Publications

This paper provides an overview of the application of reconfigurable windings in traction motors and their prospective benefits for electric vehicles. Methods of implementation discussed are series–parallel, star-delta, and tapped windings with series-parallel configurations having the greatest potential for system performance improvements.

The review provides valuable insights into the impact on e-drive system mass and volume, showing significant improvements to torque and power densities of approximately 40-80% can be achieved. Other investigations targeting motor efficiency show total motor loss reductions of approximately 30-60% are also possible.

The published literature on the topic tends to be case-specific, indicating that the generalisation of the technology applicability is lacking, with a particular need for design tools and methodologies early in the propulsion system design stage, to explore how the technology could be best applied.

This is also highlighted by the system level impacts shown in a case study on the Nissan Leaf drive, with series-parallel reconfigurable windings resulting in a 41.67% inverter current reduction while maintaining baseline performance characteristics. 

Other key challenges to the technology uptake include the cost and complexity of the reconfiguration device, however If these can be overcome, and the design advantages better understood, traction drives with reconfigurable windings show potential in reducing the size, weight, and cost of the electric machines, while improving their efficiency and power density.

The performance of electric machines for automotive applications is characterised by a high transient torque capability for low speed tractability and a large speed range of high energy conversion efficiency to achieve a desirable vehicle range. Inevitably, these conflicting requirements will introduce a compromise in the design process of electric machines and drives, generally resulting in heavier machines and overrated drive specifications.

This paper discusses the principles of reconfigurable windings, explaining how altering winding connections directly influences key machine parameters like flux linkage, inductance, and resistance. It details the necessary switchgear for series-parallel winding reconfiguration, highlighting potential advantages such as enhanced fault tolerance and emergency braking capabilities. A prototype in-wheel motor with series-parallel reconfigurable windings, developed as part of the EM-TECH Horizon Europe project, is presented.

Simulation results using the Artemis MW130 driving cycle demonstrate that an efficiency-optimized gear shifting strategy can achieve a 1.57% reduction in energy consumption and a speed range extension greater than 50% compared to a fixed winding configuration. This highlights the potential of reconfigurable windings to improve the range of Battery Electric and Hybrid Electric Vehicles (BEVs/HEVs) by reducing drive cycle losses across various speed regions.

In-wheel electric machines are a desirable form of propulsion for lightweight electric vehicles, however the torque capability and speed range is often compromised due to the fixed gear ratio and volume/mass constraints from packaging the motor within the wheel of the vehicle. Actively changing the winding configuration of an electric machine during operation can be leveraged to enhance the torque capability, speed range, and efficiency of electric machines; the characteristic performance of which can be likened to that of a mechanical gearbox.

Reducing the flux linkage of a permanent magnet synchronous machine through winding reconfiguration from star to delta can significantly improve high speed efficiency, with a 32.78% reduction in total loss without detriment to low speed torque capability. This translates to a potential 5.43% reduction in total energy consumption over a real-world drive cycle of a solar powered electric vehicle. Improving drive cycle efficiencies for lightweight electric vehicles highlights a more sustainable transportation option for climate change mitigation strategy.