Magnetisation and demagnetisation of trapped field stacks in a superconducting machine for electric aircraft

Abstract

This research presents a comprehensive and innovative approach to investigating the magnetisation and cross-field demagnetisation behaviour of high-temperature superconducting (HTS) coated conductors (CCs) in practical superconducting machines. This study introduces several novel contributions, including the operation of the machine in propulsion energy conversion mode, the exploration of harmonics interaction in a real electric machine environment involving CCs, and the extraction of these harmonics as cross-field components. A 2D electromagnetic-thermal coupled numerical model employing the finite element method (FEM) has been developed and validated against experimental data to simulate a partially superconducting machine. Upon magnetisation, the HTS stacks effectively operate as trapped field magnets (TFMs), generating rotor fields for motor operation. With a peak magnetic flux density of 462 mT of the trapped field stacks (TFSs) in the air gap, the average values of the fundamental and fifth harmonics of the tangential magnetic flux density experienced by the TFSs were observed to be 25 mT and 1.75 mT, respectively. The research has thoroughly examined the impact of cross-field demagnetisation parameters including amplitude and frequency on the demagnetisation of TFSs. Furthermore, the study has also investigated the magnetisation losses occurring in various layers of HTS tapes, encompassing the HTS layer, magnetic substrate layer, and silver stabiliser at different amplitudes and frequencies. Two tape structures, namely a semi-homogenised model and a multi-layered model, have been analysed in terms of magnetisation loss. Additionally, insights into the shielding effect and skin effect at high frequencies were obtained, offering valuable information on the performance of HTS TFSs exposed to high frequency scenarios, especially in high-speed machines for electric aircraft. The research outcomes are anticipated to provide valuable knowledge for the design and optimisation of HTS rotors employing TFSs in superconducting machines, contributing to the advancement of superconducting machine technology.

Study of the demagnetization behavior of no-insulation persistent-current mode HTS coils under external AC fields by 3D FEM simulation

“…In these applications, although the HTS rotor magnets are normally synchronous with the stator field and resemble a static field, they occasionally operate in dynamic conditions and encounter considerable non-synchronous time-varying fields, such as during the acceleration/deceleration and starting/breaking processes (which are frequent for wind turbines), electrical and mechanical vibrations, primary-coil-currentcontrol and out-of-step faults, and active short-circuits of the machine [89][90][91]. The amplitudes of the non-synchronous rippling fields felt by the HTS coils can be ∼10 mT, or even greater, depending on the design [23,42,43,92,93]. On the other hand, previous fundamental researches on the AC field demagnetization of bulks [21,94,95], stacks [22,96,97] and insulated coils [8,17,19,45] mainly focus on amplitudes in the range of ∼10 -∼100 mT.…”