Suppressing Leakage Magnetic Field in Wireless Power Transfer using Halbach Array-Based Resonators

TL;DR

Leakage magnetic fields constrain power transfer in wireless systems, and ferrite shielding adds weight and losses. The authors present a ferrite-free Halbach-inspired resonator built from interleaved helix and spiral coils and optimize its geometry with CMA-ES to suppress leakage while preserving high transfer efficiency. MoM simulations and experiments at 6.78 MHz show substantial leakage reduction (up to about 87%) with minimal efficiency loss, enabling orders of magnitude more transferable power under the same leakage field. This approach offers a lightweight, potentially hybrid shielding strategy for high-power wireless power transfer and demonstrates the effectiveness of evolutionary optimization in practical coil designs.

Abstract

Wireless power transfer has the potential to seamlessly power electronic systems, such as electric vehicles, industrial robots, and mobile devices. However, the leakage magnetic field is a critical bottleneck that limits the transferable power level, and heavy ferromagnetic shields are needed for transferring large amounts of power. In this paper, we propose a ferrite-less coil design that generates an asymmetric magnetic field pattern focused on one side of the resonator, which effectively reduces the leakage magnetic field. The key to enabling the asymmetric field pattern is a coil winding strategy inspired by the Halbach array, a permanent magnet arrangement, which is then tailored for wireless power using an evolutionary strategy algorithm. Numerical analyses and simulations demonstrated that the proposed coil structure delivers the same amount of power as spiral coils, while achieving an 86.6% reduction in magnetic field intensity at a plane located 75 mm away from the resonator pair and a power efficiency of 96.0%. We verified our approach by measuring the power efficiency and magnetic field intensity of a test wireless power system operating at 6.78 MHz. These findings indicate that our approach can efficiently deliver over 50 times more power without increasing magnetic field exposure, making it a promising solution for high-power wireless power transfer applications.

Figures (7)

• Figure 1: Suppressing the leakage magnetic field using a resonator structure inspired by the Halbach array. (a) A Halbach array composed of five permanent magnets. The magnetic field on one side is enhanced, while it is suppressed on the other side. (b) Suppressing the leakage magnetic field using resonators inspired by Halbach arrays.
• Figure 2: Structure of the Halbach array-based resonator. (a) Replacing the permanent magnets of the Halbach array with helix and spiral coils. When the coils are combined, the magnetic field is enhanced on the upper side and suppressed on the lower side. (b) Parameterization of the Halbach array-based resonator. $r_i$ are the radii of each coil, $n_i$ are the number of turns of each coil, and $h$ is the height of the resonator. (c) The Halbach array-based resonator with all helix and spiral coils configured. This structure is a series connection of five helices and spiral coils with capacitors inserted in series. (d) The position of the transmitter, receiver, and measurement plane, in which the magnetic field is measured. $g$ is the distance between the resonators, and $h_{\rm m}$ is the distance from the resonators to the measurement plane.
• Figure 3: Results of the optimization process when the measurement plane position is defined as (a) $h_{\rm m}=75\hbox{mm}$ and (b) $h_{\rm m}=150\hbox{mm}$. The plot shows the performance of the optimized structure when the weight $w$ is varied from 0.1 to 1.0. The MoM-based simulation and experiment were conducted using a design resulting from a measurement plane of 75 mm and a weight $w$ of 0.3.
• Figure 4: Resonator structure and the leakage magnetic field of (a)(b) a 300 mm diameter, five-turn spiral coil, (c)(d) the previous Halbach array-based coil Halbachcoil, and (e)(f) the proposed Halbach array-based resonator. The power delivered to the load was the same (1 W) for all configurations.
• Figure 5: Comparison of the leakage magnetic field of Halbach array-based coils (a) before and (b) after optimization. A current of 1A was applied for both cases. The optimized resonator successfully suppressed the leakage magnetic field at the edges of the structure, whereas the non-optimized structure exhibited a far-reaching leakage magnetic field.