Comparison Between Three- and Four-coil Wireless Power Transfer Systems with Resonant Coils

Abstract

In this paper, it is demonstrated that the efficiency and ability to transfer power to the load in three-coil wireless power transfer (WPT) systems are always higher than in equivalent fourcoil ones. On the other hand, it is shown that there are features attainable in four-coil WPT system that are not in three-coil ones. For instance, in a four-coil WPT system, which can be divided into source, two communication, and load circuits, it is possible to devise a method for which the maximum power transferred to the load circuit or the maximum efficiency do not depend on the mutual inductance between the two communication coils, independently of the load resistance value. The necessary conditions to achieve the above feature together with the overall circuit analysis are discussed in details and practical results presented. Index Terms− four-coil, power transfer efficiency, three-coil, wireless power transfer systems. I. INTRODUCTION Among the several forms of energy, whenever possible, the electrical one is preferable as produces less pollution comparatively, it is easier to handle, and mainly because it can be transmitted more efficiently. The usual method to transmit electrical energy from the source to the load is via cables or wires. However, from the very beginning of electrical energy distribution history, it was recognized that wireless methods to transmit it would be comparatively more convenient [1]. Nevertheless, after the pioneering work of Tesla, which used an inductive link, composed of two coils tuned at the same resonance frequency to transmit electrical energy at a given distance [2], the investigation of the so-called wireless power transfer (WPT) systems was almost neglected for several years, but by some sparse works [3]-[9]. In fact, only about a decade ago the three-[10]-[18], and four-coil [19]-[30] WPT systems had been introduced. WPT systems using more than four coils had also been investigated, but most of the research effort in the area had been focused in the three-and four-coil configurations [25], [31], [32]. Here it is important to emphasize that the three-and four-coil WPT systems are, in some aspects, similar to the two-coil WPT systems, e.g., they have one coil connected to the source and one connected to the load. The differences are that the three-coil WPT systems have one additional (communication) coil and the four-coil WPT systems have two additional (communication) coils. Moreover, following Tesla's original approach [2], all coils are tuned at the same resonance frequency and mutual inductance of non-adjacent coils are made as small as possible. Anyway, perhaps because it is a relatively recent circuit configuration, the three-and four-coil WPT systems characteristics are still object of studies. For example, in a recent paper it was demonstrated