from 1 to 1.4 with a constant total stroke of the piston,which is a significant advantage compared to other techniques.The application of asymmetric waveform excitation achieves piston offset adjustment without any significant impact on COP.The maximum value of COP 1.62 is still reached at the resonant frequency state.(4)For the problem of resonant frequency change after linear compressor adjustment,an adaptive resonant frequency tracking technique is proposed to track the resonant frequency of the system by the frequency perturbation and the compressor output power value.A control system simulation model is established to simulate and analyze the resonant frequency tracking under no-load and load.The results show that the resonant frequency tracking algorithm can achieve the resonant frequency tracking within 0.86 s.At the moment of load change,the frequency tracking tends to be disordered,with large fluctuations and sudden frequency changes.When using resonant frequency tracking adjustment,attention should be paid to the overload of system current to avoid damage to the excitation coil.(5)Design a multi-loop moving coil linear compressor,and build an electromagnetic structure testing test bench,a dynamic performance testing test bench,and a refrigeration performance testing test bench.The intensity relationship between the magnetic field strength and the axial position is given and the developed mathematical model is verified by experiments.The power loss of piston offset under different adjustment methods is compared and analyzed,and it is obtained that the loss of both adjustment methods is similar when the offset is less than 0.5 mm.At an offset of 2.5 mm,the loss generated by the DC excitation reaches 3.6 times that of the asymmetric waveform excitation.The performance of the developed prototype is also tested,and the performance comparison analysis with the conventional compressor(DF91Y)shows that the efficiency of the prototype is 11.4% higher than that of the conventional compressor when the working conditions are similar and verified the superiority of linear compressor performance.This paper has 121 figures,10 tables,and 141 references.