| Recently,electric vehicles(EVs),as an important approach to alleviating the conflicts of environment and achieving the clean utilization of energy,have gradually been accepted and promoted by various countries.The electric vehicles mostly adopt the wired charging,but the wired charging has the disadvantages of poor operability,low safety,large floor space,etc,which largely limit the large-scale promotion of the electric vehicle.In contrast,the Wireless Power Transfer System(WPTS)has strong research value and broad application prospects due to its high security,strong operability,and easy intelligence.However,during the actual use of WPTS,many factors cause significant reduction of system efficiency,which obstructs the standardization and marketization of WPTS.There are two important reasons causing the decline of WPTS efficiency:1)The temperature of each element of a WPTS is elevated under long-time high-frequency loading condition,and therefore results in the increase of system resistance;2)The inconsistency between the width of the electric vehicle and the parking space results in the horizontal and vertical offsets between the coils of the loosely coupled transformer of WPTS.For the above two reasons,this thesis mainly carries out the following work:1)In order to suppress the decline of the system efficiency caused by the increase of the system resistance,the optimal range of coupling coefficient of loosely coupled transformer is proposed for two-coil WPTS.The optimal range also can ensure the adequate active output of WPTS.This thesis further combines two approaches,simulation calculation and enumeration method,to determine the optimal range of coupling coefficient for different WPTSs at 85 kHz,55 kHz and 25 kHz.2)To improve the misalignment tolerance of the WPTS efficiency,this thesis optimally designs the three-coil loosely coupled transformer.The size of intermediate coil of the loosely coupled transformer is slightly larger than that of the primary and secondary coils,and the intermediate coil is 30 mm higher than the primary coil.In addition,the capacitance of the intermediate compensation capacitor can be changed with the offset distance between the primary and secondary coils.When the horizontal or vertical offset between the primary and secondary coils reaches 200 mm or 50 mm,respectively,the intermediate coil can move horizontally or vertically with the secondary coil.3)A 3-kW WPTS prototype is designed and established.By experiments,it is found that when the coupling coefficient of WPTS is in the optimal range of coupling coefficient,the decline of system efficiency caused by the increase of system resistance can decrease from 19.065%to 5.264%,and the active power output of system can still meet the demand,thus the validity of the optimal range of coupling coefficient is verified.In addition,by experiments,it is also measured that when the horizontal and vertical offsets of the primary and secondary coils of the three-coil loosely coupled transformer reach 300 mm and 100 mm,respectively,the efficiency decline of the three-coil loosely coupled transformer decreases by only 4.246%and 2.811%,which are much lower than the level of system efficiency decline of the existing loosely coupled transformer caused by the offset between the primary and secondary coils.These results confirm that the misalignment tolerance of the efficiency of three-coil loosely coupled transformer shown in this thesis is strong. |
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