Roadmaps explored: Making automotive electric machines cheaper, faster and sustainable
A blog post by Jon Regnart
The automotive sector is driving innovation in electric machines
The electrification of road transport is facilitating an unprecedented level of innovation in electric machines. Novel architectures, better materials and advanced manufacturing processes are all being explored in response to the diverse and demanding requirements of the automotive sector. In 2016, the Advanced Propulsion Centre’s (APC) Capability Study acknowledged that the automotive electric machine supply chain is nascent, with opportunities identified in both high volume and specialist applications. Figure 1 illustrates that while there have been some early movements in the traction machine supply chain from companies such as Toyota and Honda, the global volume of full hybrid, plug-in hybrid and full electric traction machines is still relatively small compared to engine manufacturing (approximately 77 million produced in 2016 alone).
However, as more electrified cars, buses and trucks hit the market, electric machines will play an ever greater role in creating and safeguarding jobs as global engine manufacturing begins to decline. Therefore, electric machines could provide an attractive opportunity to ensure national industries can continue to export powertrain components. In light of the importance of electric machines, this article highlights some of the most important innovations identified in the updated Electric Machine Roadmap and offers insights from the APC’s Electric Machine Spoke (led by Newcastle University) and industry experts.
The new roadmap goes beyond topology and looks at common challenges across machine types
At LCV2013 the Automotive Council launched two roadmaps in electric machines: one focussing on machines below 40kW and another for machines over 100kW. The emphasis of both roadmaps was understanding which machine designs were appropriate for different vehicle powertrain architectures. However, for the updated roadmap for 2017, the roadmapping team widened the perspective and looked beyond machine topologies. Dr James Widmer, Lecturer in Electrical Machines at Newcastle University and the APC’s Electrical Machine Spoke lead, believes broadening the focus of the roadmap is vital:
“Electric machines are the heart of every electric vehicle and the need to improve their performance, cost and sustainability is driving intense research across the automotive industry. The exciting thing is companies and academics from many different disciplines can contribute, from materials right through to manufacturing, system integration and computer modelling. As a result the key focus of this update has been to make the roadmap about the full breadth of activity, recognising that to be internationally leading we need to harness the broadest range of skills possible.”
Therefore by restructuring the roadmap into four new categories of: Machine Architectures, Machine Integration, Materials and Manufacturing and Enablers, the roadmap encompasses a wider set of technology innovations and leaves scope for a number of designs or manufacturing routes.
Cost, performance and sustainability
Three big themes underpin the updated Electric Machine Roadmap: reducing costs, improving performance (either the power density or efficiency) and reducing the negative environmental impact of electric machine production and end of life. Therefore to encourage future innovation in electric machines, the roadmap has set ambitious targets for cost ($/kW), gravimetric power density (kW/kg) and volumetric power density (kW/l). While sustainability doesn’t have a defined metric, design for recycling and life cycle impacts feature prominently in the drivers and visual roadmap. Figure 2 lists the targets set for both passenger cars and heavy duty vehicles. Different targets have been set because both vehicle types have varying drive cycles and require different electric machine designs. However for consistency, all the targets on the roadmap assume the same input voltages, material price mark-up and efficiencies based on the Worldwide Harmonised Light Vehicle Test Procedure (WLTP).
For the passenger car targets the cost, power density and drive cycle efficiency metrics represent expected improvements for a C-segment battery electric vehicle traction machine and should be all read together. Moreover, power density targets are based on continuous power output rather than peak power as this is a fairer representation of real-life operation rather than claiming a narrow peak power output. Nevertheless the roadmap recognises that greater power densities could be achieved for higher performance applications. For example, adding higher grade permanent magnets would improve machine performance but this comes at a significant cost premium in addition to the negative environmental impact of sourcing and recycling rare earth magnets. This example demonstrates the complex relationship between cost, performance and environmental impact and the roadmap tries to capture these relationships. In fact, the remainder of this article will explore technology options that address each of these three aspects but will also try and identify the various trade-offs.
How to lower the cost of electric machines?
With the exception of a few automakers, such as Tesla’s Model S and X, traction machines for current models of xEVs contain rare earth materials such as Neodymium and Dysprosium. While there are significant performance benefits in using rare earth materials, this comes at a cost premium to motor manufacturers. Figure 3 illustrates that the cost of magnets for a typical interior permanent magnet machine (found in vehicles such as the Nissan LEAF, Toyota Prius and Chevrolet Volt) represents over half of the final machine cost.
Moreover the future price of magnets is uncertain. As Figure 4 shows, China produce almost all the rare earth ores and oxides, processed metals, alloys as well as the final Neodynmium Iron Boron (NdFeb) magnets and its closest alternative Samarium Cobalt (SmCo) magnets. Therefore if the supply of NdFeB magnets is restricted or unable to keep up with future demand, this exposes the automotive industry to price increases they can’t control.
The new Electric Machine Roadmap recognises these pressures on both OEMs and the supply chain and identifies potential pathways for reducing the automotive sector’s reliance of price sensitive, rare earth materials. One short-term solution is to reduce the content of rare earth materials. For example, BMW’s approach to this challenge has been to use a permanent magnet assisted reluctance motor for the i3 and i8. Both vehicles use a motor topology that utilises permanent magnets and the reluctance effect of synchronous reluctance motors to improve performance. This creates an electric machine that exploits the higher performance found in permanent magnet machines but widens the constant-power speed range similar to synchronous reluctance machines all while achieving lower costs.
Another short-term solution for lowering costs is removing elements such as Terbium or Dysprosium from permanent magnet machines. These heavy rare earths increase the maximum operating temperature of NdFeB magnets to as high as 180° C but come at a cost. Recent figures show that the average price of Neodymium in 2016 was $69/kg but Dysprosium is significantly higher at around $250/kg. Therefore by removing Dysprosium it lowers the bill of material cost but may require better strategies to cool the rotor in order to facilitate the use of lower grade permanent magnets. Therefore a manufacturer either accepts a reduction in performance or there is a system cost implications for improving thermal management strategies.
Another strategy the roadmap identifies for reducing costs in electrical machines is finding substitutes for copper windings. Aluminium windings are significantly cheaper than copper windings, weigh half as much while offering equivalent conductivity on a mass basis (see here). However, challenges occur in commercialising aluminium windings given their inherent lower conductivity (hence more volume of aluminium being needed), higher rates of thermal expansion making packaging more complex and difficulties in terminating aluminium conductors. These factors taken together require a change in the mechanical design of electrical machines in order to accommodate the altered winding strategies for aluminium.
Longer term cost challenges
In the longer term, the roadmap identifies other avenues of research that use different machine topologies and alternative materials to reduce costs. One solution being investigated is swapping NdFeB magnets for lower cost substitutes such as iron ferrite (also known as ceramic) or Alinco magnets. As Figure 5 illustrates, ferrite magnets are lower cost with the added benefit of operating at higher temperatures compared to NdFeB magnets. However ferrite magnets also pose significant challenges to electric machine designers. The magnet “strength” (indicated by the metric MGOe) is significantly lower in both the ferrite and Alinco magnets, meaning that the volume of magnets required to generate an equivalent magnetic field increases. Furthermore both ferrite and Alinco magnets’ resistance to demagnetisation is lower, which means there is a higher risk of machine failure if exposed to significant vibration or mechanical stress.
Another long-term strategy identified by the roadmap is the proliferation of machine topologies that do not require rare earth magnets to operate. Popular front runners identified in the roadmap workshops were induction motors (used by Tesla) and switched reluctance motors (used by UK companies such as Controlled Power Technologies and Advanced Electric Machines). AC induction motors are cheaper than permanent magnet machines as they have been mass-produced for a long time. In addition they provide benefits such as lower maintenance, lower weight and have higher efficiencies across a broader spectrum of operating speeds. However compared to permanent magnet machines, they have relatively lower starting torque, are not as power dense and the speed control is more complex. While recent and future advances in power electronics could overcome some of these issues, it’s still a significant barrier for widespread adoption.
Similarly switched reluctance motors have their advantages over permanent magnet machines. For example the absence of magnets or windings in the rotor greatly reduces the requirement of mechanical retention. Therefore switched reluctance motors could be well suited for more robust applications or higher speed applications where power density is desirable. Furthermore as there are no conductors in the rotor, a lower amount of heat is generated with most of the heat being generated in the stator which is easier for the thermal management system to deal with. Nonetheless there are challenges in commercialising switched reluctance motors, namely the torque ripple and complexities in design due to their non-linear performance characteristics. While techniques for reducing these problems can be applied, they can incur a penalty in efficiency, power density or cost.
How to improve performance of electric machines?
As discussed earlier cost effective electric machines are critical for the automotive sector but enhancing the performance of electric machines is also another key element of this new roadmap. Certain applications such as 48V mild hybrids, heavy duty traction machines and high performance passenger cars require different technology approaches compared to high volume passenger cars.
One avenue of research identified in the roadmap to improve electric machine performance is using higher grade permanent magnets. This can be achieved through manipulating the chemical composition of the magnets (i.e. hydrogen ductilisation to reduce machining and sintering), making best use of current magnets by better understanding their material properties (i.e. through advanced nanostructure analysis) or using new manufacturing technologies to enhance or tailor the magnetic properties. One manufacturing innovation is using magnetic laminations instead of sintering or hot-pressed magnets. This has the potential to reduce eddy current losses in high efficiency motors which in turn reduces the heat generated and improves efficiency. Nevertheless laminating permanent magnets can be quite complicated. This is due to the brittle nature of NdFeB magnets and the manufacturing processes required to effectively achieve the desired thinness commands a cost premium which currently prohibits its use in mass market applications.
Another research area for improving electric machine performance is using alternative winding materials and strategies as a significant share of electrical machine losses occur in the copper windings. In the short term, utilising copper more effectively by optimising existing winding techniques (i.e. hairpin, concentrated and distributed) or using litz wire and laminated copper windings can help reduce losses. However to achieve the longer term power densities required for specialist applications there is an opportunity to replace copper windings. Numerous candidate materials are actively being explored, including: carbon nanotubes (either embedded into the copper or replacing copper entirely) graphene or high temperature superconductor materials. Currently research into alternative winding materials are primarily led by academia but the roadmap identifies their importance in the longer term for automotive applications.
In addition to technology and material innovations, advanced manufacturing technologies are also identified on the roadmap as an enabler for higher performance machine. In particular, the adoption of additive layer manufacturing (i.e. selective laser melting) is highlighted as impacting machine design. With current 2D design approaches there are design constraints such as available space, layer thickness or building time. With additive layer manufacturing an unlimited amount of geometries could be produced resulting in designs orientated towards performance. For example additive layer manufacturing could improve thermal management systems by placing cooling channels close to the origin of loss or applying expensive magnetic material only where it is needed.
Finally a key area of innovation is in the interaction between the power electronics community. The imminent introduction of wide band gap semiconductors in the automotive industry will enable motor inverters to achieve higher switching frequencies. This results in higher efficiencies by reducing the current ripple, which results in lower motor losses. Furthermore for motor topologies such as switched reluctance motors, the higher motor speeds and decreased audible noise (as a result of higher switching frequency) could make them more viable for automotive applications. However the use of wide band gap materials can also create other challenges if not managed appropriately. For example using wide band gap materials could impact on electrical machine lifetime due to deterioration of their insulation systems over time. Therefore there are trade offs between he switching strategy, insulation systems and the adoption of output filters. Furthermore wide band gap materials are currently expensive and currently only used in high performance applications such as Formula E. Therefore dramatic cost reductions through economies of scale are required for this to trickle down into higher volumes segments.
Reducing the environmental impact of electric machines
Electric machines that are less harmful for the environment is a key to in ensuring the longevity of the automotive sector. With regards to end of life, two routes exist for improving the sustainability of electric machines: designing electric machines that do not contain rare earth materials (which was discussed in the cost section) or creating a closed loop supply chain whereby rare earth materials are recycled and pulled back into the automotive supply chain.
Figure 6 shows the global warming potential of materials used in electric machines with NdFeB magnets clearly having the most negative impact on climate change. Therefore by reducing the environmental impact of NdFeB magnet refining and manufacturing by reusing existing magnets, it sustains the use of these magnets in automotive applications.
Dr Allan Walton, Head of Magnetic Materials Group at the University of Birmingham, believes a key enabler in establishing a closed loop recycling supply chain is to design electric machines with end of life in mind:
“As future legislation begins to encompass the total environmental impact of vehicles this will present challenges to electric machine designers. Not only will manufacturers have to design a product that delivers good performance but they must consider manufacturing efficiency and end of life. A key challenge with respect to end of life is that magnetic materials, such as permanent magnets and electrical steels, will need cost effective manufacturing processes to sort, extract and maintain material properties for second life use. The positive thing is that work is being carried out in this area with the UK taking a leading role in this research”
As it stands today, current electric machine designs such as interior permanent magnet machines are difficult to recycle as the magnets are embedded inside the rotor and glued with strong adhesives. This makes mechanical extraction processes difficult as the magnets are too brittle and can break. Heat treatment processes are available which can make the magnets more amenable to extraction. However these processes emit fumes from the heated adhesives which are environmentally unfriendly and prolonged heat can cause the magnets to uncontrollably fly off the rotor (see here). Therefore hydrometallurgical methods using solvents is what’s currently used. These methods are effective but they rely on the use of hazardous mineral acids which raise numerous problems such as: producing toxic fumes, containing the acid and managing the acid-contaminated waste material post process. In response to these challenges other extraction and recycling processes are being actively pursued. For example the University of Birmingham are recycling rare earth magnets by injecting hydrogen into the magnet which makes it is possible to break the recovered magnets down to a coarse, friable powder which can be mechanically separated from the discarded products. Other approaches, seen in the EU’s DEMETER project, have also been investigated such as using ionic liquids. However magnets are not the only area of contention. Electric machines are also designed to package in as much copper windings as possible to maximise performance. Firstly extracting the copper becomes problematic because the windings are glued in as electric machines are designed for optimal performance, not end of life. Secondly recycling the copper once extracted is costly because different recycling routes are required for electrical steels, copper and rare earth magnets.
Electric machines are poised for radical innovations
The roadmap aims to capture the breadth and depth of potential innovations in electrical machines and illustrates that there is enough work for the research and manufacturing community to tackle. Only by engaging the whole breadth of the supply chain, from material scientists to vehicle recyclers can the true potential of automotive electric motors be realised. Sandy Smith, Professor of Electrical Machines at the University of Manchester, succinctly sums up the need for collaboration to deliver next generation of electrical machines:
“A key theme that runs through the roadmap is the need for new materials and manufacturing processes to deliver the next generation of electric machines. I think it’s important that the electric machine and automotive communities look beyond their traditional supply chains and look at what other sectors are doing. This shouldn’t be just limited to familiar sectors such as aerospace and motorsport, could the automotive sector learn and work with companies from the chemical sector or materials scientists?”
Therefore with this in mind, we hope the updated roadmap will help facilitate discussions between interested companies and aid those organisations in adjacent sectors to navigate the automotive electric machine landscape. To view the detailed commentary of the new Electric Machine Roadmap, please click here.
The Advanced Propulsion Centre would like to thank all the participants who attended the Electric Machine workshop. Your input at the workshop was valuable in delivering the updated Electric Machine Roadmap. Special thanks also go to members of the steering committee who ensured the APC represented the challenges facing the community.