Enhancement of Inductive Power Transfer Technology: Iron-based Nanocrystalline Ribbon Cores

Abstract

Inductive power transfer (IPT) has been studied extensively during the last decades, particularly for electric vehicle chargers (EV). Inductive chargers offer several advantages over standard plug-in ones. First, they reduce user interaction increasing comfort and mitigating safety concerns. Furthermore, they allow for the automation of the charging process and the implementation of opportunity charging schemes. Thus, distributed charging points can be deployed in strategic locations — such as traffic lights, public and private parking places, etc.— and EVs can be charged more frequently. This reduces the depth of discharge of the battery and increases its lifespan. Furthermore, IPT systems with bidirectional power flow can facilitate the adoption of vehicle-to-grid schemes (V2G). IPT technology is reaching a mature state. Nevertheless, several aspects of the technology can still be improved. First, the state-of-the-art systems are sensitive to misalignments between the transmitter and receiver pads. Second, the complete standardization of the pad's design has not yet been achieved. Consequently, the interoperability of systems designed by different manufacturers is not yet guaranteed. Third, the detection of foreign objects between the pads is a problem that has not been completely solved. Last, the power density of the pads can still be improved. Pads are generally large and heavy which hinders the adoption of this technology. This dissertation addresses some of these problems in an attempt to enhance the state-of-the-art of IPT technology. The largest portion of this thesis is dedicated to the study of alternative core materials for IPT charging pads. In particular, nanocrystalline ribbon cores are considered a promising material. This material offers a higher saturation flux density, a higher permeability, superior thermal performance, and mechanical robustness compared to the standard MnZn ferrites commonly used in IPT systems. A feasibility analysis of this material was carried using intricate finite element models and experimental measurements. The analysis concluded that higher power densities can be effectively achieved with nanocrystalline ribbon cores. However, eddy-current losses on the outer/lateral faces of the cores were identified as problematic. This motivated a new design approach in which the unique properties of this material were considered during the design stage. Guidelines for the design of nanocrystalline ribbon cores were derived. These were applied to the design of a WPT3, 11 kW pad. These pads showed superior performance as compared to identical pads with ferrite cores. Pads with nanocrystalline cores were 2% more efficient and achieved an 11% higher coupling factor. Likewise, up to 25%, lower flux leakage was obtained. Moreover, their performance concerning temperature variation outperformed the one from ferrite cores both in heat dissipation and thermal stability. Finally, the pads were tested near magnetic saturation. Nanocrystalline cores were able to transfer more power before reaching this point. Thus, higher power densities were achieved with this material. Finally, methods for reducing the eddy-current losses in the system were tested. Ferrite shielding, in particular, was found to be an effective method to improve efficiency and homogenize the temperature distribution within the core. As a minor contribution, a control strategy that uses the dual-resonant frequency characteristic of LCCL-compensated pads is also presented. This strategy was validated experimentally, and it can be used to increase the power transfer capability of pads under misaligned conditions. Moreover, this strategy can ease the interoperability of IPT pads designed by different makers which have different ratings and dimensions.