| Permanent magnet synchronous motor(PMSM)is widely used in industrial production and life due to the simple structure and high power density,especially in the field of variable frequency air conditioning.Variable frequency air conditioning has the characteristics of automatic regulation of room temperature,brings the good experience for users,so it has gradually become the mainstream product in the field of air conditioning.However,the traditional air-conditioning drive system has low system reliability because the electrolytic capacitor with large capacitance is used as the DC-link capacitor.Therefore,the electrolytic capacitor is replaced with a film capacitor with a small capacitance value,and the PFC circuit is removed to form the air conditioning drive system without electrolytic capacitor.Due to the small capacitance of the film capacitor used,there exists the fluctuation of DC-link voltage at twice the grid frequency.And the periodic change of load caused by the special structure of the air conditioning compressor can also cause the driving system unsteadily and the rotational speed pulsation at low speed.In addition,in order to save the cost of the air conditioning system,the current sensor adopts the method of single resistance sampling for current collection,so the sampling strategy also has higher requirements.Therefore,this paper mainly studies a series of problems in the motor drive system without electrolytic capacitor.Firstly,the working principle of PMSM is analyzed,and the mathematical model of the motor in the static coordinate system and the rotating coordinate system is obtained through different expression forms of voltage vector,and the dq axis mathematical model of permanent magnet synchronous motor is decoupled through coordinate transformation.On this basis,the motor double closed loop vector control model is analyzed,and different control strategies based on double closed loop control strategy are introduced.In order to eliminate speed pulsation problem under low-frequency running status,this paper proposes a speed ripple rejection based on iterative learning control strategy.First analyze the compressor frequency runtime and the reason of the torque ripple and then get the motor load torque observation algorithm through the analysis of the motor torque equation.Adopt the sliding mode observer to observation of load torque to calculate the expect q-axis current.Then make actual q axis of expected q axis current tracking through iterative learning control algorithm.A phase current reconstruction method based on the single resistance sampling is proposed in this section.Firstly,the advantages and principles of single resistance sampling current reconstruction are analyzed,and the sampling difficulties in low modulation ratio region and sector boundary region are analyzed.Finally,PWM phase shifting mode is proposed to process PWM wave,increase the sampling time so as to achieve the minimum sampling time and reconstruct the current.Study the difficulties of current reconstruction in the over-modulated region,and put forward the current observer reconstruction strategy to process the current in the over-modulated region so as to obtain relatively accurate reconstruction currentIn order to improve the stability of motor drive system without electrolytic capacitor,active damping control strategy based on virtual capacitor is studied.From the point of view of impedance,the small signal of the motor model is modeled and its characteristics are analyzed.On this basis,the methods of improving stability,passive damping method and active damping method are introduced,and the control system is stabilized by introducing active damping method of virtual capacitance,and the system after adjusting is analyzed from the Angle of impedance.Finally,the digital algorithm of the control method is implemented on the experimental platform of 1kW air conditioning without electrolytic capacitor,and the experimental results verify the effectiveness of the method. |
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