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Compact, magnet-free motor uses inductive transmission
The German automotive technology supplier ZF has developed an electric motor which needs no magnets but is claimed to deliver a similar performance to permanent-magnet synchronous machines. Unlike other magnet-free concepts, such as separately excited synchronous motors (SESMs), ZF’s I2SM (In-Rotor Inductive-Excited Synchronous Motor) transmits the energy to produce its magnetic field via an inductive exciter in the rotor shaft. It says this makes the motor “uniquely compact with maximum power and torque density”.
This separately excited synchronous motor is an alternative to permanent-magnet synchronous machines (PSMs), the most common types of motor used in EVs. PSMs are usually based on magnets that rely on controversial and costly rare-earth materials. With the I2SM, ZF says it is setting a new standard for making e-motors that are both sustainable in production and powerful and efficient in operation.
“With this magnet-free e-motor without rare-earth materials, we have another innovation with which we are consistently improving our electric drive portfolio to create even more sustainable, efficient and resource-saving mobility,” says ZF CEO, Dr Holger Klein. “This is our guiding principle for all new products. And we currently see no competitor that masters this technology as well as ZF.”
Compared to common SESM systems, the inductive exciter can reduce losses for energy transmission into the rotor by 15%. In addition, its production CO2 footprint can be up to 50% smaller by avoiding the need for rare-earth materials.
“This uniquely compact electric motor without magnets is impressive evidence of our strategy to make e-drives more resource-efficient and sustainable, primarily through efficiency improvements,” says ZF board member, Stephan von Schuckmann.
As well as eliminating rare-earth materials in a compact, powerful machine, the new motor is also said to eliminate the drag losses created in traditional PSM e-motors. This allows higher efficiencies at certain operating points, making it ideal for long-distance, high-speed EV journeys.
Conventional SESM designs usually require sliding or brush elements to ensure that the magnetic field in the rotor is built up by a current instead of magnets. According to ZF, this results in compromises such as a dry installation space which is not accessible for oil cooling and needs extra seals. As a result, conventional SESMs need around 90mm more axial space and car-makers cannot easily change between PSM and SESM technologies in different models.
ZF says it has manged to compensate for the design-related disadvantages of separately excited synchronous machines. In particular, it has increased the torque density “significantly” by using an innovative rotor design. The integration of the exciter into the rotor without needing extra space means that there are no disadvantages in terms of axial length. In addition, an increase in the power density in the rotor improves performance.
The ability to transfer energy inductively into the rotor, without mechanical contact, is key to the ZF motor design. A magnetic field by is generated using coils, thus avoiding the need for brushes or sliprings. Furthermore, there is no longer any need to keep this area dry using seals.
As with permanently magnetised synchronous motors, the rotor is cooled efficiently by circulating oil. Compared to separately excited synchronous motors, the ZF machine needs up to 90mm less axial installation space. Despite this, it is claimed to operate at a similar level to a PSM in terms of its power and torque density.
ZF plans to develop the I2SM technology until it is production-ready and will offer it as an option in its own e-drive platform. Automotive customers will be able to choose between 400V or 800V variants, the latter relying on silicon carbide chips in the drive’s power electronics.
