ZF is developing an electric motor for series production which does not require magnets. In contrast to the magnet-free concepts of so-called externally excited electric motors already available today, ZF’s I2SM (In-Rotor Inductive-Excited Synchronous Motor) transmits the energy for the magnetic field via an inductive exciter inside the rotor shaft. This makes the motor uniquely compact 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 synchronous machines (PSM). The latter are currently the motors most frequently used in electric vehicles, but they are based on magnets which require rare earths for their production. With the I2SM, ZF sets a new standard for making electric motors both extremely sustainable in production and highly powerful and efficient in operation.
Dispensing with rare earths not only saves valuable resources in production, but also reduces dependencies in the supply chains. In addition, there are no drag losses due to permanent magnets compared to PSM. This enables better efficiency at certain operating points such as long highway trips at high speed.
Rotor design makes the electric motor compact
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, forcing compromises: A dry installation space, i.e. not accessible for oil cooling, with additional seals is necessary. As a result, these conventional FSMs take up around 90 mm more space axially. As a result, manufacturers usually 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 succeeded in compensating 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. The space-neutral integration of the exciter into the rotor means that there are no axial space disadvantages. In addition, an increase in power density in the rotor leads to improved performance.
Inductive transmission as key technology
The technological prerequisite for the ZF innovation is that energy is transferred inductively, i.e. without contact, 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 conventional externally excited electric machines, the ZF innovation 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 then 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.