Development of superconducting composite permanent magnets for synchronous motors: an enabling technology for future electric aircraft

使用燃气轮机来产生所有推力的传统飞机的持续渐进改进，从长远来看，将不足以遏制空中交通对环境的负面影响，因为空中交通正在呈指数级增长。混合动力分布式推进系统是一种新的解决方案，具有显著减少燃油消耗和二氧化碳排放的潜力。在这种方法中，电力从燃气轮机发电机传输到众多的风扇电机。超导电动机可能是唯一能够实现所需功率密度的技术，功率密度大于10kw /kg，使混合动力飞机成为可能。在这种应用中，最有前途的同步电机设计之一是在转子上使用捕获磁通的超导复合永磁体（scpm），而不是线圈。增加的间隙场允许可以实现高功率密度。该项目将以第二代高温超导（HTS）磁带堆叠的形式开发这些scpm。虽然设计用于传输电流，但高温超导胶带也可以维持与捕获磁场相对应的持续电流。剑桥大学应用超导和低温科学小组已经证明，尽管超导体的体积不到2%，但高温超导胶带的捕获场比稀土磁体高很多倍。然而，之前几乎没有将其应用于电机的工作，也没有任何原型可以提供比稀土磁铁更大的间隙场。与竞争对手的超导转子技术相比，新的SCPM技术具有三个关键优势。i)与线圈相比，电流引线的消除减少了热泄漏，简化了转子设计。ii)成堆的高温超导胶带具有高度的机械和热稳定性，因为它们含有大量的金属，这使得它们的平均故障时间很长。iii)与所有超导线圈不同，这些线圈几乎不可能淬灭（由于热失控而无法控制的超电流损失），从而防止了空中运行时转子突然失效。该项目将使用最新的商用12毫米和46毫米宽的HTS胶带创建堆栈，并研究适合同步电机的堆栈几何形状的脉冲场磁化。除了使用先前开发的框架进行有限元临界状态建模外，还将使用现有的脉冲磁场磁化系统来研究新堆栈的特性。一个主要的目标将是详细研究由于转子可以经历的振荡外加磁场对scpm的消磁效应。将实验研究将消磁效应降低到航空航天支持伙伴可接受的水平的方法。一个10千瓦的实验室规模的原型电机将围绕一个新的低温恒温器建造，以证明高温超导胶带堆作为转子永磁体的概念，并研究它们在旋转机器环境中的行为。当转子运行到20k时，峰值间隙场可达约3t。定子将包括集成的脉冲磁场磁化线圈，以在机器启动时实现实际磁化，并将由液氮冷却。系统级设计研究将与电机和航空航天工业的合作伙伴一起进行，以根据项目结果确定利用scpm的大型电机的设计可能性。

Continued incremental improvements of conventional aircraft, using gas turbines to generate all the thrust, will not be sufficient in the long term to curb the negative environmental impact of air traffic which is growing exponentially. Hybrid electric distributed propulsion is a new solution with the potential to significantly reduce fuel burn and CO2 emissions. In this approach, electrical power is transmitted from gas turbine powered generators to numerous electric fan motors. Superconducting electric motors are likely the only technology capable of achieving the required power densities, greater than 10 kW/kg to make hybrid electric aircraft feasible.One of the most promising synchronous motor designs for this application uses trapped flux superconducting composite permanent magnets (SCPMs) on the rotor, instead of coils. The increased gap fields this permits can enable high power densities. This project will develop these SCPMs in the form of stacks of second generation high temperature superconducting (HTS) tape. Although designed to carry transport current, pieces of HTS tape can also sustain persistent currents which correspond to trapped magnetic fields. Stacks of HTS tape have been proven by the Applied Superconductivity and Cryoscience Group at the University of Cambridge to trap fields many times higher than produced by rare earth magnets, despite containing less than 2% superconductor by volume. However, there has been almost no previous work on implementing them into a motor, and there are no prototypes in which stacks provide greater gap fields than rare earth magnets. The new SCPM technology has three key advantages over competing superconducting rotor technologies. i) The elimination of current leads reduces thermal leak and simplifies the rotor design compared to coils. ii) Stacks of HTS tape are highly mechanically and thermally stable due to their large metal content, giving them a high mean time to failure. iii) The stacks are almost impossible to quench (uncontrollable loss of supercurrent due to thermal runaway) unlike all superconducting coils, preventing sudden rotor failure in airborne operation. The project will create stacks using the latest commercial 12 mm and 46 mm wide HTS tape and investigate pulsed field magnetisation for stack geometries suited to a synchronous motor. An existing pulsed field magnetisation system will be used to investigate the properties of the new stacks, in addition to FEM critical state modelling using previously developed frameworks. A major objective will be to study in detail the demagnetising effects on SCPMs due to the oscillating applied fields that can be experienced by rotors. Methods to reduce demagnetising effects to levels acceptable to the aerospace supporting partners will be experimentally investigated.A 10 kW lab-scale prototype motor will be constructed around a new cryostat to prove the concept of HTS stacks of tape acting as rotor permanent magnets and to study their behaviour in a rotating machine environment. Peak gap fields up to approximately 3 T will be possible when operating the rotor down to 20 K. The stator will include integrated pulsed field magnetisation coils to achieve practical magnetization upon machine start up and will be cooled by liquid nitrogen.System level design studies will be conducted with partners in the motor and aerospace industries to determine design possibilities for large scale motors that utilise SCPMs based on the results of the project.