Novel Method and Procedure for Evaluating Compliance of Sources with Strong Gradient Magnetic Fields Such as Wireless Power Transfer Systems

Abstract

To date, the development of valid and globally universally accepted recognized methods for the accurate exposure assessment of wireless power transfer (WPT) technologies is lagging behind the rapid emergence of high power systems in the energy and automotive sectors. WPT systems based on inductive and magnetic resonance technologies generate strong but rapidly decaying magnetic fields, which often exceed reference levels (RL) in the immediate vicinity of the WPT coils by up to a factor of $>$100. Compliance testing of WPT systems with limits derived for homogeneous exposure can lead to estimations of exposure that exceed by up to 40 dB assessments of the fields induced in the human body, i.e., the basic restrictions (BR) defined by the international safety guidelines. Testing compliance with the BR is impractical for regulatory purposes due to the high costs of resources of determining the maximum exposure conditions. This paper presents a novel compliance testing method that mitigates the overestimation of the exposure while maintaining the simplicity of the testing procedure. This is achieved by using coupling transformation functions to correlate not only the amplitude and frequency but also the gradient of the incident field with the BR. These novel conservative coupling functions have been determined by means of a large scale numerical study in the frequency range 3 kHz-10 MHz supported by a physics-based approximation. The here proposed compliance testing method is still conservative in comparison to the compliance toward BR of localized sources. However, in comparison to today's practice of applying RL directly, the overestimation is strongly reduced for high gradient fields (G_n > 50 T/m/T), e.g., by more than 3000 times for field gradients of about 200 T/m/T. We have validated the method by numerical analysis of human exposure to actual WPT sources. The adoption of the new method will help to accelerate the introduction of high-power wireless charging devices in the global market.