In contrast to the magnet-less concepts of separately excited synchronous motors (SESM) that are already used in most electric vehicles, ZF’s I2SM (Induction Synchronous Motor with Rotor Excitation) motor transmits the necessary magnetic field energy through an inductive exciter located inside the rotor shaft. This gives the engine a unique, compact design with maximum power and torque.
This advanced variant of a separately excited synchronous motor is thus an alternative to permanent magnet synchronous motors (PMS). The latter are currently the most commonly used motors in electric vehicles, but they rely on magnets that require rare earths for their production. With the I2SM, ZF sets a new standard in the manufacture of electric motors with extremely high durability in the production phase and extremely powerful and efficient in operation.
“With this magnet-free, rare-earth-free electric motor, we have another innovation that constantly improves our electric drive portfolio to create even more sustainable, more efficient and resource-saving mobility,” said Holger Klein, CEO of ZF.
“This is the principle we follow when making new products. There is currently no competitor that masters this technology as well as ZF.”
Compared to conventional SESM systems, the inductive exciter can reduce rotor power transmission losses by 15%. In addition, the CO2 footprint of production can be reduced by up to 50%, thanks in particular to the magnets, including rare earths, that are used to make PSM electric motors.
“This compact, unique, magnet-free electric motor is impressive proof of our strategy to make e-drive more resource-efficient and sustainable, primarily by increasing efficiency,” said Stephan von Schuckmann, Member of the Board of Management of the ZF Group.
“We consistently use 800-volt silicon carbide technology while eliminating rare earths without increasing size or weight.”
In addition to the benefits of eliminating rare earths, I2SM eliminates the resistance losses created by traditional PSM electric motors. This provides increased efficiency at key operating moments, such as long motorway journeys at high speed.
Advanced rotor design makes the electric motor very compact
To ensure that the magnetic field in the rotor is built up by current instead of magnets, conventional SESM designs require, in most cases today, sliding elements or brushes, which forces compromise, as a dry (i.e. inaccessible) installation space is required for oil cooling using additional seals. Conventional EMSs therefore take up about 90 mm more axial space. Therefore, manufacturers cannot flexibly choose between PSM and SESM variants in the planning of the chosen model.
In order to offer competitive advantages to separately excited synchronous motors, ZF was able to compensate for the design disadvantages of conventional synchronous motors. Thus, the torque intensity has increased significantly compared to the state of the art, thanks to an innovative rotor design. The spatially neutral integration of the exciter into the rotor means that there are no disadvantages in terms of axial space. In addition, an increase in rotor power leads to improved performance.
Inductive excitation as a key technology
The technological premise for the ZF innovation is that energy is transferred inductively, i.e. without mechanical contact, into the rotor, generating a magnetic field through the coils. Thus, I2SM does not require brush elements or slip rings. In addition, it is no longer necessary to keep this area dry with seals. As with the permanent magnet synchronous motor, the rotor is efficiently cooled by circulating oil. Compared to the usual separately excited synchronous motor, the ZF innovation requires up to 90 millimetres less axial installation space. However, in terms of power and torque intensity, the ZF innovation works at the level of a PSM.
ZF intends to develop I2SM technology to production maturity and offer it as an option within its own e-drive platform. Customers in the passenger car and commercial vehicle segments can choose between a 400-volt or 800-volt architecture for their applications. The latter is based on silicon carbide chips in power electronics.
About ZF
ZF is a global technology company providing systems for cars, commercial vehicles and industrial technology, enabling the next generation of mobility. ZF allows vehicles to see, think and act. In the four technology areas of Vehicle Motion Control, Integrated Safety, Automated Driving and Electric Mobility, ZF offers comprehensive product and software solutions for established vehicle manufacturers and newly emerging transport and mobility service providers. ZF electrifies a wide range of vehicle types. Through its products, the company contributes to reducing emissions, protecting the climate and improving safe mobility.
With around 165,000 employees worldwide, ZF reported sales of €43.8 billion in fiscal 2022. The company operates 168 production sites in 32 countries.
ZF in Romania
ZF employs around 5,000 people in six locations in Romania (Bucharest, Timisoara, Roman, Oravita, Lupeni, Marghita). These include ZF R&D Tech Center Timișoara, which was spun off from the automotive division of Romanian engineering services provider BeeSpeed and has been part of ZF since January 2019. BeeSpeed has already been working for the ZF Active Safety Systems division (formerly part of TRW) since 2008.
