The demand for electric vehicles (EVs) has been increasing due to the global warming issue. On the other hand, a major issue for EVs is the excessively long charging time. Therefore, it is necessary to develop a wired power transmission system of 100 kW class for EVs. However, such systems have inherent issues, including the risks of electric shock, discharge, and wear on the charging connectors. Therefore, there is a growing need for large-capacity wireless power transmission (WPT) systems capable of transmitting several 100 kW. On the other hand, to achieve 100 kW-class WPT systems for EVs, it is necessary to lower the operating frequency to below 10 kHz. This is crucial for reducing both the switching losses in the power converter and the eddy current losses in the coils [1]. However, conventional large-capacity and low-frequency WPT systems using copper coils have the problem of temperature rise due to heat generation within the coils. Thus, we have been investigated a several 100 kW-class WPT system for EVs in which a high-temperature superconducting (HTS) coil is installed only on the ground side [2]-[3]. In order to realize the high energy density WPT system using HTS and copper coils for EVs, it is important to install ferrite cores on the back of the coils. Particularly for the ground-side HTS coil, ferrite cores may also need to be cooled. On the other hand, the magnetic properties of ferrite cores in the kilohertz frequency band at temperatures of liquid nitrogen and the AC loss characteristics of the HTS coil with ferrite cores for the WPT system have not been satisfactorily clarified.
In this presentation, we focused on a toroidal sample of ferrite cores commonly used as back yokes in WPT systems and evaluated its magnetic properties in the kilohertz frequency band at liquid nitrogen temperatures. We also measured the AC loss of the HTS coil under three different conditions: without ferrite cores, with ferrite cores placed inside the cryostat (ferrite cores and HTS coil at liquid nitrogen temperature), and with ferrite cores placed outside the cryostat (ferrite cores at room temperature and HTS coil at liquid nitrogen temperature). The measured results of these experiments will be presented.
[1] K. Ukita, T. Kashiwagi, Y. Sakamoto, and T. Sasakawa, “Evaluation of a non-contact power supply system with a figure-of-eight coil for railway vehicles,”Proc. IEEE PELS Workshop Emerging Technol.: Wireless Power, pp. 1-6, 2015.
[2] Y. Inoue, R. Inoue, H. Ueda, and S. B. Kim, “Basic Study of a Wireless Power Transmission System Using Superconducting Coil as a Ground-Side Coil for Electric Vehicles,” IEEE Trans. Appl. Supercond., vol. 33, no. 5, pp. 1-4, 2023, 5400605.
[3] R. Inoue, T. Iwamoto, H. Komoda, H. Ueda, and S. B. Kim, “Basic Experimental Study on a 1 kW-Class WPT System Using HTS and Copper Coils for EVs,” IEEE Trans. Appl. Supercond., vol. 34, no. 3, pp. 1-4, 2024, 5901305.
Keywords: AC loss, HTS coil, ferrite, wireless power transmission