Abstract Body

The AC loss property of the REBa₂Cu₃O_y (REBCO, RE = rare earth, Y, Gd, Eu, etc.) tapes is complex because of anisotropy originated by its crystal structure. In superconducting machines for propulsion system on electric aircraft, the AC loss causes a quench and decreases the propulsive efficiency of entire aircraft, therefore, it should be reduced. Additionally, quantitative estimation of the AC loss is required to optimize the design of lightweight cooling system. For that purpose, we have studied the AC loss property of REBCO tape.

Our previous studies have focused on the AC loss generated in the armature windings of fully superconducting synchronous machines because it is most dominant in all losses in the machine. In contrast, the AC loss in the field windings is negligibly smaller than that. However, even such a small loss might cause a serious problem in terms of cooling rather than efficiency. Specifically, our recent study resulted that the temperature rise of the field windings is higher than in case of armature. We suppose that the refrigerants for the armature and field are liquid nitrogen and gas helium respectively. The gas helium is used to reduce windage loss. The gas refrigerant has lower capacity of cooling than liquid, therefore, this leads such a result. We suppose that the superconducting windings are cooled to about 64 K. In such a temperature range, slight variation of temperature can cause considerable variation of critical current, I_C, of the REBCO tape. Thus, also in case of field windings, reduction and quantitative estimation of the AC loss are very important. The AC loss property in conditions of field windings is more complex than in case of armature. The magnetic field applied to the armature is only AC field, in contrast, that in case of field windings is a combination of AC and DC field. In this study, we measured and clarified the AC loss property of the REBCO tape as supposing such conditions. Additionally, we proposed a method which estimates the AC loss quantitatively. Subsequently, we estimate the AC loss in fully superconducting synchronous machine.

We obtained the AC loss property by experiment which measures the magnetization of the REBCO tapes in magnetic field containing DC component, B_DC, by using saddle-shaped pickup coil method. As a result, we found that the AC loss for small amplitude increases with increasing B_DC. This means that B_DC causes extra heat. This emphasizes our suggestion of importance of quantitative estimation of the AC loss in field windings. Additionally, we found that when the AC losses normalized by I_C(B_DC)² are plotted against the field amplitude normalized by I_C(B_DC), the AC losses curves coincide with one curve regardless of BDC. Based on such a property, we propose an estimation method of AC loss of REBCO tapes under field composed of AC and DC component.

Utilizing such an estimation method for AC loss, we can quantitatively calculate the heat induced even in the field windings of fully superconducting synchronous motor/generator. Subsequently, we can discuss the design of the motor/generator in terms of cooling based on quantitative consideration of the heat in the machine. Thus, such a method can promote development of superconducting motor/generator for electric aircraft. For example, to optimize design of cooling system for superconducting machines in perspective of lightweight, it is required to estimate the losses in the machines accurately.

Acknowledgment

This work was supported by JSPS KAKENHI Grant Number JP21K20432.