ZF's magnet-free EV motor is more efficient and sustainable

The rotor's internal capacitive current transmission unit allows for extremely small e-motor architecture.
Jijo Malayil
ZF's magnet-free EV motor
ZF's magnet-free EV motor

ZF   

To supplement the transition to EVs by offering to make it more compact and competitive, ZF has now introduced a new space-efficient magnet-free motor that offers more efficiency and sustainability. 

The German firm, which specializes in making automotive components, has achieved this by integrating its inductive transmitter into the rotor itself. The design promises to offer performance on par with permanent-magnet synchronous machines (PSMs). 

According to ZF, its I2SM's (In-Rotor Inductive-Excited Synchronous Motor) magnet-free design also requires fewer rare earth elements, increasing supply security and sustainability. 

Better packaging and efficiency

Most of the current EVs use motors known as PSMs, which require rare earth materials for their production. On the other hand, traditional magnet-free systems have extra physical components, such as the sliding rings or brushes required to feed electrical current into the rotor windings. This leads to an increase in weight and size, adding wear and tear risk due to friction. 

According to ZF, such separately excited synchronous motors (SSEMs) design calls for a dry installation room that has extra seals and is not accessible for oil cooling. Therefore, typical SESMs require an additional 90 mm of axial space. As a result, manufacturers typically find it difficult to easily switch between PSM and SESM versions in their model planning.

ZF's compact design is made possible as their inductive exciter is located inside the rotor shaft of I2SM, which provides the energy for the magnetic field. Coils are used to create a magnetic field while energy is delivered inductively, or without mechanical contact, into the rotor. Therefore, brush components or slip rings are not necessary for the I2SM. Additionally, there is no longer a requirement for seals to maintain the area's dryness. Because of this, the motor is very small and has a high power and torque density.

The I2SM also removes drag losses produced by conventional PSM e-motors in addition to the advantages of removing rare earth minerals in a small and powerful design. This property makes high-speed highway rides more efficient for EVs. 

"The inductive exciter can cut energy transmission losses into the rotor by 15% when compared to conventional SESM systems. Additionally, the production's CO2 footprint, which is exacerbated by PSM e-motors in particular since their magnets use rare earth minerals, may be cut by up to 50%," said ZF in a statement. 

The development of magnet-free designs ensures less reliance on China as most resources are mined and processed there. According to various estimates, China accounts for over 90% of the world's supply of rare earth elements, thereby giving Chinese EV automakers an advantage over manufacturers of conventional EV motors.

Regarding the future of The I2SM motor, the technology will be developed by ZF to production maturity and made available as a choice inside its own e-drive platform. Then, for their respective uses, customers from the passenger car and commercial vehicle categories can select between a model with a 400-volt architecture or with an 800-volt architecture. The power electronics of the latter rely on silicon carbide chips.

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