Design considerations to obtain a high figure of merit in circular archimedean spiral coils for EV battery charging applications

Abstract

It is fairly well-known in circuit theory that the product of magnetic coupling coefficient and system quality factor is the single most important figure of merit (FOM) for optimizing the design of series-series-compensated resonant inductive coupling coils. Coils with high value of FOM can operate efficiently under misalignment conditions, such as poor coupling systems. This would be the case in transit systems with on-road/in-motion charging systems using inductive power transfer (IPT). Archimedean spiral coils are widely used for stationary charging of electric vehicles (EVs). This is due to ease of manufacturing, lower material cost, symmetric coupling profile, ease of experimental characterization, and very well-known magnetic characteristics. In this paper, a comprehensive analysis of Archimedean spiral coil structure has been presented, with the aim of establishing key design parameters, leading to high FOM. Knowing the key parameters in order to adjust the coupling coefficient and system quality factor gives a designer abundant options to design coils that can handle misalignment, and at the same time, minimize losses. For this purpose, 2D analysis of a range of coils with difference geometric parameters have been performed. Finite element analysis (FEA) has been used to establish key coil design parameters, in order to obtain a high value of FOM. Results of this design and analytical work will lead to efficient IPT coil design methodologies, which in turn will lead to considerable cost and energy savings. Due to their scalable and modular nature, this work is also applicable to any lumped system specifically utilizing Archimedean spiral coils.