Relaxation of Stress Concentration by Applying Clothoid Curve at the Intersection of Straight and Curved Sections in REBCO Racetrack Coil

Rare-earth barium copper oxide (REBCO) racetrack coils can be used for superconducting rotating machines to achieve higher power density than conventional ones. Unlike solenoid coils, winding turns in a straight section of a racetrack coil can easily move outward generating stress concentrations at the intersection of a straight section and a curved section by winding tension and strong Lorentz force. Abrupt change in curvature at intersections of straight section and half circle even create singularities in terms of mechanical stress. Clothoid curves, whose length and curvature are proportional, are often used to create smooth changes in curvature, making all higher order derivatives of curvature continuous. This study investigates the relaxation of stress concentration by applying Clothoid curve to a REBCO racetrack coil for 500 kW class superconducting motors. Non-linear anisotropic equivalent mechanical properties of the stacked REBCO tape are used for precise and efficient analysis. The modification of the coil shape will be applied to the detailed design of the 500 kW superconducting motor field coil.