Understanding Low-frequency Non-radiative Power Transfer

Introduction

In the past decade, the increased interest in wireless power transfer technology is evident both from the technical and consumer perspectives. Improved system efficiencies due to the emergence of resonant transfer of power have engendered the rapid increase of applications that are emerging for this technology. As a consequence of the increasing ubiquity of these applications and the electromagnetic radiations that might be emitted, more concerns are being raised on their environmental friendliness. This has led to increased studies on the impact of this technology on the biosphere, especially on humans.

Different technologies are usually summed up into the term, “wireless power transfer technologies” resulting in complexities and errors in studying their environmental and human impact. This is because the frequency of operation over a distance from the source varies considerably. Hence discussing the technology without separating them along this breakdown could be misleading. One goal of this write up is to examine a subset of this technology, the inductive wireless transfer technologies. The focus will be on their non-radiative characteristics with respect to distance from source and frequency. Such a discourse might bring clarity not only to engineers in the field and consumers of these wireless products, but might also help with the regulatory requirements for this technology and its applications.

In an inductive power transfer system, an alternating electromagnetic field due to an alternating current in a transmitting system of coils enables voltage to be induced in the receiving coil. This is based on Faraday’s law of electromagnetic induction. It is these electromagnetic fields in the vicinity of the power transfer system that raises the question of the radiation emitted by such systems; how they could be tested and/or limited. In order to examine the claim of non-radiative power transfer of this technology, a non-mathematical look at the solutions of Maxwell’s equations and some electromagnetic wave theory will facilitate the analysis.