Abstract Body

Superconducting REBa₂Cu₃O_y(REBCO, RE= rare earth elements or Y) wires are a possible candidate for several superconducting power applications such as electric rotating machines, power transformers, and applications in magnets for fusion and accelerators. Some of these applications require the enhancement of the current capacity of REBCO wires. For instance, MW-class rotating machines, particularly for propulsion systems in aviation applications, require a current capacity of several kA [1]. Therefore, we have been developing a cabling design for the transposed parallel conductors to realize large current capacity in superconducting armature windings. In the parallel conductor design, the REBCO wires are insulated from each other, except for both terminal ends, and transposed during the winding process to obtain uniform current distributions by the cancellation of the interlinkage magnetic flux between the wires.

Recently, we have investigated the current distribution properties among multi-strand parallel REBCO wires in armature windings without a magnetic shield such as a back yoke. As a result, almost uniform current distributions among the transposed two- and four-parallel wires were demonstrated by small sample coils in those conditions [2, 3]. However, practical superconducting motors employ a magnetic shield to reduce or remove leakage magnetic fields outside of the motors. For propulsion motors for aviation, the active shield composed of superconducting wires is also a possible candidate for magnetic shields because of its lightweight and strong shielding effect properties. Several design studies for superconducting motors employing superconducting active shields showed the possibility of a high output power density of over 40 kW/kg [4, 5]. The spatial distribution and strength of the magnetic fields at the armature windings should be affected by the presence and kind of magnetic shield. This study aimed to reveal the influences of magnetic shield on the current distributions among the multi-strand parallel wires. The current distributions were calculated by a simplified analytical method. Analytical expressions for magnetic fields were derived based on Laplace’s equation in cylindrical coordinates, and consequently, the self and mutual inductances of the wires were calculated. The individual strand current contributions among the constituent wires were assumed to be determined only by the self and mutual inductances. The modeling armature windings composed of 2-4 parallel wires were assumed under a single-phase arrangement of armature coils for a two-pole rotating machine. In addition, the magnetic shields were assumed to be a back yoke thick enough not to magnetic saturation, and a fully reflecting active shield for simplicity.

The calculated current distributions among the 2 and 4 parallel wires introducing the optimal transpositions without a magnetic shield were uniform current distributions. On the other hand, for turn number of 16 turns, the current distribution rates of two parallel wires with no-transposition were 75.8% for the inner wire and 24.2% for the outer wire. The current distribution rates of four parallel wires with no-transposition were 70.4%, 10.4%, 10.3% and 8.9%, respectively. The current distributions with the magnetic shield will be investigated and compared with the no-magnetic shield case. In addition, the influences of distance between the armature windings and magnetic shields also will be investigated. The detailed analytical results will be discussed at the 36th International Symposium on Superconductivity in New Zealand.

References

[1] S. S. Kalsi, R. Badcock, J. Storey, K. A. Hamilton, and Z. Jiang, “Motors employing REBCO CORC and MgB2 superconductors for AC stator windings,” IEEE Trans. Appl. Supercond., vol. 31, no. 9, Dec. 2021, Art. no. 5206807.
[2] S. Miura et al., "Development and assessment of simplified analytical method for current distribution among REBa2Cu3Oy parallel conductors in armature windings for fully superconducting rotating machines," Supercon. Sci. Technol., vol. 36, no. 6, 2023.
[3] A. Kobun et al., "Basic Concept for Uniform Current Distribution in Parallel Conductors by Introducing a Small Number of Transpositions in REBCO Armature Coils," IEEE Transactions on Applied Superconductivity, vol. 33, no. 5, pp. 1-6, 2023.
[4] T. Balachandran et al., "A superconducting air-core machine for aircraft propulsion," IOP Cinf. Series: Matl. Sci. Eng., vol. 756, 012030, 2020.
[5] R. Sugouchi et al., "Conceptual Design and Electromagnetic Analysis of 2 MW Fully Superconducting Synchronous Motors With Superconducting Magnetic Shields for Turbo-Electric Propulsion System," IEEE Transactions on Applied Superconductivity, vol. 30, no. 4, pp. 1-5, 2020.

Acknowledgment

This research was partially supported by the Japan Society for the Promotion of Science (JSPS): Grant-in-Aid for Scientific Research (JP23H00187), and the New Energy and Industrial Technology Development Organization (NEDO).