Mutual Inductance Measurement in Wireless Power Transfer Systems Operating in the MHz Range

This tutorial focuses on the problem of the experimental measurement of the mutual inductance (or, equivalently, of the coupling factor) of two coupled coils designed for a Wireless Power Transfer (WPT) system. Although many instruments and techniques are available for this purpose, in the literature, no clear guidelines defining the most reliable technique exist, and the vast majority of papers propose ad-hoc measurement approaches without clearly explaining pros and cons of each specific choice. The contribution of this paper is to fill this gap by i) reviewing the available methodologies that can be used to measure the mutual inductance, ii) highlighting the issues that may arise, and iii) identifying the most suitable technique for a particular use case. More specifically, to increase the impact of our analysis, we focus on the case of a couple of WPT coils for biomedical applications designed to work at 6.78 MHz, which is a setting both very common and quite tricky for the direct measurement.