Next generation of rare earth free electric drives considering mass production features
The expansion of the electric vehicle (EV) market will impact the socio‐economic dimension of the transport sector. The introduction of new technologies for energy storage and innovative powertrain concepts plays a critical role in the development of the EV market. Permanent Magnets for electric motors rely on elements such as Nd or Dy to achieve outstanding specific power and efficiency. The main drawback is the critical supply risk associated to them, as these materials are mostly imported from outside the EU. While depending on critical materials like heavy rare earths, the EU is vulnerable to supply bottlenecks of several key materials needed in wind, photovoltaic and electric vehicles technologies and EU resilience to potential supply issues will deteriorate if mitigation measures are not taken. Hence it appears that substitution is the most effective measure to improve the EU resilience to anticipated supply bottlenecks. The design of the ReFreeDrive electric drives has taken a premise of the reduction of use of materials, as more than half of the final price of the motors is formed by raw materials cost. ReFreeDrive motor topologies have good room for cost reduction by off-setting permanent magnet use. Through their configurations these machines are not only rare-earth magnet free, but also share common features that can be exploited during the design step, as well as in the manufacturing process. In addition, one of the key avenues for cost reduction is the reduction of size through the implementation of technical solutions such as higher rotational speed or compact hairpin windings, among others. This project has studied and developed simultaneously two solutions for the power traction system of electrical vehicles: 1) induction machine (IM) with a) fabricated and b) copper die-cast rotor 2) two types of synchronous reluctance (SynRel) machine, a) pure SynRel without any permanent magnets and b) with the assistance of ferrite based permanent magnets. The project has developed the new machines for two use cases (75kW and 200kW), with a final in-vehicle validation of the 75kW prototypes. Hence the project evaluated 8 electrical motor use cases in total. ReFreeDrive proposes a scalable design: by changing only the stack length and ampere-turns, a good range of power levels can be obtained with the same material quality and radial dimensions. In addition, to achieve a design that meets economic requirements, both drive technologies share the same power electronics and control algorithms for each use case (medium power range, high power range). For all motor topologies analysed within our project, we set quite challenging objectives to be achieved compared to existing solutions (e.g. Tesla S60) in both power classes: - 30% more specific torque - 50% less direct motor losses - 50% more power density in the power electronics - 15% lower production costs in a mass production scenario. Experimental testing of the developed motor technologies has been carried out, validating the proposed design methodologies and testing the performance of the motors with a series of test campaigns that added powertrain components until full integration into a real vehicle. The peak performances were: >Pure SynRel 75/200 kW: efficiency 94.2/96.0 %; spec. power 3.9/5 kW/kg; spec. torque 8.6/6.9 Nm/kg. >Ferrite-assisted SynRel 75/200 kW: peak efficiency 96.4/96.3 %; spec. power 3.6/3.8 kW/kg; spec. torque 7/7.9 Nm/kg. >IM 75/200 kW: efficiency 94.0/92.7 %; spec. power 2.3/5.5 kW/kg; spec. torque 5.2/7.8 Nm/kg. Both IM and SynRel machines proved to be promising solutions for mass production in automotive applications, as both show good performance based on low-cost materials while eliminating potential supply risks associated with critical rare earth materials. Further innovation activities will enhance the motor performances and cost-effectiveness in order to allow their massive market uptake.