IAA 2023: ZF comes with e-motor without magnets and rare earths

ZF develops an electric motor without magnets. In contrast to the magnet-free concepts of so-called externally excited electric motors already available today, ZF's "In-Rotor Inductive-Excited Synchronous Motor" (I2SM) transmits the energy for the magnetic field via an inductive exciter inside the rotor shaft. This results in a uniquely compact motor with maximum power and torque density. This further developed variant of a separately excited synchronous motor (FSM) is thus an alternative to the so-called permanent magnet excited synchronous machines (PSM).

PSMs are currently the motors most commonly used in electric vehicles. However, they are based on magnets, the production of which requires rare earths. "With this magnet-free e-motor without rare earths, we have another innovation with which we are consistently trimming our electric drive portfolio to sustainable, efficient, and resource-saving mobility," says Dr. Holger Klein, CEO of ZF. "And we currently see no competitor that has mastered this technology as compactly as ZF." Compared to common FSM systems, the inductive exciter can reduce losses during energy transfer into the rotor by 15 percent. In addition, the CO2 footprint in manufacturing, which in the case of PSM-E motors is caused in particular by magnets with rare earths, can be reduced by up to 50 percent. Dispensing with rare earths not only saves resources in production, but also reduces dependencies in the supply chains

"This uniquely compact electric motor without magnets is impressive proof of our strategy to make e-drives more resource-efficient and sustainable, primarily by increasing efficiency," says Stephan von Schuckmann, member of the Board of Management. In doing so, ZF relies on efficient 800-volt silicon carbide technology and at the same time dispenses with rare earths without increasing the dimensions or weight." Compared to PSM, there are no drag losses due to permanent magnets. This allows for better efficiency at certain operating points such as long highway trips at high speed.

To ensure that the magnetic field in the rotor is built up by current instead of magnets, the FSM concept currently still requires sliding or brush elements in most cases, which force compromises: A dry installation space, i.e. not accessible for oil cooling, with additional seals is necessary. As a result, conventional FSMs take up around 90 mm more space axially. As a result, manufacturers generally cannot flexibly vary between PSM and FSM variants in their model planning without additional effort.

In order to be able to offer these advantages of separately excited synchronous machines competitively, ZF has compensated for the design-related disadvantages of common separately excited synchronous machines. In particular, the torque density has been significantly increased compared to the state of the art thanks to an innovative rotor design. Due to the integration of the exciter into the rotor without affecting the installation space, there are no axial installation space disadvantages. In addition, an increase in power density in the rotor leads to an improvement in performance.

The technological prerequisite for the ZF innovation is the inductive, i.e. contactless, transfer of energy into the rotor, where it generates a magnetic field by means of coils. Thus, the I2SM does not require any brush elements or slip rings. Furthermore, there is no longer any need to keep this area dry by means of seals. As with permanent magnet synchronous machines, the rotor is efficiently cooled by circulating oil. Compared to common externally excited electric machines, the ZF unit requires up to 90 millimeters less axial installation space. In terms of power and torque density, however, the ZF innovation operates at the level of a PSM.

ZF plans to develop the I2SM technology to production maturity and offer it as an option within its own e-drive platform. Customers from the passenger car and commercial vehicle segments can choose between a variant with 400-volt architecture or with 800-volt architecture for their respective applications. The latter relies on silicon carbide chips in the power electronics. (aum)