AC Loss Characteristics in Kilohertz Frequency Band of a High-Temperature Superconductor Coil for a Wireless Power Transmission System

Improvement of transmission efficiency is an important issue in a large-capacity wireless power transmission (WPT) system using copper coils for automobiles and electric railways. We have investigated the application of a high-temperature superconducting (HTS) coil to a high-power WPT system operating in the frequency region of 10 kHz. We measured the transport current losses of GdBCO and Bi2223 tapes and the total ac losses of GdBCO and Bi2223 coils in the kilohertz frequency band. We analyzed the current distribution within the tape using a finite element method (FEM) program. When the current load factor was low and the frequency was above 1 kHz, the ac losses per cycle increased with the frequency. This is because the eddy current loss in stabilized layers was larger than the hysteresis loss at the frequency of above 1 kHz. One the other hand, the ac losses per cycle were constant regardless of the frequency at the high-current load factor. This is because the hysteresis loss was larger than the eddy current loss regardless of the frequency. We investigated the maximum transmission efficiency at a resonance frequency in an HTS WPT system and a Cu WPT system. Although the quality factor of the HTS coil gradually decreased with the frequency and the operating current, the maximum transmission efficiency of the HTS WPT system was much higher than that of the Cu WPT system. We believe that the HTS coil is suitable for a large-capacity and high-efficiency WPT system in the frequency region below 10 kHz.