Posted by bestouff 11 hours ago
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 electrical contacts, which are worn out by friction, so such motors require frequent maintenance for changing the brushes.
It is possible to make brushless EESMs, but they require a rotating transformer and a semiconductor rectifier inside the rotor.
The second disadvantage is a lower efficiency than with permanent magnets, which cannot be improved so much as to match PM motors, because the electrical currents that circulate through the rotor windings must generate heat. The lower efficiency also makes cooling more difficult.
Renault says that their EESMs have an efficiency of 92%. This is a good efficiency, even if not as good as attainable with permanent magnets. Losing a few percents in efficiency is an acceptable compromise for avoiding the use of expensive and supply-constrained chemical elements.
What I wonder is whether Renault reaches this 92% efficiency with EESMs having brushes, or with brushless EESMs, and this is what I would have liked to read on the parent Web page.
Brushless EESMs usually had a lower efficiency, so 92% would be impressive for them, while it would look normal for EESMs with brushes.
If Renault has succeeded to make a brushless EESM (i.e. maintenance-free) with an efficiency of 92%, that is something worth to brag about. Otherwise, making a traditional EESM would not be great news, because everybody has avoided those because of the maintenance problem.
> Group will gradually embed new technological improvements from 2024 on its EESM: stator hairpin, glued motor stack, *brushless* and hollow rotor shafts.
[0] https://www.evspecifications.com/en/news/6ec9484
That said, what sibling says about the maintenance problems is very true. :-/
Those 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.
It's worth noting that some of the biggest motors have always been designed this way, because the size of magnets required would make them both too expensive and dangerous, and still not powerful enough for their size; a field coil can generate a field that's only limited by the current and resistive heating of the winding, but rare earth magnets have fixed limits on field strength.
1. a plank to form the base
2. several 6 inch nails
3. wire
4. a tin can (as a source of sheet metal)
5. tape
No magnets. But it worked perfectly fine when connected to a dry cell. Adventurous science lad that I was, I decided it would work better when connected to AC. So I attached a power cord and plugged it in.
A loud vibration ensued, and then it burst into flames. My mom wasn't happy.
Three holes were punched in the house by the branches, 1-2 inches in diameter. What to do, what to do. I took a coke can, slit it and unrolled it into sheet metal. Then cut a disk bigger than the hole, and epoxied it into place. Worked like a charm, and cost nothing.
I've used coke can metal for shingles and flashing, too. They don't rust.
Mine was a bit fragile, and the first gust of wind shredded the sticks and plastic film.
But it was still fun!
As a teen I built a flame thrower. No, I'm not going to explain how to build one. My dad told me that God looks out for little boys, because otherwise they'd never survive to adulthood.
When I was 9, I found a book of his "Rocket Manual for Amateurs". The opening sentence was something like "if you're fascinated by things that burn and explode, this book is not for you." Who could resist a teaser like that? I promptly read it cover to cover. He wouldn't let me buy any of the necessary materials.
All big generators have an exciter coil that is used to generate the magnetic field. It has the advantage of allowing voltage regulation through adjustment of the field, rather than after the fact, which would be far less efficient.
In both motors and generators, there is an efficiency hit related to the need to supply power in order to generate the field, but when you scale up the system, it actually becomes more efficient to use the electromagnet. With the rare-earth mineral shortage, it makes even more sense.
A permanent magnet motor uses permanent magnets on the rotor, but an electrically excited synchronous motor has an electromagnet on the rotor. This requires a rotating electrical contact which has normally been made with slip rings and carbon brushes. These wear over time and need replacement.
Most large electric generators are externally excited synchronous generators using carbon slip rings, so it's a well understood field.
This can be made contactless using inductive coupling and a rectifier - since inductive coupling needs AC but the excitation coil needs DC - at the expense of some efficiency.
You can see the efficiency difference - Renault claim 92% efficiency but permanent magnet motor EVs have touted efficiency over 95% in the motor.
The lower efficiency means a lower range for the same battery, which is why the companies that have used them in the past, like Tesla, have abandoned them.
Permanent-magnet motors have the highest possible energy efficiency, followed by electrically-excited synchronous motors, than by the induction motors mentioned by you.
Both permanent-magnet motors and induction motors do not contain parts that need frequent maintenance, while this property is more difficult to achieve for electrically-excited synchronous motors.
It's like how laptop power bricks used to be big and get hot, and now they aren't and don't.
They've been used to great success since we had the needed power electronics to drive the electric trains of Europe.
I would assume the innovation here would need to be making it small and efficient for any meaningful torque output? Usually when you see claims of a 93% efficient electrical motor its the result of taking an absolute beast of a 2kW machine and operating it at 400W. Does anyone have insights into what Renault are doing here?
It's safe to say the companies are not in the market bracket, no?
BMWs have a terrible record for needing expensive repairs.
I know you shouldn’t rely on anecdote, but it seems I do.
> BMWs have a terrible record for needing expensive repairs.
For example there was that case of the car that needed an entire new sealed €5k battery controller because it was in a minor crash and blew a fuse.
My garage charges 50% more for labour on EVs. I'm sure part of that is price discrimination but I bet part is also because working on them is more difficult. I would not be surprised if they need to pay more for access to the manufacturer's diagnostic tools too, which are becoming increasingly required.
If you get into an accident or let the bmw get into disrepair via neglect, yeah it’s not cheap to clean up. Body work is expensive on any car though, and I don’t have sympathy for people who own higher-end cars and don’t take care of them, they deserve to pay the price for it.
What other wear and tear things are expensive?
Recently, there was a problem with the engine misfiring but it was $200.
LA, California
However, comparing prices between cars nowadays is a complicated matter. BMW's iX1 and iX2 (they use the BMW EESM motors) theoretically cost about €55k, but they have been very recently available to lease for about €250 euro per month - so pretty much for the same price as the cheapest electric Renault if leased.
Renault has also been thumbing China recently for undermining EU manufacturing as well [0] while China has returned to using Wolf Warrior diplomacy against Europe [1][2][3][4] using the same rhetoric that the Trump admin uses.
Of course, under the Xi admin China's foreign policy has always viewed the EU as inferior and a has-been [5] and has become an active participant in the Ukraine War [6][7].
Europe might not be able to trust the US, but it can't trust China either.
[0] - https://www.reuters.com/world/china/renault-ceo-asks-eu-enco...
[1] - https://www.globaltimes.cn/page/202605/1361926.shtml
[2] - https://www.chinausfocus.com/finance-economy/dear-brussels-d...
[3] - https://www.globaltimes.cn/page/202605/1362161.shtml
[4] - http://news.china.com.cn/2026-06/10/content_118541873.shtml
[5] - https://fddi.fudan.edu.cn/_t2515/57/f8/c21257a743416/page.ht...
[6] - https://www.reuters.com/business/aerospace-defense/russians-...
[7] - https://www.pravda.com.ua/eng/news/2026/06/12/8039041/
What EU states are now lobbying for is if BYD wants to sell an EV in the EU, it should include European originated parts. Just assembling a knockdown kit in Hungary whose parts were all manufactured in China is not "Made in Europe". If BYD or MG wants to sell a BYD or MG car in the EU, they should source the battery pack and powertrain from the EU.
Alternatively, the PRC can drop similar origination requirements from it's domestic market.
The reality is the PRC won't back down, so they will be tariffed by the EU, especially as the EU has lost patience with the PRC due to their active involvement in the Russia-Ukraine War [0], attempting to use diplomatic immunity to kidnap a French national [1], and attempting to embargo the EU's rare earth imports [2].
Additionally, it's easier for the EU to push back against China versus the US while also winning brownie points in the US.
[0] - https://www.reuters.com/business/aerospace-defense/russians-...
[1] - https://www.lemonde.fr/societe/article/2024/07/02/deux-espio...
[2] - https://www.reuters.com/business/autos-transportation/china-...
Is that why Renault EVs (R5, Twingo) are wholesale developed in China? Doesn't seem very ex-to me, more an in- type of strategy.
Sharing platforms isn't something EU manufacturers are opposed to, but they do not want to be dependent on Chinese supply chains. That is the crux of ExChina, especially as the majority of an EV's value is derived from the battery and powertrain.
Still, presumably Mercedes ambitions are for few motors than BMW or Renault.
Renault is going after the consumer market with these motors, where minimising cost and maximising availability is more important than pushing past 95% efficiency or cramming a 700kW power output in a motor that is small and light enough to fit inside of a wheel hub.
Technically the brushes can wear out, although there are claims they are good for 150,000-250,000 miles it seems.
Brushless DC motors don't arc -- because they switch stator polarity with electronics that sense the position of the rotor without rubbing parts. (They can also fine-tune the stator current spikes to make the motor very efficient over a wide speed range, which brushed DC motors cannot do.) The lack of arcing is more important than the fact that they don't have rotating contact points.
Brushed AC motors have rotating contact points (slip rings) but they don't arc (ideally), so the contact points don't degrade as fast as brushed DC motors do. But they do carry a lot of current because their purpose is to energize the rotor. Brushed AC motors are not ideal, but making an AC motor "brushless" is not nearly as big a win as making a DC motor brushless.
Wait. You're saying DC motors require current that's constantly switching polarity? So they're sort of really AC internally?
Yep. All motors require constantly changing current. The distinction between AC and DC motors is whether you feed the motor externally with current that is already alternating sinusoidally, or whether the motor itself turns external DC into some kind of AC.
Kind of interesting for a professionally branded company to use "..." like that
You are unlikely to see a vehicle with sodium batteries until after that happens, and it needs to be significantly less than LFPs as you Na batteries have more weight per Wh. I believe they also have a shorter lifespan (but not NMC short). Edit correction, looks like CATL is promising 15000 cycles, which is much longer than LFPs which usually come in at 7000 to 10000.
It seems far more likely to me that if the Na prices tank, you'll probably first see them deployed as grid and home battery solutions.
CATL already put sodium ion in cheap cars. And there are other benefits to this type of battery like a wider range of operating temperatures that cover essentially all of the extreme temperatures you'd find in the arctic and the hottest deserts.
I would not be surprised to find some of these batteries in big semis a few years down the line when the cost benefits make the space/weight sacrifices worth the trade off.
But you are right that domestic and grid storage are also going to be huge use cases.
EESMs are primarily manufactured by European OEMs (ZF, MAHLE, Schaffler, AEM) and their Indian JV partners (Sona Comstar, Sterling, and the India branches of the OEMs listed). Both have been blocked via export controls from accessing battery tech from China over the past few years, and a major reason for the push for EESMs was for an ex-China supply chain, especially after China began export controlling rare earths to the EU [6].
Additonally, Chinese and American EVs tend to use PMSMs unlike European and now Indian EVs. Also, the EU is cracking down on automotive exports (cars and OEMs) from non-FTA states as part of the EU Industrial Accelerator Act (which btw has made China go ballistic [2][3][4][5]).
On the other hand, they will most likely use Japanese or Korean solid-state batteries as Idemetsu Kosan is in the process of mass producing them [0][1] as is LG [7], and both Japan+SK are FTA partners with the EU.
[0] - https://www.chiyodacorp.com/en/projects/solidelectrolytefaci...
[1] - https://battery-tech.net/battery-markets-news/idemitsu-kosan...
[2] - https://www.globaltimes.cn/page/202605/1361926.shtml
[3] - https://www.globaltimes.cn/page/202605/1362200.shtml
[4] - https://www.globaltimes.cn/page/202605/1362161.shtml
[5] - https://www.ft.com/content/5903318c-319b-426e-b05d-062f7620f...
[6] - https://www.reuters.com/world/china/eu-lawmakers-rebuke-chin...
[7] - https://blog.lgchem.com/en/2026/03/25_solid_state_battery/
Advantages:
- Not subject to the price and supply chain volatility of rare earth permanent magnets.
- For highway dominant drive cycles, the cycle efficiency of EESMs can be higher than state of the art IPMSMs. EESMs tend to have their best efficiency at moderate torques and high speeds because of their excellent field weakening characteristics. I tend to think that they would be a good fit for application in class 8 trucks or as auxiliary motors in automobiles with two powered axles.
- The output torque doesn't necessarily decrease with rotor temperature. In IPMSMs the permanent magnet flux linkage decreases with rotor temperature.
- At least theoretically, with proper control, it is possible to operate EESMs with unity power factor and decrease the kVA rating of the stator inverter.
- If there is a stator inverter fault, there are schemes to denergize the rotor which have some safety implications.
Disadvantages:
- DC current needs to be transferred to the rotating field winding. For automotive applications this tends to be done either with brushes and slip rings or brushlessly using a high frequency transformer with a rotating rectifier. In either case additional power electronics and other components are needed for the field power transfer and control which reduces some of the potential cost savings of the elimination of the permanent magnets. If brushes and slip rings are used with oil spray/oil jet cooling of the rotor they need to be sealed in a separate compartment. I am a little surprised that Renault has stuck with brushes and slip rings versus an inductive high frequency transformer solution. I think this has limited their power density.
- For very torque dense machines, cooling the rotor field winding is challenging, and in my opinion is best accomplished by oil spray/oil jet cooling.
- It is difficult to reach the same maximum speeds as IPMSMs in an automotive package size. The rotor field winding retention system to keep the field turns from moving into the airgap at high speeds needs considerable attention during the design.
- The overall axial length of the non-active region of EESMs is typically longer than IPMSMs because of the field winding end turns and field excitation system.
- EESM efficiency is dominated by the manufacturable slot fill of the field winding.
- High performance current/torque regulation is considerably more difficult.
High performance EESMs have been used in aerospace generator applications for decades, albeit with a different rotor excitation system than what is used in automotive applications. Renault (and their supplier Continental) really led the commercialization of EESMs into automotive mass production. Now BMW has followed suit and multiple suppliers have EESM designs (Mahle, ZF, etc.) GM had a really nice EESM design and high frequency transformer excitation which they published back in 2014. My colleagues and I built several generations of EESMs as part of U.S. Dept. of Energy projects (https://www.osti.gov/servlets/purl/1837809) and I think they have their place as EV traction motors for certain applications.
You can switch a motor without permanent magnets to "idle mode".
I understand in Tesla dual motor configurations, the front motor is without magnets. The excitation field will be turned on when you need extra power, but at crusing speed it does not cause extra "drag". From one teardown I've seen, they even went so far to use cheaper and less efficient IGBTs for the front drive, and more efficient SiC Mosfets for the rear motor (in the same vehicle!). If you need extra acceleration briefly, lower efficiency can be accepted.
(I have a Renault EV and it’s excellent. Aside from the motor technology, it’s relatively light, has a heat pump as standard, and a good-sized battery).