| In the field of permanent magnet synchronous motor control,system reliability,life time and power density have become important issues.Electrolytic capacitors are the core component in traditional motor drives for the DC link voltage stabilization,but they have the disadvantages of large volume,short life time and the explosion risk.Compared with electrolytic capacitors,film capacitors have the advantage of longer life time.The electrolytic capacitorless motor drives with slim film capacitor have long life time and high power density,which are suitable for the electric drive application of fans,pumps and compressors.At present,the control technologies of electrolytic capacitorless motor drives are in the early stage and there are some technical problems existing in harmonic suppression and stable operation,which include LC resonance,beat phenomenon suppression,flux-weakening control and DC-link anti-overvoltage stable operation.This paper focuses on the above core issues to figure out the mechanisms and solutions,which provides the theoretical and technical support for the application of electrolytic capacitorless PMSM drives.As LC resonance is prone to occur due to the significant reduction of the capacitance in the electrolytic capacitorless motor drive,harmonics at the inherent frequency of the system will be amplified,thus aggravating the fluctuation of DC-link voltage,and reducing the power quality at the grid side,and even causing the system instability.By extracting the harmonic components of the inherent frequency of DC-link voltage around the resonant frequency,a harmonic suppression strategy based on impedance rebuilding at the inherent harmonic frequency is proposed.The proposed method only rebuilds the impedance of the inherent harmonics near the resonant frequency,which can effectively reduce the additional disturbance energy of the motor side.In addition,the non-ideal characteristic of the motor system is considered in the process of driving system modeling to further improve the accuracy of impedance matching and the effectiveness of damping control.The proposed resonant suppression method can effectively suppress the harmonics in DC-link voltage and the grid current,and the power quality of the grid side and stability have been significantly improved.Influenced by the DC-link voltage fluctuation and sampling delay,the beat frequency problem exists in the drive system.By analyzing the harmonic characteristics of the motor current,the mathematical expression of the beat frequency envelope is obtained,and the mechanism of beat phenomenon is revealed.In order to suppress the system beat phenomenon,it is necessary to regulate the harmonics at the frequency of DC-link voltage in motor current,and a suppression method based on DC-link voltage prediction is proposed.By extracting the six times frequency component of the DC-link voltage in the grid side and predicting the component according to the DC-link voltage periodicity,the accuracy of the sampling value of the DC-link voltage is improved.The method is simple to implement and does not require high periodicity of the DC-link voltage.Experimental results show that the method can suppress the beat phenomenon caused by the DC-link voltage fluctuation and sampling delay,and improve the current quality of the motor side.Due to the large periodic fluctuation of the DC-link voltage in the electrolytic capacitorless motor drive,the traditional flux-weakening control strategy has some problems such as low voltage utilization and large torque ripple.Therefore,a flux-weakening control strategy based on dynamic adjustment of maximum voltage boundary is proposed,which consists of two modes: minimum DC-link voltage control and extended DC-link voltage control.In the initial stage of flux-weakening,the valley of the DC-link voltage is used as the control boundary to reduce the torque ripple of the motor.When the motor current reaches the amplitude limit,if the drive output capacity needs to be further improved,the DC-link voltage fluctuation component is used for regulation.Experimental results show that the maximum voltage boundary can be smoothly adjusted,the stator voltage vector can be extended to the maximum voltage hexagonal inscribed circle in the extended DC-link voltage control mode,and the torque ripple can be reduced to 2% in the minimum DC-link voltage control mode.When the electrolytic capacitorless motor drive operates in the regenerative braking state such as deceleration or load reduction,the voltage fault is easy to emerge in the DC-link.In order to ensure that the drive is reliable and stable,an anti-overvoltage control strategy based on stator current vector orientation is proposed according to the theory of motor loss based on the analysis of the impact of capacitance on the braking performance.By restricting the stator voltage in the direction of the stator current,the energy flow can be controlled to achieve the goal of preventing the DC-link voltage rise.In order to further improve the dynamic performance of motor braking process,a dual anti-overvoltage control strategy based on parameters self-regulation is proposed.The controller parameters can be automatically selected according to motor operating conditions to maintain the preset amplitude-frequency characteristics of the control loop.The direct and quadrature currents can be used to adjust the DC-link voltage at the same time,which can effectively reduce the energy control error of the motor,improve the regulation ability of the DC-link voltage,and ensure the reliable regenerative braking of the motor.Finally,the system integration of control strategies is carried out,and the above digital control algorithms are realized on the platform of the electrolytic capacitorless motor drive.The test of the grid side power quality,the test of motor side performance,the experiment of speed regulation and the experiment of regenerative braking are carried out,which lay a theoretical foundation for the further popularization and application of the electrolytic capacitorless motor drive. |
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