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

Ilaria Liorni, Tomasz Lisewski, Myles Capstick, Sven Kuehn and Niels Kuster, IEEE Transactions on Electromagnetic Compability 2020, Volume 62, Issue 4, pages 1323-1332, August 2020, online 08 August 2019, doi: 10.1109/TEMC.2019.2924519

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.

The scientific and technical impact of the study can be summarized as:

• A new test method for for determining conservative coupling factors for magnetic resonance WPT systems that are generally applicable and rely solely on the incident field amplitudes, the frequency, and the gradient of the field at the closest location of the body (in the frequency range 3 kHz – 10 MHz) is proposed
• • Conservative coupling factors are derived by exposing the computational human anatomical models of the virtual population (ViP) to different sources at different locations with respect to the body
  • • The unnecessary overestimation of the new method is reduced by 35 dB (enabling >3000 higher power to be transmitted) for gradients of about 200 T/m/T and it is expected to decrease even more when further increasing the gradient
    • • The new method represents an easy-to-use approach for conservative estimation of the exposure to any WPT system by directly checking the compliance with BRs of all current safety guidelines. Further analyses will be needed to estimate potential exposure modifications due to human posture or contact of the human body with metal or other conductive objects during WPT exposure

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