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Aviation electric propulsion is characterized by distributed propulsion, flexible maneuverability, and high efficiency with low pollution. The aviation superconducting electric propulsion system based on the concept of direct-drive power generation from turbines, which is one of the most promising solutions for meeting the power demands of high-power aviation electric propulsion aircraft. This system uses an aviation turbine engine to directly drive a superconducting generator for efficient power generation, which is then supplied to a superconducting motor via superconducting cables to drive the fan and produce flight thrust. By decoupling the turbine from the fan and connecting a high-speed turbine to a high-speed superconducting generator, the core machine power density is enhanced. A high-efficiency superconducting motor drives the fan for large bypass ratio propulsion, which together significantly reduces fuel consumption and promotes the development of green and low-carbon aviation.
The superconducting homopolar generator(SHG) capable of operating at extremely high speed shows the potential for application in the aviation turbo-electric system. Compared to the conventional superconducting generators, SHG has the following merits: i) solid rotor allows it to operate at high-speeds. ii) HTS winding is stationary mounted on the stator, which simplifies the cooling structure and improves the reliability. The stationary superconducting excitation coil is directly exposed to a complex external vertical alternating field. The superconducting coil carrying direct current will generate AC losses, including dynamic losses and magnetization losses, which are detrimental to the operation of the superconducting excitation magnet. Especially under high-speed and high-frequency operating conditions, the alternating losses of the superconducting excitation magnet are more significant.
To meet the load requirements of electric aircraft at multiple voltage levels, the generator is connected to an AC/DC rectifier. The air-gap flux density of a permanent magnet generator cannot be adjusted, and active power switches must be used, which brings issues of reliability and additional losses. In contrast, the air-gap flux density of a superconducting homopolar generator based on electrical excitation is fully controllable. Therefore, AC/DC rectifiers can be constructed using uncontrolled diodes in the power generation system of a superconducting homopolar generator. Compared with active power switches, the use of uncontrolled diodes can improve efficiency, reliability, cost-effectiveness, and power density. In this case, the response of the output voltage is achieved by controlling the excitation current. However, to quickly establish a steady-state output voltage, the excitation current needs to reach the target value in a short period of time (usually in milliseconds). The rapid change of the excitation current poses higher AC losses for the superconducting excitation winding and increases the risk of quenching in the superconducting excitation winding.
So this paper will investigate the electromagnetic characteristics and AC loss conditions of the superconducting excitation magnet in the superconducting homopolar generator under rapid excitation rates. The models of the superconducting homopolar generator and the superconducting excitation magnet have been established. The critical current of the superconducting excitation magnet and various alternating current (AC) loss conditions are analyzed under different output voltage response speeds. The temperature distribution and temperature rise due to AC losses are also studied. Finally, the fastest excitation rate of the superconducting homopolar generator after suppressing AC losses is explored. This research provides meaningful reference value for the safe and reliable application of superconducting homopolar generators in aviation turbine direct-drive power generation units.
