Segmental rotor switched reluctance machines for use in automotive traction

Abstract

This thesis explores the development of the Segmental Rotor Switched Reluctance Machine (SRM) to provide electric vehicle traction. This electrical machine, which has a topology distinct from the conventional SRM and has been previously shown to offer enhanced torque density, is selected based on its potential to offer a low cost, sustainable alternative to today’s state-of-the-art electric vehicle traction motors. With the launch, as long ago as 1997, of the Toyota Prius Hybrid Electric Vehicle and of the more recent Nissan Leaf Electric Vehicle in 2010, volume produced vehicle traction drives are an established reality. However hurdles remain in order to reduce the cost of electric and hybrid electric vehicles so that they become cost-competitive with more conventional vehicles. From an electrical machine perspective, one clear cost driver stands out; the rare-earth metals which form the key ingredient in today’s class leading electrical machines. These materials are both expensive (>100USD/kg) and, as was seen in 2011 / 2012, subject to significant price volatility. Equally the mining and refinement of rare-earth materials, such as Neodymium, Dysprosium and Samarium, has been shown to have a much higher environmental footprint than that of the other materials typically used in electrical machines. Beyond the elimination of rare-earths, the thesis looks to further improve the sustainability and cost of the Segmental Rotor SRM. Copper conductors, expensive and difficult to recycle at an electrical machine’s end-of-life, are replaced by more easily recycled aluminium. Aluminium windings are compressed, prior to assembly with the electrical machine, in order to achieve very high fill factors to overcome their relatively low electrical conductivity. Methods are also sought to reduce overall material waste and simplify assembly processes; these include computer based optimisation of the motor structure along with the use of modular manufacturing techniques. With the Nissan Leaf’s Neodymium Iron Boron based Interior Permanent Magnet machine selected as a comparator, an 80kW Segmental Rotor SRM is constructed and tested. The design is shown to have promise and a number of industrially funded follow-on projects are now underway in order to develop the technology further for use in a volume electric vehicle application.