| Dynamic wireless charging(DWC)technology realizes real-time energy replenishment during the driving process of electric vehicles(EVs)in a non-contact manner,and provids a fundamental solution to the bottleneck problems of EVs such as insufficient range,high battery costs and frequent parking-charging.As the core component for energy transmission in DWC system,the magnetic coupler(MC)is the critical determinant of the system’s transfer performance as well as its economy,safety and reliability.Longitudinal flux stretched track magnetic couplers(LFSTMC)has excellent application prospects due to its large air gap,high power density and low magnetic leakage radiation.However,the variation of the mutual inductance between the transmitter and receiver coils while EV is in motion makes the output power of LFSTMC fluctuate greatly,which affects the stable transmission of energy.This paper takes LFSTMC as the research target,and focuses on the output fluctuation problem during the driving process of EVs from four aspects:mathematical model establishment,mutual inductance calculation method,output fluctuation mechanism and output fluctuation reduction methods.In order to clarify the analytical relationship between the structural parameters of the magnetic coupler and the output fluctuation characteristics and transmission performance of DWC system,a mathematical model reflecting the analytical relationship between the structural parameters of the magnetic coupler and the mutual inductance is investigated.Starting from single-phase LFSTMC,the magnetic field distribution generated by transmitter coil on the receiver plane is revealed,and the analytical expression for the magnetic field is given.The mutual flux linkage is calculated and then the mutual inductance calculation model under different magnetic coupler’s structural parameters is established.This model reveals the analytical relationship between structural parameters and mutual inductance based on the proposed structural-parameter calculation functions.It has the advantages of clear physical concepts and limpid modelling pathway,and can provide a theoretical guidance for the analysis of output fluctuation characteristics,mutual inductance calculation and structural parameter design of magnetic couplers.In addition,the mathematical modeling method is extended to multi-phase LFSTMCs by taking a three-phase bipolar structure as an example.A DWC experimental platform was built and the correctness of the proposed mutual inductance calculation model was verified.In order to further solve the problems of large simulation workload and long calculation time in the process of mutual inductance calculation,a fast mutual inductance calculation method of LFSTMC is studied.Firstly,a fast mutual inductance calculation method for single-phase LFSTMC is proposed.This method uses the product of structural-parameter calculation functions to calculate mutual inductance,and can replace the parameter sweep in the finite element simulation method by changing the values of each calculation function,and thus improving the speed of the calculation.The proposed calculation method is extended to multi-phase LFSTMC,and a correction method for the structural-parameter calculation functions in multi-phase structure is proposed.The calculation speed of this method and the finite element simulation method in analyzing the output fluctuation characteristics is compared,the results show that the proposed method can reduce the calculation time from hundreds of hours to a few minutes while ensuring the same accuracy as the simulation method.Based on the structural-parameter calculation functions,the design guidelines for each structure parameter in the design process of LFSTMC are given.A prototype was built for experiments,and the results showed that the maximum error between the mutual inductance calculation results and the measurement results was 1.56%,which proves the feasibility and accuracy of the proposed calculation method.Based on the proposed mutual inductance calculation model,the output fluctuation mechanism and the fluctuation characteristic of single-phase LFSTMC are investigated,it is pointed out that the cosine distribution characteristic of the magnetic field’s vertical component B_z along the driving direction is the ultimate cause of the output fluctuation.Then,the analytical expression for the traveling wave magnetic field in three-phase LFSTMC is derived,and an output fluctuation analysis method based on magnetic field decomposition is proposed.The traveling wave magnetic field generated by the three-phase transmitter coils is divided into fundamental and harmonic components for analysis,and it is pointed out that the 5th and above harmonic components of the traveling wave magnetic field are the ultimate cause of the output fluctuation.The effects of magnetic coupler’s structural parameters on the output fluctuation characteristic is analyzed,and it is pointed out that the ratio of air gap to pole pitch is the core factor in determining the fluctuation characteristic of the three-phase meander type structure;while the transmitter rail core has a large effect on the output fluctuation characteristic of the three-phase bipolar structure.The calculation method of the output voltage based on the traveling wave electromotive force is proposed,and the output fluctuation characteristics of the three-phase bipolar structure and meander-type structure are compared.The results show that the bipolar structure has a lower output fluctuation ratio when the relative air gap is short.The correctness of the theoretical analysis was verified through the DWC experimental platform.In order to solve the output fluctuation problem of the DWC system with single-phase LFSTMC,a hybrid-windings receiver with low output fluctuation ratio is proposed.This receiver consists of a full-pitch winding and a mixed-pitch winding.Compared to the conventional dual-DD receiver,it can reduce the overall length by 20%with the same output power and output fluctuation ratio,and solve the problem of cross-coupling between the two receiver windings.To solve the output fluctuation problem of the DWC systems with three-phase LFSTMC,the mechanism of reducing output fluctuation is analyzed,the results point out that eliminating the harmonic magnetic field generated by the transmitter coils and eliminating the induced voltage induced by the harmonic magnetic field in the receiver coil are the two fundamental mechanisms.On this basis,three specific output fluctuation reduction methods based on relative air gap optimization,distributed transmitter coils and short-pitch transmitter coils are proposed from the perspective of eliminating harmonic traveling wave magnetic fields.The working principle,application field and design criterion of each method are illustrated respectively.From the perspective of eliminating harmonic induced voltages,the output fluctuation reduction methods based on distributed receiver coils and short-pitch receiver coils are proposed,and the design criteria and application fields of each method are given.An experimental platform was built to verify the feasibility and effectiveness of each method.The research in this paper provides a mathematical model,a calculation method and mechanism promulgation for the analysis of the output fluctuation in DWC system,and lays a theoretical foundation for the design of output fluctuation reduction methods.In addition,the calculation methods of mutual inductance and induced voltage proposed in this paper can also provide theoretical guidance for the design and optimization of the structural parameters of the magnetic couplers. |
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