How Does a 120 kW EV Motor Without Rare-Earth Magnets Perform? Insights from a 2026 Review

This article is reposted from Juncai Magnetic Applications, with all rights reserved by the original author; the original link is available in the "Sources" section at the end.

The export controls on rare-earth magnets are expanding from the automotive sector to robotics. According to a review published in the IEEE Transactions on Transportation Electrification by teams from Khalifa University and the University of Belgrade, a "magnet-free" route that is already deployed in commercial vehicles has been systematically examined: the wound rotor synchronous motor (WRSM).

The Bottom Line First Already Deployed in Vehicles Among electric vehicle models, the proportion of WRSMs increased from 3.0% in 2023 to 17.1% in 2024; the highest mass-produced version has reached 360 kW and 17,000 rpm. Strong at High Speeds It can directly adjust the rotor magnetic field, allowing efficiency and power factor in the high-speed region to exceed those of permanent magnet motors of the same class; however, permanent magnet motors remain more efficient in the medium-to-low speed and rated operating conditions. Challenging Rotors Rotor winding heating and high-speed mechanical stress are significant. In a case study design, to keep the winding temperature below 160 °C, the continuous torque was limited to 200 Nm, which is lower than the 225 Nm set during electromagnetic optimization.

01 | An Alternative Path Beyond Magnets Today's electric vehicles mostly use interior permanent magnet synchronous motors (IPMSM). Neodymium-iron-boron magnets are embedded in the rotor, offering high power density and high efficiency, but they are inseparable from rare-earth materials. Rare-earth supply is currently in the spotlight. According to industry reports, Elon Musk recently stated that magnet export controls delayed Tesla's Optimus robot project by several months; China's expanded export control measures, which are currently suspended, will expire on November 10 this year. Consequently, "motors without magnets" are regaining attention from automakers. One route already deployed in vehicles is the WRSM. Its principle is the same as that of permanent magnet motors, except that the magnets on the rotor are replaced with energized coils. Since the magnetic field is "created" by current, rare earths are naturally not needed; the current is adjustable, meaning the magnetic field strength is also adjustable. This is the most fundamental difference between it and permanent magnet motors. This route is not new. Renault has been using it since 2011, with the first model at 44 kW and 12,000 rpm; subsequently, Smart, BMW, Nissan, Rolls-Royce, and Mini have adopted it successively. When reading the chart in the original text, note that the vertical axis represents the proportion of "number of vehicle models," not sales volume. Blue permanent magnet motors have accounted for over 75% since 2016; orange induction motors dropped from 58.8% in 2012 to 5.1%; green WRSMs jumped to 17.1% in 2024. Performance is also catching up. BMW's fifth-generation electric drive has achieved 140-360 kW and 17,000 rpm. The trade-off is the need to supply power to the rotating coils. Current mass-produced vehicles rely on slip rings and carbon brushes. Carbon brushes wear out, generate conductive dust, and require additional axial space. ZF, Valeo, and Mahle have all planned inductive brushless solutions for 800 V platforms, with power planned up to 350 kW. However, the authors point out after literature review that such wireless excitation has not yet found commercial instances.

02 | How Much is the Performance Difference? First, look at power density, i.e., how many kilowatts can be output per liter of "effective volume." The authors compiled parameters from dozens of research prototypes and commercial motors. In the original chart, the horizontal axis is the base speed, the vertical axis is power density, and the yellower the color, the higher the peak power. The overall trend moves towards the upper right: the higher the speed and the larger the motor, the higher the power density. In terms of specific values, the range for WRSM research prototypes is very large, at 0.4-48.9 kW/L; for commercial permanent magnet motors, it is 21.8-36.0 kW/L. The only commercial WRSM with volume data recorded is 10.6 kW/L, located at the diamond point near 4,000 rpm in the chart. Another major factor widening the gap is cooling. Air-cooled WRSMs only achieve 1.3-12.6 kW/L, while those using oil spray, hollow shafts, or direct rotor cooling reach 11.3-48.9 kW/L. The reason is that the rotor coil itself is a heat source. Rotor losses in permanent magnet motors are relatively small, whereas WRSMs require continuous current supply to the coils. Research cited in the paper shows that hollow shaft cooling can increase the allowable rotor loss by about 50%. The long suit of WRSMs lies in high speeds. The magnetic field of permanent magnet motors is fixed; at high speeds, additional current must be applied to "weaken" the field, increasing copper losses. WRSMs can simply reduce the excitation current directly. In the authors' comprehensive scoring, permanent magnet motors rank first in power density, while WRSMs rank first in high-speed region efficiency and power factor.

03 | The Ledger of a Case Study Motor To illustrate these trade-offs, the authors designed an 800 V motor with a rated speed of 6,000 rpm, an outer diameter of 230 mm, and a core length of 240 mm. The application requires 120 kW, corresponding to 191 Nm of continuous torque, and the optimization threshold was raised to 225 Nm to leave a margin. The optimized scheme has an effective mass of 57.25 kg, an average torque of 229.5 Nm, and a torque ripple of 3.38%. But when put into the thermal model, problems arise. To keep the winding below 160 °C, the rotor excitation current density must be reduced from 10 A/mm² to 7.96 A/mm², and the continuous torque drops to 200 Nm. Fortunately, this is still higher than the 191 Nm requirement. If the current is allowed to overload by double, the peak torque can reach 433 Nm, and the peak power 186 kW. Reading the efficiency map: the horizontal axis is speed, the vertical axis is torque, and the redder the color, the higher the efficiency. The deepest red area is concentrated in the high-speed section above about 8,000 rpm, which corresponds to the advantage of adjusting excitation; the efficiency is lowest in the extremely low-speed area on the far left. The authors also point out that efficiency is similarly high during medium-to-low speed and light load, because the excitation can be reduced to lower iron losses. Comparing this chart with the previous one: at low speeds, the excitation copper loss increases layer by layer with torque; after passing the rated speed, it generally falls back because the motor actively reduces the excitation current. High speed also tests the structure. Centrifugal force will pull the rotor coils. One study calculated that the local stress at the end of the core is 3.45 times that of the middle. Only after adding end covers, end plates, and fixing strips in the same study was 15,000 rpm achieved. Several limitations must be clearly seen. First, this is a review; the performance ranges come from different teams and different test conditions, so they cannot be directly compared horizontally. Second, the case study motor was optimized at only a single operating point. The thermal constraints were verified post-hoc, AC losses were not calculated, and no prototype was physically tested. Third, the authors point out that there is no clear evidence of mass-produced versions exceeding 15,000-17,000 rpm; the more reinforcements added, the more complex the assembly, and manufacturing costs are rarely evaluated in the literature.

Labrix Insights WRSMs are not laboratory concepts; BMW, Renault, and Nissan have all mass-produced them. Their way of bypassing rare earths is "trading magnets for current." The price to pay is passing three hurdles: rotor heating, high-speed structure, and slip rings. Whether they can replace permanent magnet motors on a large scale will depend on when brushless excitation and rotor cooling truly mature.

Sources

• EE Times China (2026-10-24)
• Original link: How Does a 120 kW EV Motor Without Rare-Earth Magnets Perform? Insights from a 2026 Review
• https://www.eet-china.com/mp/a533599.html