Liangchen Li

Nanocrystalline alloy is considered as the potential alternation to ferrite due to its higher saturation flux density and better ductility in application of loosely coupled transformers (LCT) in wireless power transfer systems. However, to reduce the eddy current loss, it generally needs crushing and compressing processes, which may lead to obvious nonlinear characteristics and affect both equivalent conductivity and permeability. In this study, the influence of the nonlinear characteristic on the maximum output power of LCT without saturation is first revealed, at the same time, the influence of the lamination coefficient of nanocrystalline core and its equivalent conductivity on the efficiency of LCT are also analyzed by finite element analysis. Then, a determination method of critical conductivity for LCT with the nanocrystalline core is also proposed. On this basis, a selection method of nanocrystalline material for LCT in the wireless power transfer system is also proposed considering nonlinear characteristics and laminated structure. Experimental validation was performed on 3-kW and 7.7-kW prototypes, and the results show that the nanocrystalline core with higher permeability approaches saturation more quickly as the primary current increases, and the critical conductivity determination method is also verified. In addition, with identical initial permeability, the higher efficiency of LCT can be reached by reducing the lamination coefficient.

Relay coils can effectively increase the transmission distance of wireless power transfer (WPT) systems. However, the poor transmission efficiency hinders the application of this structure. Based on the modeling of series-compensated WPT systems with relay coils, this study analyzes the drawbacks of the conventional compensation method and reveals that system efficiency may not be optimal when resonance is achieved in each coil. Then, a nonresonant compensation method for efficiency improvement and better output performance is proposed. With this method, the compensation parameters of the relay and receiving coils can be optimized to minimize losses in the coupling coils and power devices, while the compensation parameter of the transmitting coil is determined by the zero-voltage switching demand of the high-frequency inverter. Furthermore, the applied performance of the proposed method is investigated by finite element analysis considering misalignment tolerance. Finally, experimental validation is performed on a 1-kW WPT prototype. The results show that, compared with the conventional system, the dc–dc efficiency can be improved by 3% and 2.5%, under alignment and 200-mm misalignment, respectively.

For an inductive power transfer (IPT) system, ferromagnetic materials such as ferrite and nanocrystalline are generally adopted to enhance the coupling and misalignment tolerance between the primary and secondary coils. However, it may decrease the transmission efficiency of IPT system due to the different magnetization and loss characteristics of ferromagnetic materials. Therefore, it is necessary to investigate material performance accurately under the real scenario of high frequency supply. In this study, a mathematical model of the IPT system is firstly established and a new test method combining 3-D finite element analysis (FEA) for magnetization and loss characteristics of ferromagnetic materials in IPT system is proposed and the loss variations of different ferromagnetic materials are also revealed. With the measured magnetization and loss data, misalignment tolerance of magnetic coupler with different ferromagnetic materials is investigated by 3-D FEA. It is found that, when the magnetic field conforms to the paramagnetism of the magnetic material, misalignment tolerance can be improved effectively and the higher efficiency of magnetic coupler can be reached under misalignment conditions. Experimental validation is performed on a 1.2-kW coupler prototype under 200-mm misalignment between primary and secondary coils. With the ferrite and nanocrystalline cores of horizontal and vertical laminations, efficiency of magnetic coupler decreases by 0.457%, 1.392% and 1.352%, respectively, and misalignment tolerance of coupler can be improved effectively.