As electric vehicles continue to grow in popularity, key design challenges remain to increase range and reduce costs. A strong focus here is on high efficiency of the electric powertrain. At the same time, there is a clear trend towards highly integrated electric drive units (EDU), consisting of inverter, electric motor, transmission and heat exchanger providing advantages regarding package, weight and power density.
One key development trend for inverters is the technology shift from silicon insulated gate bipolar transistors (Si-IGBT) to silicon carbide (SiC) metal oxide semiconductor field-effect transistors (MOSFET). The benefits of SiC technology are clearly observable in the low power range which is the dominant operating regime during everyday driving. With intelligent control strategies, like variable switching frequency and discontinuous modulation, even further optimisations can be achieved. On the other side, higher switching speeds causing high voltage gradients are more challenging in regard of electromagnetic interference and isolation.
Other important design trends are the realisation of cooling structures directly in the direct bonded copper (DBC) substrate, as well as a higher integration level and modularity of the power modules. Meanwhile, there are also technologies options such as three-level gallium nitride (GaN) technology implementation on DC-DC converters and traction inverters, two-level SiC with advanced gate drivers or soft switching technology and current source inverters with dual blocking devices.
The electric motor is the main loss contributor in EDUs. Optimisation of electric motor efficiency is key for improving the vehicle’s driving range. For traction drives, it is beneficial to have a clear differentiation between prime movers and secondary axles. While the prime movers need to have good efficiency over the whole operating range; the secondary axles have a stronger focus on minimising cost and drag losses. Due to the lower drag losses, externally excited synchronous machines (EESM) and induction machines (IM) are the main motor technologies for secondary axles. The market for prime movers is seeing a wide diversity in motor topologies. In the past, permanent magnet synchronous machine (PMSM) has been the main technology for prime movers, whereas today the share of EESM models is growing.
With higher speeds, higher e-motor power can be achieved with the same amount of raw materials, since the torque depends on the volume—a benefit for the ecological footprint. A major disadvantage of high-speed motors is the loss density. As higher power—and subsequently higher losses—are focused on a relatively small volume, cooling becomes even more important.
Oil cooling allows the coolant to be applied directly to the active parts of the electric motor. Hence, manufacturers are faced with a wide variety of cooling solutions such as a water jacket, stator oil cooling channels, shaft cooling, spray ring cooling, and rotor spray cooling. Correctly understanding the pros and cons of the different solutions and the application case itself will be a key success factor for high-speed electric motors with high continuous power ratings.
Array of tech development
In terms of the EDU, single-speed transmissions are fully sufficient for the majority of passenger car applications, as the electric machine’s speed range is increased. The heavy-duty vehicle segment requires EDUs offering multi speed transmissions. This is due to the large spread between the vehicle launch capability required and the maximum speed needed. The majority of these applications need powershift transmissions to meet the continuous torque requirements of the vehicle.
With higher speeds, higher e-motor power can be achieved with the same amount of raw materials
Park-by-wire technology is also becoming mandatory for locking devices. Some electric drives have transmission-side parking locks, while other vehicle applications use vehicle braking systems. All-wheel drive applications include a permanent electric drivetrain and an electric drive unit that is used temporarily. These temporary units have disconnecting devices for the use of electric machines with higher drag torque. Here, a smooth but fast re-engagement must be ensured.
Torque-vectoring capable electric drives, meanwhile, are becoming increasingly popular in sports cars and luxury class vehicles. These electric drives are challenging from a functional safety point of view, as the mechanical differential is replaced by an electric machine control system.
All in all, there is plenty to keep the industry occupied when it comes to next generation e-motor and inverter tech development.
About the author: Norbert Alt is Chief Operating Officer of FEV Group