Superconducting Electric Aircraft Powertrain Mass Reduction by Wireless Rotor Energisation
Authors:   , Hamilton Kent 
Published: March 2023

<p><b>The European Union have set an ambitious goal of reducing aviation emissions by 25% by 2050, as commercial aviation is currently responsible for 2% of carbon dioxide equivalent emissions globally. Electric vehicle propulsion is an effective method of reducing emissions and operating costs that has been applied in marine, rail, and road transportation. Electric propulsion of large commercial airliners is not possible with conventional electric motors due to the large mass of the required steel components. Superconducting motors can be constructed without steel, and are the only method of achieving sufficiently compact and lightweight motors to enable large scale electric aircraft propulsion. However, the superconducting components of these motors must be cryogenically cooled during operation, which both adds mass to the powertrain and reduces efficiency. The goal of the work presented in this thesis is to increase the effective power to mass ratio of superconducting motors by reducing the required cryogenic cooling power. </b></p>
<p> A significant contributor to cryogenic loading is shown to be the rotor current supply leads. These leads supply constant electric current to the superconducting field coils, but also provide a low thermal resistance pathway for heat to enter the cryogenic envelope from the surrounding environment. Replacing these leads with wireless field coil energisation eliminates the thermal pathway, and removes electrically resistive components from the superconducting field coil circuit. As variations in efficiency and mass of any aircraft component effect overall efficiency and power per unit mass, component level digital powertrain models are built and combined with aircraft model to simulate domestic flights within New Zealand. Three types of hybrid aircraft powertrains are simulated, each with cryogenically cooled superconducting motors. Conventional fuel is simulated in the first and second models, with cooling supplied by mechanical cryocoolers in the first model, and by liquid cryogen in the second. The third model simulates a hydrogen fuelled powertrain where the liquid hydrogen fuel is also used as liquid cryogen for motor cooling. </p>
<p> Using these models, wireless field coil energisation is shown to reduce powertrain mass and required power, even where this energisation method is as little as 5% efficient. Although the difference due to cryogenic load is only slight where cooling is provided by liquid cryogen, wireless field coil energisation both reduces powertrain mass and increases efficiency. This results from replacement of the field coil current supplies with lightweight wireless supplies, and mass reduction through removal of the copper supply leads. Practical characterisation of a demonstration wireless superconducting supply shows efficiencies of up to 9% are possible with existing designs, and suggests a conduction cooled variant may achieve up to 14% efficiency. This is above the efficiency required for this to be an attractive alternative to conventional copper lead energisation. </p>
<p> The second half of this thesis describes the design of a modular AC Homopolar motor capable of 20,000–30,000 RPM operation, and a conduction cooled wireless current supply capable of energising the field coils of this motor. The AC Homopolar motor uses a non-rotating field coil to magnetise a solid steel rotor. Although this motor is not capable of achieving the high power to mass ratio required for aircraft propulsion, the modular design enables development and demonstration of superconducting motor technologies without each requiring a bespoke motor design. </p>
<p> Through development of this motor, it is discovered the fully laminated AC Homopolar stator architecture described in literature may not be required as much of the stator is not exposed to alternating magnetic flux. The AC Homopolar motor is redesigned to include a stator which is only partially constructed of laminated steel sheets. This hybrid stator is less expensive to manufacture due to both the reduced material cost, and the capacity for the solid steel stator components to be manufactured using conventional subtractive manufacturing methods. </p>
<p> Finally, a conduction cooled wireless current supply is designed to match the energisation requirements of the AC Homopolar motor. Based on the earlier characterisation work, the conduction cooled supply is estimated to be 14% efficient when delivering the 200 A required by the AC Homopolar motor field coil. Prior to manufacture, the design is validated through electrical, magnetic, and thermal simulations. </p>
<p> Components of the AC Homopolar motor and wireless supply have been manufactured. The wireless superconducting supply is fully assembled and awaiting characterisation. The AC Homopolar motor is currently being assembled and expected to be commissioned later this year.</p>