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

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.

The proposed research project seeks to develop superconducting composite permanent magnets in the form of stacks of high temperature superconducting tape for motor applications. The new permanent magnets have the potential to be one of the enabling technologies for future environmentally friendly hybrid electric aircraft.
In the past decade, most forms of transport have taken a significant shift towards electric power. Electric and hybrid electric road vehicles are a commercial reality, and almost all new large commercial ships are electrically propelled. Electric propulsion for civilian aircraft is at an earlier stage of development but is now gaining serious momentum, attracting design studies from industry and academia. This project can help achieve the very high power densities and efficiencies demanded by the aerospace industry for electric fan motors.
Air traffic worldwide is projected to grow by 5% each year for the foreseeable future. Therefore improving the fuel efficiency of future civilian aircraft is critical to reducing emissions and addressing the long-term rising cost of fossil fuels. Superconducting electric motors are seen as the only class of motors capable of achieving the required specific powers greater than 10 kW/kg. The superconducting composite permanent magnets developed in this project will allow new superconducting rotor designs that have great potential to be used in future aircraft fan motors. It therefore has an impact on the Flightpath 2050 targets set by the EU and the European aviation industry to significantly reduce CO2 emissions and noise by 2050 compared to the 2000 baseline. Reducing the environmental impact of civilian aircraft and the resulting societal benefit is therefore the largest overall impact of the project but also a long-term one. Proving the fundamental feasibility of HTS component technology at an early stage will ensure enabling technologies are ready for application by 2040. The low technology readiness level and disruptive nature of hybrid electric aircraft require partnership with large organisations that have a long-term commitment to Flightpath 2050 targets to enable impact. Partners Airbus Group Innovations UK and Rolls Royce will support the realisation of these long-term goals as well as being beneficiaries through technology transfer. This may contribute to securing the future of the UK aerospace industry, currently the second largest globally. Collaboration with the industrial project partners will allow growth of the research to larger scale prototypes which will be enabled by design studies integrated into the project.
As well as the larger scale economic impact expected from the success of superconducting motor technology for aerospace, other applications may also benefit from the development of high field permanent magnets. These include electrically propelled ships, wind power generators, magnetic separation and small-scale NMR/MRI. The superconducting motor, generator and cable market is expected to grow to £2.2 billion in the UK, with superconducting motors predicted to have the biggest market penetration (> 70%) by 2025. This growth will enable the research to contribute to a low carbon economy and to meeting the UK's carbon reduction targets.
Postdoctoral and student researchers working on the project, will gain significant experience of superconducting motor design and experiments, which translate to valuable skills for the UK aerospace industry and feed the people pipeline needed to grow the UK's emerging electric propulsion activities. The staff of project partners will also gain knowledge of the capability of the latest superconducting materials and specific challenges in applying them to rotating machines. A broad range of transferrable skills will be developed from collaborative work, including commercial awareness for university staff, and also communication skills from reporting results to academic, industrial and student audiences.