Recently, global warming due to an increase in CO2 emissions has become a serious problem. In the aircraft industry, electric aircraft applied superconducting technology has been developed for reducing CO2 emissions. This study focused on current distribution in superconducting armature coils composed of REBCO wires for fully superconducting synchronous motors for the electric aircraft system.
The REBCO superconducting armature coils for aircraft applications are required an operating current of 1000 A or more according to the design study of superconducting motors [1]. These values exceed a critical current (IC) of single REBCO wires in operating magnetic fields, for instance, IC of REBCO wires with 4 mm-width at 64K in 2 T under field angle of perpendicular to tape face, is about 200 A[2]. Therefore, the armature coil is composed of a parallel conductor in which stacked multiple REBCO wires. However, if the armature coil has the parallel conductor without transpositions, the balance of the inductance between parallel conductor collapses. The resistance component of a superconducting armature coil operated in a cryogenic state becomes almost zero, so the inductance component becomes dominant in determining the current distributions. As a result, parallel conductors in the armature coil have uneven current distributions.
In our previous study, armature coils composed of 2- and 4-strand parallel conductors realized uniform current distributions among parallel conductors by applying the concept of uniform current distribution between groups(CUCG)[3]. CUCG achieves uniform current by introducing transpositions to the innermost and outermost layers. This method can apply for only 2n-strand parallel conductors(n=1,2,3, and so on). The number of parallel conductors is determined based on the ratio between the critical current (IC) and the transport current. If 6-strand parallel conductors of armature coils are required due to magnitude of current, this cannot apply CUCG because 6 is not a power of 2. To achieve a uniform current, it is necessary to increase the number of parallel conductors to 8-strands. However, AC loss becomes 4/3 times and a deterioration in manufacturability. Also, focusing 3-strand parallel conductors, it is difficult to achieve uniform current compared to 2n-strand parallel conductors. Because single phase of armature coil is composed of two double-pancake winding, it is difficult to create symmetry with an odd number of parallel conductors. Therefore, symmetry of 3-strand parallel conductors was created by applying transposition among the winding process of the armature coils. It is assumed that combining this method with CUCG will lead to uniform current method for 6-strand parallel conductors. This study aimed to achieve the uniform current distribution ratios with less than 5% deviation. For simplicity, the current distribution rates among parallel conductors were calculated in the single-phase(U-phase) armature coil conditions. U-phase were composed of two racetrack coils U+ and U-. These coils were connected, and the same direction of the magnetic field generated.
3-strand parallel conductors achieved uniform current when the number of turns is a multiple of 3 by using a unique transposition technique. The total number of transpositions are 5. Uniform current distribution ratio within ±5% is achieved 14 turns or more. Next, 6-strand parallel conductor achieved uniform current when the number of turns is a multiple of 3 by combining two methods. The total number of transpositions are 7. Uniform current distribution ratio within ±5% is achieved all turn numbers. The detailed transposition configurations and patterns will be discussed on ISS 2023.
[1] R. Sugouchi, et al., IEEE Trans. Appl. Supercond., vol. 30, no. 4, 2020, Art. no. 3601905.
[2] H. Sasa et al., IEEE Transactions on AppliedSuperconductivity, pp. 1-1, 2022, doi: 10.1109/tasc.2022.3160660
[3] A. Kobun, et al., IEEE Trans. Appl. Supercond., vol. 33, no. 5, 2023, Art. no. 5200406.