Research On High Power Factor Variable Frequency Drive System Without Electrolytic Capacitor

Permanent magnet synchronous motors are widely used in civil appliances and other fields due to their small size and simple structure.However,the traditional variable frequency drive system using large-capacity electrolytic capacitors in the DC link will bring about problems such as increasing the system volume,reducing the life span,and reducing the power factor on the grid side.The capacitor-less variable frequency drive system using small-capacity film capacitors can significantly improve the grid-side power factor while reducing the system volume and cost.In this paper,the problem of power coupling and DC bus voltage overshoot of the electrolytic capacitor variable frequency drive system is studied,and reasonable and effective solutions are proposed.In this paper,the permanent magnet synchronous motor mathematical model and the influence of the capacitance of the DC bus on the variable frequency drive system are revealed.The simulation experiment proves that the capacitor-less capacitor variable frequency drive system is beneficial to improve the grid-side power factor.By analyzing the working principle of the electrolytic capacitor-free variable frequency drive based on the traditional double closed-loop control,the traditional problems of severe power coupling between the input side and the load side and the low dynamic reliability of the system when the motor speed changes are studied.Aiming at the serious problem of power coupling between the input side and the motor side of the system,a high power factor control strategy on the grid side is proposed.By analyzing the power requirements of the input side and the motor side of the system,the feedback value of the speed loop is changed to the average value within half a cycle of the input voltage to eliminate the influence of motor speed fluctuations,and the input current loop is added to the q-axis current control which realizes the control of the input side current.In order to suppress the influence of the stator current distortion on the input current when the system enters the overmodulation state,the field weakening control is added on the basis of the d-axis current control to ensure the smooth operation of the motor.The effectiveness of the proposed control strategy is verified by simulation.Aiming at the problem that the energy on the motor side enters the DC bus when the motor is changing speed,causing the bus voltage overshoot,a DC bus voltage overshoot suppression strategy is proposed.By analyzing the stability of the DC bus voltage,the phenomenon of the DC bus voltage impact when the motor speed changes is studied.By using the PI controller based on BP neural network in the speed loop,the system changes the controller parameters according to the given speed and feedback value,reduces the system’s sensitivity to speed changes,t hereby suppressing the overshoot of the bus voltage,which is verified by simulation The effectiveness of the proposed control strategy.On the basis of theoretical analysis and simulation analysis,an experimental platform is built for further verification.The experimental results prove that the grid-side high power factor control strategy can effectively improve the grid-side power factor,and the DC bus voltage overshoot suppression strategy can effectively suppress the voltage overshoot and improve the reliability of the system.