Trends & innovations in Electric Drive Units for lower cost & improved performance
by Thomas Frey, Head of E-drive / Innovation at AVL.
Electrification is entering a new market phase. At this turning point into its next chapter, a significant shift in focus for electrified vehicles can be observed. It is a shift away from technology feasibility demonstration, premium vehicles or small series development towards kick-off of mass production, technology commercialization and consequently more affordable and technology optimized vehicles. For some time, production volumes and customer acceptance of electric vehicles has widely increased as more and more vehicles have a sufficient driving range (>400 km) and very good driving performance.
Nevertheless, most current generation EVs are still considered to be too expensive or less attractive when compared with combustion engine cars. Consequently, a reduction in cost and improved performance is paramount to ensure successful and sustainable growth of the market.
Integrated electric drive units (EDUs) combine electric machine, transmission, differential and usually the power inverter into one easy to install unit. In contrast to past EDUs with separate and standalone components, this integration has resulted in reduced weight and dimensions, fewer connections, cables & interfaces as well as an overall lower cost. Also, vehicle assembly is more efficient through the packaging advantage of an integrated EDU.
In addition to integration, several other technology trends on the EDU sub-component level are expected to bring costs down and boost performance further to make EVs even more viable. New, faster and more efficiently switching semiconductors like SiC & GaN. are entering automotive applications or soon will do. Even though these new semiconductors are currently still more expensive than Si-based technologies, cost savings in other parts of the system due to their unique characteristics can compensate for the higher semiconductor cost. Examples include a reduction of DC link capacity, simpler cooling, fewer required EMC (electromagnetic compatibility) measures and a reduction in size.
Multiphase approaches result in a reduction of phase currents enabling higher power applications at lower voltage levels, such as 48 V. Overall, a diversification in voltage levels will result in more cost optimized solutions because components can be better adapted to the specific performance requirements of the different vehicle applications. Greatest component availability currently exists for passenger cars around 400 V with low cost due to economies of scale for mainstream and mass volume vehicles. A strong short-term momentum for electrified vehicles market growth will come from 48 V based applications because of their safer voltage level, no/less isolation requirements as well as cheaper and simpler integration into existing IC based platforms. This offers a relatively fast and cost-efficient reduction of CO2 emissions for OEMs to comply with emission legislation. On the other side, 800 V systems come with significant advantages such as minimized charging times enabling comfortable long-distance trips, highest power & torque but also lower weight and reduced dimensions of relevant powertrain components. Consequently, 800 V or higher is the preferred voltage level for performance and commercial BEVs.
AVL pioneered this technology with a first 800 V based demonstration vehicle (AVL Coup-E 800) back in 2012 and has since then continuously evolved respective inverters, e-motors and EDUs. For some time now, AVL has experienced growing customer interest in 800 V not only for commercial vehicles but also for passenger cars.
Considering increasing component power densities, EMC progressively faces more challenges to fulfill regulatory requirements, to maintain overall system reliability and at the same time to keep cost at the lowest feasible level. As a result, we see a clear need and trend to take EMC optimization into account at the earliest possible development phases. AVL has developed dedicated EMC simulation tools allowing early guidance and design optimizations for inverter developers to greatly reduce EMC issues. This is achieved through advanced inverter layouts and incorporation of passive but also active filters. The latter can contribute to significant cost and volume reduction compared to passive elements.
Optimal electronic hardware cooling is critical for reliable performance and durability. Innovative cooling concepts (e.g. air-cooling or common cooling loops instead of separate cooling for inverter and e-motors in integrated EDUs) offer significant potential for overall system simplification and thereby cost and volume saving.
As with e-motors, corresponding similar trends for power electronics such as multiphase and integration exist. Often, performance of e-motors is constrained by the cooling capability of the machine or unwanted over-dimensioning is needed to fulfill peak vs continuous power requirements. Hence, advanced and efficient cooling technologies play a critical role and AVL has found an excellent solution with its directly stator cooled designs where heat is removed where it is mostly generated. In addition, large cost contributions for e-motors result from manufacturing technology as well as type and quantity of materials used. Therefore, new advanced winding technologies for simpler manufacturing or substitution of costly rare earth magnet materials are important e-motor trends for cost reduction.
Relevant innovations include hairpin windings, advanced or new non-permanent magnet motor architectures or a replacement of magnets in PMSM (permanent magnet synchronous motors) machines with less rare earth or fewer magnets. Like the boost of vehicle performance with 800 V technology, AVL believes that significantly increased e-motor speeds up to 25,000 – 30,000 rpm or possibly higher will become the success factor for more power dense, lighter and cheaper e-machines. With this approach, material effort in the e-motor is reduced by size reduction. To maintain the required power with the smaller machine size the rotational speed must be increased. A thorough and detailed design approach and trade-off is necessary to avoid the use of expensive technology in bearings, lamination, windings and magnets.
Just as we successfully pioneered 800 V in the AVL Coup-E 800 back in 2012, AVL is currently finalizing a new demo vehicle featuring AVL’s latest high-speed electric drive unit prototype including most of the trends and innovations as described here. This EDU is designed for 800 V DC voltage, includes a dual SiC inverter, has two PMSM machines with rotational speeds up to 30,000 rpm and a dual transmission to support full torque vectoring. A single-speed layshaft transmission was chosen for best overall efficiency, simplicity and cost. In addition, the e-motor stator is directly cooled and the overall cooling system including inverters and transmission is co-optimized. The dual inverter with common DC-link in one housing operates with variable switching frequencies up 2 x 20 kHz and interleaving strategy for smallest possible DC-link capacities. The EDU delivers a total of 2 x 200 kW and 5,000 Nm wheel torque for 10 s. Integration into various vehicles is simple because of the small size, compactness and power density. Future variants for lower power and torque requirements based on same technology can be derived for versatile and flexible application in less demanding vehicle applications. Overall, this high-speed EDU results in a very powerful attractive package with best in class power densities and an estimated cost reduction up to ~11% compared to conventional EDUs.
All above trends and innovations on both EDU and component levels are likely to power a more accessible electrified future with the next generation of electric vehicles. Customers want short charging times and greatest possible vehicle range at an acceptable cost.
The key to design and enable such EDUs are system understanding, methods and tools that support the design and innovations as described before. AVL offers the full range of simulation, testing, engineering capabilities and experience from past projects to successfully drive these innovations and bring them into the market.
AVL is the technology development partner to not only engineer the propulsion technologies of tomorrow, but also to make them ready for series production utilizing the company’s global presence and independent access to a broad network of suppliers.