No abstract

A quench is the transition from a superconducting state to a normal conducting state. Elucidating the mechanism of quenching caused by mechanical disturbances (training quench) requires a detailed understanding of the stress state in the coil during excitation. In this study, we developed a large-scale analysis method that can precisely analyze where strain energy is concentrated and loss energy is generated due to thermal stress and electromagnetic force in superconducting coils by considering the detailed structure of superconducting wires and the wire alignment disorder caused by constraints during winding. The results showed that strain energy in the resin is concentrated in the region of overlap between the strain energy distribution caused by the macroscopic deformation of the coil and the geometrically inhomogeneous region, including the wire transfer zone. Furthermore, comparative verification using acoustic emission measurements to determine damage locations suggests that quench is also concentrated at the transfer zone in the actual coil, thus demonstrating the validity of the analysis. A sub-model of the area around the transfer zone, which was found in the large-scale analysis, was created, and crack propagation analysis was conducted using the phase-field method. Crack propagation in the resin and increased deformation of the wire due to weakening of the supporting rigidity of the wire caused by the crack propagation were observed.

Prognostic health monitoring technologies for power electronic systems assess their performance degradation, load histories, and degrees of fatigue in order to increase maintenance effectiveness, reliability design methods, and equipment availability under conditions of actual use. To improve reliability and reduce downtime, prediction of reliability in terms of thermal fatigue life under field conditions is important, as is the use of load and health monitoring data from the field in cases of performance degradation during use, maintenance, and field failure. The fatigue life of solder joints is also affected by whether the load history waveform is symmetric or asymmetric. In this paper, we propose a novel health monitoring method for thermal fatigue failure corresponding to time-dependent inelastic strain response, such as in asymmetric cycles, by use of a surrogate model obtained by a finite element method-based thermal stress simulation. We applied this method to an insulated-gate bipolar transistor power module capable of monitoring module temperature, electrical performance, and number of revolutions of the cooling fan. With the proposed method, inelastic strain cycles and thermal fatigue life distribution of solder joints could be estimated from their temperature monitoring history. The method was judged to be useful for assessing thermal load histories and estimating thermal fatigue life in prognostic health monitoring.

Acoustic emission (AE) measurement is a non-destructive testing method that can detect high-frequency elastic waves generated by mechanical events inside an object, and is able to localize the wave source by using multiple sensors. In order to determine the mechanism causing the quench phenomenon in superconducting coil, we applied the AE method to a small ellipse-shaped coil in a training experiment. Six AE sensors were installed on the coil surface before the coil was cooled to 4 K in a vacuum vessel and excited. The current to the coil was controlled to produce a gradual increase until the quench occurred. The entire training experiment ended after over 10 quenches and all AE events that occurred in each training cycle were measured. The location of the AE source inside the coil was calculated from the difference in arrival time between sensors, and the results were plotted on an expansion plan of the coil. High-amplitude AE events appeared in areas assumed to have sufficiently high internal stress to cause concentration of strain energy.