ZF Introduces Complete E-Drive System for Passenger, Commercial Vehicles
Braided windings in motors that are cooled directly, and other innovations, net a 50% power increase over current systems.
ZF Group said a new e-drive system architecture that is more compact, offers greater energy density and is easily adaptable will appeal to manufacturers introducing new battery electric vehicles when it is ready for production in 2025. The company recently showed key details of the new modular-concept system that can be configured to run at 400V or 800V. Executives speaking to global media in a virtual workshop said the customization-friendly system that includes matched motor, inverter, gearbox and software would offer shorter development times for OEMs.
“ZF is positioned very well to meet both current and future challenges,” said Markus Schwabe, product line manager of electric systems. ZF, traditionally a transmission powerhouse, has been making increasing investments in electrification. In 2019 it began supplying the entire powertrain for the Mercedes EQS SUV. “We come from the system perspective, and we bring a huge system expertise but know how important it is to understand every component,” he said, adding that engineers focused on creating improvements in four metrics: cost, EV range, torque and power density. The system is designed to be as plug-and-play as possible. ZF predicts that 80% of customer uses can be met directly off the shelf, with the remainder being easily customized to fit any application. Schwabe also said that the company expects 800v systems to quickly outpace the number of orders for 400v systems. Otmar Scharrer, head of electric drive technology development, said that the compact nature of its e-drive system and the internal interfaces of individual components means that a la carte component modifications can be made easily. “The designs also have very high structural rigidity, which results in superior NVH behavior,” he said.
One key to the new technology is that the different needs of, say, 400V and 800V systems, is handled on the controller chip and via inverters with individual power switches. The company asserted that this architecture results in fewer components than today’s power modules. “We can serve different market requirements faster and more precisely,” Sharrer said.
The system starts at 100 kW (134 hp) per axle and can go up to 300 kW (402 hp).
Braided wiring benefits
On its new motors, ZF engineers said that the new system of braided wiring was a large part of increasing the power density of its systems. “Hairpin wiring, the state of the art today, was introduced five to seven years ago, [and] requires hundreds of laser welding points,” said Roland Hintringer, head of e-motor product lines. He said ZF’s braided wiring is formed and braided in one step, saving time and requiring less space, with a winding head of 15mm versus 28mm for hairpin winding. “It only requires 24 welding spots,” he said. The braiding also reduces the raw material used by 10%.
ZF engineers focused heavily on cooling as another path to greater power. In addition to the water-cooled motor housing, cooling oil is pumped not only around the metal of the stator, but is pumped through slots in the braided windings themselves. “The oil flows through the stator and has direct contact with the copper” where the highest heat is generated, Hintringer explained. He said the result is an 85% increase in peak performance, a 50% increase in power and also results in use of only 1% of the rare earth materials than used in current technology.
High power density
Getting power to the wheels is a new coaxial reduction gearbox that draws on the company’s planetary-gearbox expertise to use two planetary gears to generate the desired axle ratio and a 50/50 power split. The bonus is that it also includes a fully integrated differential function, said Robert Peter, head of engineering product line for axle drives. Peter said that compared to two-stage offset helical gear solutions that have been in use since 2011, the new gearbox requires 25% less installation space, has higher power density and offers a 10% reduction in weight with 20% lower transmission losses. NVH is on par with offset-type gearboxes or better, he claimed.
“Our transmission knowledge helped us address... the contradiction between improved NVH and lower weight,” Peter said.
Engineers also said that the new inverter design is more fully integrated into the rest of the powertrain. They predicted that by the end of the decade, units driven by silicon carbide chips, superior to silicon, would be about 60% of the market. In addition to the inverter, ZF is making a high-voltage DC-DC converter for use with fuel cells. The company says the converter achieves an efficiency of 99.6% and that software controlling the entire system is designed for flexibility to allow tuning to different driving characteristics.