Electric propulsion for aviation requires high power density and light weight all-superconducting motors and generators. However, superconductors in the armature windings of rotating machines carry AC currents under rotating magnetic fields and large AC loss generated in the armature windings poses a great challenge for the cooling system. Therefore, AC loss reduction in superconducting armature windings is one of critical tasks to underpin the application. Obvious conductor choices for the armature windings are REBCO coated conductors and multifilamentary MgB2 wires. However, YBCO wires may not be the best suitable choice due to their limitation in achievable filament size and difficulty in twisting the filaments. On the other hand, MgB₂ wires operating at 20 K are promising for the armature windings. The filament size can be as small as 10 mm which can result in considerable hysteresis loss reduction. Twist pitches have been demonstrated as low as 5 mm, and the copper can be moved, compared to DC MgB₂ wires. However, coupling loss in the normal conductors in the MgB₂ wires might be the dominant loss component for the application due to the high operating frequency. Therefore, one of urgent tasks is to accurately estimate the AC loss in the MgB₂ wires at actual operating conditions. There have been some analytical equations developed for AC loss estimations in MgB₂ wires. However, the analytical equations have not been validated by experimental results. Furthermore, the analytical equations have limitations for complex wire compositions and operational conditions. Therefore, we need experimental AC loss data in the MgB₂ wires operating under real operating conditions and simulation tools which are validated by experiment and can be extended to predict AC loss for more complicated wire structure and operating conditions which cannot be achieved by experiment.

In this work, we describe the measurement system under development at Robinson Research Institute, Victoria University of Wellington (VUW). The system when it is fully developed has a temperature range of 15 K – 80 K, magnetic fields up to 500 mT, and frequencies between 15 Hz – 200 Hz. The cooling will be enabled by circulating helium gas cooled by two powerful cryocoolers. The magnetic field will be generated by an air-core magnet wound with copper litz wires operating at liquid nitrogen temperature. AC loss in MgB₂ wires will be measured using the measurement system. In addition, we will experimentally evaluate AC loss in the MgB₂ wires at temperatures < 20 K by combining coupling loss and hysteresis loop measurement results. At VUW, we will carry out magnetization loop and critical current measurements on the MgB₂ wires to obtain hysteresis loss at operational temperatures and magnetic fields. At Kyoto University, using the state-of-the-art equipment, we will measure 4 K coupling loss in the MgB₂ wires in the frequency range of 10 Hz – 10 kHz. The extrapolated AC loss results at 20 K in the MgB₂ wires will be compared with the measured results using the measurement system at VUW. Furthermore, AC loss in the MgB₂ wires under rotating magnetic field will be measured using the high dB/dt AC loss test rig at AFRL. The amplitude of the rotating magnetic field is 0.59 T and the temperature range of the measurement system is 10 K - 77 K when fully developed. The measured AC loss data in the MgB₂ wires at AFRL will be compared with the measured AC loss results using the other two measurement methods mentioned in the above. We will also present our 3D AC loss simulation in the multi-filament MgB₂ wires using COMSOL Multi-physics. Actual 3D filament shape and size, the resistivity in the sheaths and barrier will be considered in the simulation. Finally, we will compare AC loss in 3 MW all-superconducting motors using MgB₂ wires and striated REBCO conductors as a case-study.

Acknowledgment
This work was partly supported by the New Zealand Ministry of Business, Innovation and Employment under the Advanced Energy Technology Platform program “High power electric motors for large scale transport” contract number RTVU2004 and and partly supported by the Royal Society of New Zealand Catalyst: Seeding New Zealand – Japan Joint Research Project Programme contract number E4153.

Keywords: AC loss, MgB2 wires, REBCO conductors, All-superconducting rotating machines