Method Research On Wireless Power Transfer Based On Magnetic Field Concentration And Demagnetization Suppression Mechanism

Compared with the traditional inductive wireless power transfer technology,magnetic resonant wireless power transfer(MRWPT)has been widely applied in various fields such as electric vehicle battery charge,internal medical devices supply and powering domestic appliances because of its longer transfer distance and higher efficiency.However,the magnetic coupling between the transmitting resonator and the receiving resonator decreases with the increase in the transfer distance.The transfer efficiency drops when the transfer distance is extended to be long due to the weak coupling,which is unable to meet the practical requirement of the remote power supply.Magnetic coupling between resonators can be efficiently enhanced in the MRWPT system when the high permeability material,such as ferrite core,is supplemented to the resonant coil,and thus power can be transferred to a long transfer with relatively high efficiency.However,a demagnetizing field and core loss are generated during magnetization which limits the enhancement of the magnetic coupling.Thus,this thesis is focused on investigating demagnetization mechanism of the ferrite core and then proposes a design method of resonator structure based on magnetic field concentration and demagnetization suppression mechanism,aiming to enhancing the magnetic field concentration and weakening the demagnetizing field,and thus the efficiency of the MRWPT system is increased in the long transfer region.The contents of this thesis are as follows:Firstly,based on the equivalent circuit theory,the circuit model of the MRWPT system with air core and with ferrite core are analyzed,and the power transfer efficiency for the two system is calculated.Furthermore,the load characteristic of the MRWPT system using ferrite core is explored and the requirement of load for improving transfer efficiency is derived when adding the ferrite core to the system,which lays the theorical foundation for the optimum design of the resonator structure.Secondly,to solve the problem of the strong demagnetizing field in the traditional cylindrical ferrite core,the demagnetization mechanism of the ferrite core is investigated,the demagnetizing factor of the traditional ferrite core is calculated and the main factor affecting demagnetizing factor is expounded.Then a novel resonator structure with ferrite core is proposed which is based on the magnetic field concentration and demagnetization suppression characterization.The analytical expression of mutual inductance for the coil with different resonator structure are compared.Furthermore,the effect of the winding method and the structure parameters of the novel resonator structure on the system transfer characteristics are analyzed,and the optimal winding method and the ferrite core parameters are determined.After that,the robustness and the core loss for the two systems using cylindrical ferrite core and novel ferrite core are further explored.The robustness is comprised of the pitch of the receiving coil,the radial misalignment of coils,and the load variation.Finally,the experimental prototypes are established to validate the mutual inductance characteristics,the transfer efficiency characteristics and the voltage across the load for the system with air core resonator structure,with cylindrical ferrite core resonator structure and the novel resonator structure.The superiority of the proposed resonator structure with ferrite core in improving performances of MRWPT system is further verified.The experiment result indicates that for the transfer region of 10~100 mm,the average transfer efficiency of the system using proposed resonator structure is higher than that for the system using cylindrical ferrite core resonator structure by 80% and is approximately two times larger than that for the system with air core resonator structure.