Research On Electrolytic Capacitor-Less Permanent Magnet Compressor Drives For Air-conditioner Applications

With the development of modern motor drive system,the lifetime and cost of AC motor drive system have become important issues.Aluminum electrolytic capacitors are widely used in air conditioning compressor drives because of the larger capacity and higher cost performance,whereas their lifetime are closely related to the environment temperature,which has become one of the main weakness of the drive system.The electrolytic capacitor-less drive system adopting the small film capacitor instead of DC-link electrolytic capacitor can significantly improve the operating lifetime of the system.At present,there are still some technical difficulties in the electrolytic capacitor-less permanent magnet compressor drive system,including the control of high input power factor,mitigation of LC resonance between the input inductance filter and the DC-link film capacitor,stabilization control method and beat phenomenon suppression of the drive system.The above techinques have been investigated in this dissertation.Effective solutions have been proposed,which benefits promoting the electrolytic capacitor-less drive system applications.In order to improve the input power quality of the air conditioning permanent magnet compressor drive system,the high power factor control can be realized by regulating the inverter power,which is based on the analysis of the grid input side and the inverter output power characteristics.As the DC-link capacitance value of the electrolytic capacitor-less drive system is significantly reduced,there is a strong coupling relationship between the energy of the grid input side and the inverter output side.The inverter power control loop based on proportional resonant(PR)controller is established to regulate the inverter power be a sinusoidal wave,and the theoretical design method of the parameters is given to make the PR controller achieve a high gain at the desired frequency,which could enhance the performance of the inverter power control.Further,the DC-link voltage closed-loop control method is adopted to optimize the generation of the motor voltage,which improves the inverter power control effect to meet the high power factor requirements for air conditioning applications.As for the LC resonance issue between the input inductance filter and the DClink film capacitor of the drive system,the negative effect on the system stability is analyzed.The system damping increases after applying the feedback control loop to mitigate the LC resonance.The drive system model is established in the frequency domain,and the feedback signals with different control laws are evaluated.The effect of the LC resonance mitigation,the complexity of the algorithm realization and the robustness to parameters variation are considered comprehensively,and the proportional DC-link voltage feedback based resonance mitigation method is adopted consequently.In addition,an active damping control method is proposed applying a virtual resistance in series with the input inductance,i.e.,the resonance mitigation method based on inductor current feedback.It has been proved theoretically that the inductor current feedback based active damping control method is equivalent to the specific DC-link voltage feedback based active damping control method.Experimental results verify the effectiveness of the proposed resonance mitigation method,which meets the harmonic standard of IEC-61000-3-2.The influence of motor parameters on the system stability is ignored in the conventional ananlysis when the motor is regarded as a constant power load.In this dissertation,the impedance characteristics model of the system is established,and the stabilization control method based on the impedance model is proposed.In the related researches,the inverter side is equivalent to the constant power load to analyze the drive system stability,ignoring the influence of motor parameters,control mode and control parameters.Hence the impedance characteristics model of the grid input side and the motor output is estabilished to analyze the drive system stability.The internal stability mechanism of the drive system and the harmonic characteristics of the grid side current are revealed from the impedance perspective.The drive system can be regarded as a cascade of the grid input side and the inverter output side.Then the stability condition of drive system can be obtained through the stability criterion of the cascade system.Further,a stabilization control method based on impedance model is proposed to improve the stability of the electrolytic capacitor-less air conditioning permanent magnet compressor drive system.Due to the energy coupling characteristics between the grid side and inverter side of the electrolytic capacitor-less drive system,the beat phenomenon occurs at specific operating frequencies.In order to suppress the low frequency oscillation of the drive system caused by the beat phenomenon,the control method to eliminate the harmonic components with the same frequency of the load torque is essential to be studied.The beat phenomenon leads to the increase of system loss,voltage and current stress,additional noise,and even mechanical resonance,which weakens the operating stability of the drive system consequently.By analyzing the effect of the fluctuated DC-link voltage and load torque on the drive system,the mechanism of beat phenomenon is revealed,and the low-frequency oscillation characteristics of grid current and stator current caused by beat phenomenon are analyzed further.Based on the above analysis,a novel beat phenomenon suppression method is proposed.It could avoid the interaction of different frequency signals caused by the coupling of the grid side and the inverter side,which is realized by eliminating the signal components with the same frequency of the fluctuated load torque.Experimental results show that the proposed method could effectively suppress the beat phenomenon of the drive system.Based on the above analysis,a prototype of the electrolytic capacitor-less compressor drive has been developed for the air-conditioner applications,and the algorithm of the proposed method is realized in the prototype.Experimental results are carried out to verify the effectiveness of the proposed method.