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

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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.