Frequency splitting suppression in wireless power transfer using hemispherical spiral coils

In wireless power transfer (WPT) systems, frequency splitting phenomena always occur when the transfer distance is too small to obtain the maximum output efficiency in the working frequency. In this paper, a hemispherical spiral coil structure is proposed to suppress frequency splitting phenomena by containing the coupling the system without changing the original system size and transfer distance. Meanwhile, compared with the planar system, the hemispherical system can maintain high power and transfer efficiency and has better angular misalignment tolerance. A WPT system consisting of a hemispherical spiral coil and a planar spiral coil resonating at 110 kHz called the single hemispherical spiral coil WPT (SHWPT) system and another with two hemispherical spiral coils called the double hemispheric spiral coils WPT (DHWPT) system are suggested. Analytic models of these two structures are established to analyze the influence of the coil height on the frequency splitting. The critical coil height is closely related to the critical coupling that provides a reference for the design of the coil. Finally, the experimental results show that the hemispherical coil structure can suppress the frequency splitting to ensure the transmission stability of the WPT system. In the SHWPT system with a 1 cm inner diameter and a 13 cm outer diameter, the critical height of the coil is 5.2 cm, which can suppress frequency splitting when the transmission distance is 0 cm. In the DHWPT system, the critical height of the coil is 4.3 cm.