Electric vehicle propulsion has settled around permanent-magnet synchronous motors because they deliver excellent efficiency and power density, but that performance is bought with dependence on rare-earth magnets whose price, supply and extraction footprint sit outside any single automaker’s control. The industry’s alternatives, from reluctance machines to electrically excited rotors, prove that magnets can be removed, but removing them tends to relocate the engineering problem into current, heat and control rather than eliminating it. This piece traces where that trade-off comes from and where a genuinely system-level design, Cooled Motors’ UESM among them, closes it instead of shifting it.
Understanding Electric Motor Technologies
EV traction motors split into two broad families: synchronous machines, whose rotors lock onto the stator’s rotating field, and induction machines, whose rotor currents are induced by that field. The dominant synchronous design is the permanent-magnet synchronous motor (PMSM), where magnets embedded in the rotor supply its field, delivering high efficiency, power density and precise torque control. That performance is why PMSMs sit at the centre of EV propulsion today, but it also ties motor output directly to rare-earth magnet cost, supply security and extraction impact [1].
The alternative to a magnet-dependent rotor is not a single technology but a design space. Induction motors sidestep rare earths entirely, while electrically excited synchronous motors replace the permanent rotor field with an electrically generated one; both approaches shift the engineering challenge from magnet material to how the field itself is produced and controlled. Permanent-magnet machines remain the default traction baseline, but rare-earth-free synchronous designs are now demonstrating performance targets that rival them [2].
Limitations of Traditional Motor Designs
Rare-earth magnets give EV designers compact, high-flux rotors, but that compactness comes bundled with a material that has a narrow operating margin. Elevated temperature and fault currents can irreversibly demagnetize permanent magnets, which constrains allowable operating conditions and limits fault tolerance [1]. Magnet losses add a second rotor-side burden: as the conductive magnets move through the stator field, they generate circulating eddy currents that produce heat without contributing torque. Segmenting the magnets into smaller blocks reduces these currents [2].
The same dependence carries beyond the motor itself: the motor industry relies on a rare-earth mining, refining and magnet-manufacturing base that is geographically concentrated, exposing it to supply-chain vulnerability [3]. A durable EV motor architecture therefore needs to decouple high performance from the magnets themselves, not merely reduce how much magnet material each motor uses.
Why Performance Compromises Arise
Permanent magnets deliver compact torque density, but that density is rented from a thermal margin fixed by chemistry. At the temperatures a traction motor reaches under sustained load, Nd-Fe-B magnets lose coercivity; preserving magnetic performance at those temperatures can require higher-temperature magnet grades and additional heavy rare-earth content [4].
Removing the magnet does not remove the heat, either. Copper and iron losses still convert to heat that must be extracted before insulation, bearings or magnets impose a lower continuous-power ceiling. Addressing that heat commonly means adding liquid cooling, which itself adds pumps, plumbing, maintenance intervals and additional failure points to the drivetrain [5]. The resulting compromise is not simply efficiency versus cost: it is a constrained trade among sustained power, thermal hardware, magnet specification and operating expense, and any one variable improves only by worsening another unless the system is redesigned as a whole.
Alternative Designs: Challenges Faced
Removing rare-earth magnets does not, on its own, remove the thermal constraint described above; it relocates it. Switched-reluctance, synchronous-reluctance and induction machines generally need more current to produce comparable torque, which raises I²R losses in their windings. Torque ripple and vibration in these designs can also create localized heating, so the challenge is not simply dissipating more heat overall but keeping temperature distribution controlled across the whole machine [1].
Electrically excited alternatives introduce a related version of the same trade-off: rotor current generates heat in a location that is harder to cool than the stator. Astemo identifies rotor heating from current flow as a challenge shared by induction and wound-field motors, and addresses it in its own rare-earth-free design by immersing the coils directly in oil to remove heat at the source [6]. Alternative architectures shift where the problem sits; they do not automatically resolve the underlying performance-cost balance.
Cooled Motors: Solving the Performance Bottleneck
The performance bottleneck in magnetless design is not the absence of magnets; it is the habit of treating excitation, control, winding architecture, cooling and manufacturing as separate problems solved one at a time. Cooled Motors’ UESM integrates them: rotor field strength is generated and controlled electrically, while the motor still runs on a standard three-phase inverter and carries no added rotor power electronics or extra rotor components. That integration matters because, as the alternatives above show, magnetless machines otherwise pay for magnet removal in higher current and concentrated I²R losses that cap continuous output.
The result is a rare-earth-free synchronous motor built as a drop-in replacement rather than a new propulsion architecture, requiring no platform-level inverter change. This directly targets the trade-off the rest of the field has been managing rather than solving: material independence traded against usable power density. Incumbent hybrid-drive manufacturing, meanwhile, continues to scale around established, magnet-dependent motor supply chains [7], which is precisely the dependency this architecture is built to avoid rather than inherit.
Conclusion: The Need for System-Level Thinking
The trade-offs traced through this piece point to one lesson: magnet removal, current, heat, control and manufacturing are coupled variables, not independent design choices, so improving one in isolation just moves the constraint elsewhere. Rare-earth-free machines commonly carry higher loss density and tighter thermal-control demands, which is why closing the gap to permanent-magnet performance requires multidisciplinary optimization rather than a single substitution [1].
That coupling is the real standard against which any rare-earth-free motor should be measured: does it preserve efficient, continuous EV performance without simply exchanging magnet dependency for excess current, heat or added electronics? System-level engineering is what makes that trade explicit and solvable, and it is also what lets a design keep the supply-chain, cost and production-footprint benefits of eliminating permanent magnets rather than trading them for a different constraint. For engineers evaluating architectures and investors evaluating suppliers, that systems-level integration, not the simple presence or absence of a magnet, is the meaningful signal of technical maturity.
Sources
- Machines — mdpi.com
- IEEE Spectrum — spectrum.ieee.org
- Navigating the rare earth magnet conundrum — kearney.com
- pmc.ncbi.nlm.nih.gov
- Aem to unveil rare earth free electric drive system at ivt expo — automotivepowertraintechnologyinternational.com
- Astemo Stories — astemo.com
- Astemo Invests $112M in Kentucky Hybrid Motor Expansion – The EV Report — news.google.com