Research On Control Strategy Of Electrolytic Capacitor-less Compressor Drives For Air-conditioner

In traditional variable frequency air-conditioner system,large volume of electrolytic capacitors are utilized at the dc-link to decouple the energy and stabilize the dc-link voltage,which results the grid input current distortion and difficult to meet the EN61000-3-2 harmonic emission standard.Moreover,electrolytic capacitor has many shortcomings,such as high cost,short lifetime and high failure rate.This thesis takes electrolytic capacitor-less PMSM drive for air-conditioner as research object,focus on the control strategy,aims at obtain a high grid input power factor and meet the EN61000-3-2 harmonic emission standard.In this thesis,mathematic model of the drive system without electrolytic capacitor is established.Influence on the diode-rectifier circuit and system impedance due to the small capacity film capacitor are analyzed.The small capacity film capacitor at the dc-link can not keep the voltage constant,and the effects of the fluctuated dc-link voltage on the grid input power factor and the speed operation range are analyzed.Due to the remarkable decrease of dc-link capacitor,the gird input power and the inverter output power are coupled together,therefore,the key of the electrolytic capacitor-less drive system is to control the inverter output power,and inverter output power control strategy based on high power factor control is studied.A motor torque loop is cascaded between the speed loop and current loop,and the motor torque controller is utilized to allocate the motor current reference and instructions for design the gain of the controller is given.Considering the negative impedance caused by the fluctuated dc-link voltage will affect the stability of the drive system and the LC resonance induced by the dc-link capacitor and the grid side inductance will increase harmonic current.An active damping method is utilized to offset the effect of negative impedance,reduce the harmonic current caused by LC resonance and improve the stability of drive system by adding virtual resistance on the system.To verify the control strategy,experiments on a 1.5P electrolytic capacitor-less PMSM drive for air-conditioner are conducted.The experimental results show that the studied control strategy can improve the grid input power factor,suppress the LC resonance and meet the EN61000-3-2 harmonic emission standard.