Renault argues that the small efficiency penalty (roughly 1-2 percentage points vs PMSM) is worth eliminating dependence on Chinese rare-earth refining. By using wound rotor coils energized through brushes and slip rings, they generate magnetism on-demand rather than baking it in, which they frame as a deliberate trade of peak benchmarks for a cleaner dependency graph.
By submitting Renault's technical explainer to Hacker News where it gained 314 points, bestouff surfaced the rare-earth-free architecture as a noteworthy engineering decision. The high score signals community interest in supply-chain-conscious hardware choices over pure performance optimization.
The editorial highlights that EESMs can de-excite the rotor field at low torque while PMSMs cannot turn their magnets off, meaning PMSMs always pay iron losses. At highway cruise this narrows the headline efficiency gap considerably, suggesting peak-dyno numbers misrepresent the real-world energy picture.
The editorial reframes Renault's motor choice in software terms: a PMSM is like pinning your stack to a single closed-source vendor with export controls, while an EESM is the bring-your-own-magnetism version. The argument is that dependency-graph thinking — familiar to developers — now drives mechanical engineering decisions at automakers.
Renault published a long-form technical explainer detailing the electrically excited synchronous motor (EESM) it ships in the Megane E-Tech and Scenic E-Tech. The headline number: zero rare earths in the rotor — no neodymium, no dysprosium, no terbium, none of the elements that 90%+ of the global EV industry currently depends on China to refine.
The trick is old physics dressed up for 2026. Instead of embedding sintered permanent magnets in the rotor (the dominant PMSM architecture used by Tesla, BYD, Hyundai-Kia, and basically everyone else), Renault wraps copper windings around the rotor and energizes them through brushes and slip rings. Current flowing through the rotor coils generates the magnetic field that the stator's rotating field locks onto. The magnetism is on-demand rather than baked in.
The wound-rotor approach was the standard topology for industrial synchronous motors for most of the 20th century. It got displaced in automotive applications because rare-earth magnets are simpler, lighter, and slightly more efficient. Renault's pitch is that the simplicity premium isn't worth the geopolitical premium anymore.
This is a software-flavored decision dressed in copper and steel: Renault chose the architecture with worse peak benchmarks because the dependency graph is cleaner. A PMSM motor is the equivalent of pinning your production stack to a single closed-source vendor with export controls. An EESM is the bring-your-own-magnetism version: more moving parts, slightly worse at the top of the dyno chart, but you own the supply chain.
The efficiency gap is real but small. Independent teardowns of the Megane E-Tech put the EESM at roughly 96% peak efficiency vs. 97-98% for comparable PMSM designs from Tesla and Hyundai. At highway cruise the gap narrows further, because EESMs let you de-excite the rotor field at low torque — PMSMs can't turn their magnets off, which means they're always paying iron losses. BMW's iX3 quietly made the same bet with its fifth-gen drivetrain. Mahle and ZF have both demonstrated brushless EESM variants that eliminate the slip rings entirely.
The community reaction on Hacker News (314 points) split predictably. The materials-science crowd pointed out that "rare" earths aren't actually rare — they're geologically distributed but commercially concentrated because China spent two decades subsidizing the refining capacity nobody else wanted. The hardware crowd noted that slip rings and brushes are wear items in a drivetrain that's supposed to be maintenance-free for 200,000 miles. Both observations are correct. Renault's counter is that the brush assembly is designed as a service-replaceable cartridge, and that the slip-ring failure mode is gradual degradation rather than the catastrophic demagnetization PMSMs suffer at high temperatures.
The broader pattern is what's interesting. Over the last 18 months, Western industrial firms have started treating rare-earth dependency the way SREs treat single points of failure — something to engineer around even when the workaround is more expensive. Apple is publicly funding rare-earth recycling at MP Materials. The Pentagon dropped $400M into the same facility. Stellantis filed patents on ferrite-magnet motors. Volkswagen's Trinity platform reportedly evaluated EESM. The decisions look like supply-chain insurance policies priced against a tail risk that suddenly stopped being tail-shaped.
If you ship anything that touches hardware procurement — cloud infrastructure with custom silicon, robotics, drones, satellite components, even commodity datacenter motors — the Renault piece is a case study in how to write a decision memo for choosing the harder architecture. The argument isn't "our motor is better." It's "our dependency graph is auditable, and the BOM doesn't have a kill switch in Beijing." That framing maps directly onto how developer-platform decisions are increasingly being made: "is this thing on the export-control list, can the upstream vendor be coerced, what happens if I have to swap it under duress."
For anyone building agent infrastructure or AI hardware, the analogy is sharper than it looks. The TSMC/Nvidia/HBM stack has roughly the same shape as the rare-earth-magnet stack circa 2020: one geography, a handful of vendors, ferocious lead times, and the assumption that the political environment that made it cheap will keep it cheap. Renault's bet is that you should design the EESM equivalent now, before the option premium balloons.
The practical takeaway: the next time someone on your team picks the dominant architecture because the benchmarks are 2% better, ask what the dependency graph looks like under stress. Renault's engineers gave up 1-2 percentage points of peak efficiency for an architecture that has no single point of geopolitical failure. That trade is going to keep showing up — in motors, in chips, in models, in everything that has to ship under conditions the spec sheet didn't anticipate.
Expect a wave of EESM and ferrite-assisted motor announcements over the next two years as European and US OEMs catch up to what Renault, BMW, and ZF have already shipped. The interesting question isn't whether rare-earth-free becomes the default — it's whether the same pattern shows up in compute. The first hyperscaler that publishes a credible "we can run this workload on any of three independent silicon stacks" architecture will have done the AI-infra equivalent of what Renault just published.
A historical pioneer in the complex technology of electric motors without magnetsThose who know the history of electric machines will find the title and verbiage very amusing. Motors with no permanent magnets were the first practical ones, and at this point wound-rotor motors are over a century old.
BMW also makes rare-earths-free motors for their EVs and - at this very moment - theirs are far more advanced. They offer almost twice the power (up to 300kW vs 160kW) and are on a 800v architecture.
Clearly making a motor with induced magnetic fields both for the stator and rotor isn't the innovation here, since a large fraction of industrial motors do not have permanent magnets.I would assume the innovation here would need to be making it small and efficient for any meaningful torque outp
It's interesting that this is a brushed design. In the RC car community, brushless motors are generally regarded as superior, but those of course have the rare earth magnet problem.Technically the brushes can wear out, although there are claims they are good for 150,000-250,000 miles it seems.
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Unfortunately, their Web page does not say a single word about the important problems of their motors.The electrically excited synchronous motors have been known forever, but they had not been used in EVs because of 2 disadvantages.The first is that traditional EESMs require brushes, i.e. sliding el