Design And Research On Wireless Power Transmission With Multi-coil In Electric Vehicle Based On LCC Topology

With large-scale application of electric vehicles,charging technology that has received much attention has become a research hotspot.Charging method of electric vehicles is no longer an ordinary wired charging.Development of Wireless Power Transmission(WPT)technology offers new possibilities for charging battery of an electric car.In order to improve output power of WPT system,this thesis studied resonance compensation structure and system control method.Firstly,main circuit topology is studied based on analysis of basic structure of typical wireless charging system.Especially in design of resonant network,a new LCCS-LCC three-coil transmission system is formed by adding a relay coil based on bilateral LCC resonance compensation structure.By analyzing equivalent circuits of the two compensation structures,theoretical conditions that output power of the three-coil system is higher than that of two coils are obtained.Then,characteristics of constant current,power efficiency and soft switching of LCC-S-LCC compensation structure are analyzed from both theoretical and simulation aspects.It is proved that the three coils improve power while retaining advantages of bilateral LCC.Secondly,charging mode and control strategy of WPT system are analyzed with considering characteristics of lithium battery.Designing constant voltage and constant current charging strategies to reduce switching losses as a control target.A control strategy combining frequency shifting and phase shifting is proposed,and the phase shift angle tracks input impedance angle of resonant network.The goal of minimum frequency increase is realized on basis of ensuring soft switching of the whole charging process.After analyzing implementation method of relevant module in Simulink,the charging process of variable load is simulated to prove effectiveness of above control strategy.Finally,with regard to the three-coil transmission system and its control method proposed in this thesis,main circuit and auxiliary circuit are designed respectively to built an experimental platform with maximum charging power of 300 W.Comparing output power of three-coil transmission system and two-coil transmission system,result shows that the three-coil transmission system is improved by 24%;In constant current charging stage,the charging current of three-coil transmission system is controlled between 2.94 A and 3.27 A with increased load resistance from 15Ω to 33Ω.and average value of the charging current is significantly high than two-coil transmission system.Compared with two-coil transmission system,three-coil transmission system is more efficient with heavy load;At the same time,in experimental of constant voltage charging of three-coil transmission system,the current is adjusted between 3A and 0.5A with frequency combined with phase shift control strategy,and switching losses are significantly reduced compared to phase shift control or frequency control alone.