In wireless power transfers (WPTs), it is challenging to obtain a constant output of power (COP) and constant transmission efficiency (CTE) in large coupling variation ranges. In this study, the eight-coil WPT system achieves a uniform magnetic field (UMF) in the transmitter and receiver sides using two transmitting (Tx) coils and two receiving (Rx) coils, respectively. COP and CTE are then achieved with large coupling variation ranges. The circuit model and equations of the transmission characteristics are first obtained based on the structure and working principle of the Helmholtz coil. The model of the mutual inductance and equation of the impedance coupled factor are then developed. The laws of the transmission characteristic are also determined by adopting a simulation tool and equations of the transmission characteristics. Finally, the eight-coil WPT experimental system is designed. In a fixed-frequency mode, the COP and CTE are achieved when the coupling and misalignment distances are changed within a quarter or one-fifth of the relay coil diameter, respectively. This topology provides an efficient solution for problems faced in practical applications, such as wireless chargers of kitchen appliances and automatic mobile robots of small size.

In recent years, wireless power transfer (WPT) technology has entered the application stage. However, the inevitable presence of foreign objects (FO) in the magnetic coupling area during the operation of WPT systems, especially ferrite FO, may result in system detuning and safety incidents. To address this issue, this paper studies the mechanism by which the ferrite FO affects the WPT systems, providing a foundation for the future development of an FO detection device. Using a series–series topology WPT system as an example, a loop model incorporating ferrite FO is first developed to analyze the impact on the output power (OP) and transmission efficiency (TE) when the ferrite FO enters the magnetic coupling area. A 3D electromagnetic field simulation model of the magnetic coupling mechanism is then developed using finite element simulation software to study the impacts of FO of different sizes and positions within the magnetic coupling area on the WPT system. Finally, an experimental platform is constructed for experimental analysis and validation. An approach to detect the ferrite FO is provided by measuring the input power (IP) and TE. The WPT system is protected immediately by shuting down the transmitter if IP and TE sharply change over 15% in a few seconds.