On the Design of Planar Arrays of Nonresonant Coils for Tunable Wireless Power Transfer Applications

In this article, we introduce a novel array-based system for inductive wireless power transfer (WPT) applications and a procedure for its design, which can be interpreted as a generalization of the traditional three-coil systems where the intermediate passive resonant coil (i.e., the transmitter) is replaced with an array of nonresonant concentric loops. We demonstrate that the new WPT array system provides flexibility with respect to the characteristic parameters of the inductive link (i.e., efficiency and gain), thus allowing for their optimal tradeoff depending on the specific WPT application: this is achieved by controlling the current amplitude flowing in each element of the array through the addition of an appropriately selected reactive load. Furthermore, we demonstrate that it is possible to obtain, with the WPT array, focused magnetic field distributions, leading to a potential reduction in the receiver dimensions. Such configuration can be made effectively equivalent to a traditional three-coil system in terms of physical dimensions and performance, through the addition of an appropriate reactance to each loop of the array. To demonstrate the validity and usefulness of the proposed system and design procedure, we first performed extensive numerical simulations of a traditional three-coil system and compared its performance with different array configurations. We then fabricated and experimentally characterized several prototypes, observing an excellent agreement between the measurements and simulations. The innovative features of the proposed design can be extremely useful in many WPT applications requiring a fine optimization in efficiency, gain, and field distribution.