Air-gap Electrical Windings for Light-weighting of Future Electric Propulsion Systems

Student thesis: Doctoral ThesisDoctor of Philosophy (PhD)


The drive towards hybridisation and electrification of air transport requires electrical machines and accompanying energy storage systems to be incorporated into existing gas turbine power trains or to be implemented as the sole aircraft propulsor. This relieves the thrust load from conventional kerosene fuelled engines reducing noise and the production of carbon dioxide and nitrous oxide in line with targets set as part of the Paris 2050 Agreement. With the efficiency of aircraft being especially sensitive to weight, it is crucial that the power density of the electrical machine, energy storage and power electronics unit offset the consequent increase in system mass. As such, in the interim organisations including the UK Aerospace Technology Institute and NASA have articulated ambitious mass power density targets and volumetric power density target of circa 20 kW/kg and 50 kW/l, respectively, by 2035, a significant rise from that exhibited by current electric propulsion systems. Typically, these systems comprise electrical machines with high mass density steel core packs, which improve rotor and stator magnetic coupling but account for a significant portion of machine mass. An alternative topology, known as an airgap winding machine, eliminates the stator electrical steel teeth resulting in a lightweight design, hence, improved mass power density. Challenges of the topology include limited torque capability due to reduced achievable magnetic loading and the exposure of the winding layer to the rotor reaction torque which would typically be borne by the stator electrical steel teeth. Therefore, this thesis investigates a composite winding layer configuration designed to exhibit low mass and high mechanical strength whilst maximising electromagnetic and thermal performance.
With the aim of assessing the feasibility of the composite winding design, multiphysics analysis covering the electromagnetic, thermal, and mechanical domains is performed and, where possible, analytical methods are used to reduce computational costs and to develop an intuitive mathematical understanding of system behaviour. The proposed design methodology is used to investigate the airgap winding machine design with system requirements constrained to the specification of an existing conventional iron-cored permanent magnet generator, enabling a direct performance comparison to be made.
Aspects of the design approach are experimentally evaluated with reduced-scale composite winding samples representative of the full composite winding layer. Sample manufacture enabled precise control over material volume and position and various composite matrix arrangements were characterised both mechanically and thermally via a shear strength testing rig and an experimental heat flow meter, respectively.
In collaboration with the National Composites Centre the manufacture of a composite airgap winding stator is demonstrated using novel resin transfer moulding techniques, and the prototype is tested to validate the derived electromagnetic and thermal models.
Date of Award3 Oct 2023
Original languageEnglish
Awarding Institution
  • The University of Bristol
SupervisorNick Simpson (Supervisor) & Phil H Mellor (Supervisor)


  • Airgap Winding
  • Slotless
  • Composite Winding
  • Electrical Machine

Cite this