Research On Control Strategy For Electrolytic Capacitor-less Permanent Magnet Compressor Drive System

In recent years,the electrolytic capacitor lifetime has become one of the important factors for the permanent magnet compressor drive system failure in household air conditioning.Developing the key technology of "electrolytic capacitor-less drive system",replacing electrolytic capacitor with small film capacitor can effectively extend the lifetime of capacitor,and improve the drive system reliability.However,the use of thin film capacitors not only causes the periodic change of dc-link voltage,which may lead to the failure of the traditional power factor control method;but also cause inductance capacitance resonance and motor speed periodic fluctuation,which makes it difficult to achieve sensorless control and affect the drive system stability.In view of the above problems,the main investigations of this dissertation are as follows:As the dc-link capacitance decreases and the traditional power factor control fails,a power factor control method based on torque closed-loop was proposed.Firstly,the relationship between power factor,DC-link capacitance and minimum voltage is analyzed.Based on studying inverter torque characteristics,a linear inverter-motor torque model is established.By designing the torque control loop,torque regulator and setting control parameters,the q-axis current is generated to produce the sinusoidal motor torque.An average voltage constraint strategy is used for flux weakening control to generate the d-axis current.Therefore,by controlling the d-axis and q-axis currents to generate sinusoidal grid current,the high power factor control at the grid side is realized.As the limitation of torque control bandwidth may affect system power factor,a voltage vector correction strategy is adopted to realize the motor torque feedforward control,which enhances the torque control response and further improve the system grid power factor.A voltage feedback resonance suppressing method based on damping current correction was proposed.On the basis of discussing the resonance mechanism at grid side in electrolytic capacitor-less drive system,the damping feedback variables and damping control modes are analyzed,it is concluded that the current proportional feedback and voltage proportional feedback active damping control meet the resonance suppression requirements.By comparing current reconstruction and current sampling damping control,it is found that the current problems exist in DC-link voltage feedback damping method.On this basis,the damping current polarity is changed according to the inverter current direction,then,the voltage feedback damping control is modified to make the grid current more smooth when crossing zero.At the same time,the damping current over-sampling method is used to improve the damping control accuracy.Without adding any external circuit,the proposed method not only effectively suppressed the resonance at grid side,but also improved the system power factor.Considering the influence of the periodic DC-link voltage on motor speed and position estimation,a sensorless control method based on Iterative Sliding Mode Observer(ISMO)is proposed.Firstly,the characteristics of rotor angle and speed identification errors of the electrolytic capacitor-less drive system were analyzed,and a full-order sliding mode observer model based on the permanent magnet synchronous motor extended back electromotive force(EEMF)was established.On this basis,considering the delay time of EEMF and position estimation,the proposed ISMO observer is adopted to expand the observer control bandwidth,which reduces the tracking error of position estimation.In order to solve the problems of phase lag and amplitude attenuation of speed estimation,a digital phase-locked loop based on Proportional Integral resonance(PIR)regulator is adopted to enhance tracking performance of speed estimation,which further improves the position estimation accuracy.Finally,taking the permanent magnet compressor of air conditioner as the typical application object,an electrolytic capacitor-less permanent magnet compressor drive system based on TMS320F28075 is built.The above three control methods were integrated into the designed platform to verify the whole machine.The experimental results show that the drive system can adjust the speed smoothly within the range of high energy efficiency ratio(EER),the power factor is up to 0.987,and the grid current can meet the IEC61000-3-2 standard,which verifies the effectiveness of the proposed control method.