I. Introduction
The so-called “racetrack” coil (Figure 1(a)) is a type of electric magnet commonly used for electric rotating machines. It has been widely adopted even in high-temperature superconductor (HTS) rotating machines, mostly for electric propulsion and wind turbine, in expectation to reduce the overall system volume and weight [1]–[8]. Meanwhile, motivated by recent notable achievements in no-insulation (NI) REBCO high field magnets [9]–[13], several research groups have started active studies on rotating machines employing NI REBCO racetrack coils [14]–[19].