An electric motor engineered by Germany-based ZF Friedrichshafen AG has no use for magnets. In contrast to the magnet-free concepts of separately excited synchronous motors (SESM) 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 advanced variant of an SESM is thus an alternative to 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 Earth materials for their production. With the I²SM, ZF is setting a new standard for making e-motors both extremely sustainable in production and highly powerful and efficient in operation.

An inductive current transmission unit inside the rotor enables ultra-compact e-motor design. Source: ZF Friedrichshafen AG

Compared to common SESM systems, the inductive exciter can reduce losses for the energy transmission into the rotor by 15%, according to ZF. In addition, the carbon dioxide footprint in production, which arises with PSM e-motors in particular due to magnets including rare Earth materials, can be reduced by up to 50%.

In addition to the benefits of eliminating rare Earth materials in a compact and powerful package, the I²SM eliminates the drag losses created in traditional PSM e-motors. This enables better efficiency at certain operating points such as long highway trips at high speed.

In order to offer the 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 an improvement in performance.

The technological prerequisite for the ZF innovation is that energy is transferred inductively -- without mechanical contact, into the rotor, generating 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 permanently magnetized synchronous motor, the rotor is efficiently cooled by circulating oil.

Compared to common separately excited synchronous motor, the ZF innovation requires up to 90 mm less axial installation space. In terms of power and torque density, however, the ZF innovation operates at the level of a PSM.