May 8, 2017

EMRAX Motor Teardown

** 12/5/2017 Update **
A number of people noticed that this post disappeared for a while.  Here's why.  Skip down for the actual teardown

TL:DR, Emrax is a shitty company.  They seem to have no/poor protection of their IP, and are willing to bully college students to keep pictures of their motors off the internet.

On July 4, I received the following message through the contact form on this site, typos and all:

Dear Mr. Ben,
I am a representative of EMRAX company. We have come across your web site, where you have published disassembled EMRAX motor. We want you to removie this article. We are signing Non-disclosure agreement with every our customer. I believe that also Massachusetts Institute of Technology (MIT) has signed it. Consider this agreement and remobe the article as soon as possible. I am waiting for your reply.
If you have any further questions feel free to contact me.
Hmm, sounds fishy.  I don't buy it.  My response:

This particular motor was not purchased by myself or by MIT - it was donated to the MIT electric vehicle team by a company a number of years ago, and lent to me by the the electric vehicle team.  I do not believe that I am held to any agreement in the NDA signed by the original purchaser.
There was some more back-and-forth.

thank you for your reply. NDA is valid also for any third parties that are involved. Please remove the article as soon as possible.
You have not provided me with any evidence that I need to remove the article.  At the very least, could you send me a copy of the NDA?
sorry for delayed response. We have summer holidays there days, so only a few people are working now. I have forwarded this email to our Sales department. We will look into our archive of NDA-s, because it was written some time ago. We will also send it to MIT.
I have attached our “General Terms and Conditions” – they are also published on our web site. Please take a look at the Item 15 – Intellectual Property Rights. They are also published on our website.  Article should be removed as soon as possible.
Here's the General Terms and Conditions document.  Which not an NDA.


That is not a copy of the NDA.  14(c) in the "general terms and conditions" implies the NDA is a separate document.   
Furthermore, your "general terms" define "intellectual property rights" as:
"any patents, trademarks, registered designs and all applications for their registration, copyrights or design rights or any rights similar to these rights. " 
I do not believe that photographs of the inside of the motor qualify as your intellectual property.  I am not infringing upon any patents, copyrighted material, or trademarks of yours.

After this, I don't hear anything from Emrax for a while.  I'm pretty sure they have no legitimate claim for asking me to take down these pictures, and even if they did, I'm not sure how they would enforce them.

For a little while I thought I was done with this nonsense, but then I got an email from the captain of MIT's FSAE team, saying that they were trying to purchase a custom-wound motor from Emrax, but emrax was refusing to sell to them until I removed this post.

I sent this to Emrax:
I have just received word from the MIT FSAE team that you are withholding the sale of a motor to them until I remove the article, on the grounds that my pictures violate the NDA FSAE signed when they first purchased a motor from you.  I would like to reiterate that I am not and have never been affiliated with the MIT FSAE team, and the motor I took apart was not purchased by or ever used by MIT FSAE.   
Using MIT FSAE to bully me into removing the article is unfair to FSAE, and reflects quite poorly on you as a company.
Unsurprisingly, I didn't hear anything back from Emrax, but I did remove this post, because I didn't want to screw over the FSAE team.  While it seems Emrax has no actual legal grounds for making me remove these pictures, they do have the right to refuse to do business with someone.

Now FSAE has their custom Emrax motor, so I'm putting this back online, along with the story of how shitty Emrax is.  So if you or your friends are ever thinking about doing business with Emrax, beware.


And back to the teardown...

The EMRAX series of motors are some really impressive axial-flux motors, designed in Slovenia, for applications like ultralight aircraft.  Thanks to their absurd datasheet numbers, they seem to be the favorite choice of FSAE teams everywhere.

The particular model we took apart (228LV) turns out to be particularly miserable to control.  It's combination of ultra-low resistance (1.12 mΩ) and inductance (10 uH) means you would need to switch ~100 kHz at rated voltage, in order to have reasonable amounts of current ripple just from the PWM, which is kind of unreasonable for silicon MOSFETs or IGBT's of sufficient current and voltage rating.  So it's a great motor, but not very useful unless you can afford to build a GaN-FET or SiC-FET inverter for it.  Since CRC has two of these motors, Bayley and I decided it was worth the risk of disassembling one, to learn what magic-sauce goes on inside.

Here's the motor we started out with.  There was a layer of blue tape around the outside to stop metal shavings from getting inside the motor during bench-testing.  Like any good hobby-grade motor, the rotor and housing are full of holes for weight reduction and airflow.

After pulling off the stator of the resolver, we ran into this really obnoxious locknut, which was peened over into a keyway on the shaft to prevent you from removing it.  Fortunately, the steel isn't particularly hard, and a diamond-coated dremel grinding attachment made short work of the peened over bits.

Here's the rotor of the resolver.  The bolt on the left has a tapered head, so as you tighten it, it drives the split in the shaft apart, so it grips the inside of the motor shaft.

Next step was pulling the rotor.  This is always hairy business with axial flux motors, because of the huge magnetic forces holding the halves of the motor together.  In the past, this has been done at MITERS like this, but there ended up being a better way for this motor.  First, we pulled the set-screws along the outside if the rotor.

I drilled and tapped a 5/8" -11 thread into the end of the shaft that came with the motor, and used the bolt like a crank-extractor to pull the top half of the rotor away from the steel motor shaft.

Bolt all oiled-up and ready for cranking:


And, here's the magic-sauce revealed.....

wait a second, there isn't any magic-sauce.  This is pretty much exactly what I was expecting.  The motor has a YASA-style construction, with independent segments of steel lamination and winding, all fixed to an aluminum hub.

Everything on the inside is thoroughly doused in epoxy and some red stuff I've seen in other motors before but don't know the name of:

And here's the magnet array.  Each magnet is made from 3 sub-magnets, and they're epoxy coated rather than nickle-coated.  As an astute reader pointed out, this is to reduce the eddy current losses in the magnets (see YASA analysis).  Ease of assembly, maybe?  There's no halbach array, or any other magnetic fanciness going on, at least as far as I can tell.

Before taking the motor apart, I thought there might be a hallbach array, since the entire rotor appeared to be aluminum, but no flux seemed to leak out the back of the rotor.  However, scraping off some of the epoxy between the magnets, it looks like there is indeed some steel behind the magnets:

Zoomed in view.  I can't tell if those are actual laminations, or just coarsely-turned steel, but it definitely feels like steel.

And here's the fully exposed stator.

Both halves of the rotor had an "N" scratched into the anodizing by hand, presumably to help line up the magnets on the two halves of the rotor.

Here's the aluminum rotor-ring:

Happy motoring!


  1. The red stuff is probably insulating varnish.

    1. Back in the day in the US electrical industries, we used "Glyptal 1201" which is a polyester (actually alkyd enamel) insulating paint, or "varnish" like JP said above with 1.5kV electrical resistance per 0.001 inch (mil) of coating thickness. This is probably very much like what Emrax used on the windings.

    2. HA - by "resistance" I meant dielectric strength, slip of the mind...

  2. Can you disassemble the rotor itself? it seems they use some aluminum wedges between each stator poles. I am curious how to keep the stator poles from pushing out when there is huge magnetic force from the rotors

  3. Hello,
    Can you just take a pin or a screwdriver and Check where is the center of the magnet?
    Is it only at the center of the middle magnet only or does the pin/screwdriver centers at the other two magnets too

    1. The 3 magnets are all magnetized axially in the same orientation.

    2. Thanks,
      still wondering how the flux doesnt leak outside the aluminium plate. Are the magnets mounted on pieces of iron, as the back iron is not visible?

  4. Hi, thanks for a super interesting disassembly! By any chance were you able to measure the magnetic field density on the surface of the magnets?

    1. Hi,
      I haven't had a chance to do that, but I think it's safe to assume that the magnets are grade N42 or higher.

  5. Hi
    Thanks for the pictures and the comments
    Could you confirm if there are two discs (each with 3x20 magnets) .. 2 faces
    Is it possible to have the axial thickness of the magnets
    And how many turns per coil for your opened EMRAX (228LV)

  6. The magnets are segmented in order to reduce eddy currents, see YASA's analysis.

    This motor design looks very similar to the one in the linked article. Makes you wonder what emrax is trying to protect exactly...

    1. Thanks for the link to the YASA paper. That makes sense, and also explains the coating on the magnets. I didn't realize nickle-coated magnets were so lossy.

    2. Nickel-coated magnets are much more common and easier to get. I guess that bonding them to the steel with insulating glue, like a good epoxy, would do the trick.

    3. The link below might explain why the rotor iron is so coarse.

  7. Hi
    Great work and Thanks for the pictures and explaining them
    Could you confirm if there are two discs (each with 3x20 magnets)
    Is it possible to have the axial thickness of the magnets
    And how many turns per coil for your opened EMRAX (228LV)

  8. Excellent article!
    Thank you
    Isn't it possible to remove the other face also?
    There seems to be a base/plate for the stator poles

  9. Thanks for the pictures and explaining them and Stator fixing method is unique

  10. Being somewhat of a newb why would "you would need to switch ~100 kHz at rated voltage, in order to have reasonable amounts of current ripple". Any good references ? Wouldn't there just be higher losses and bigger external components at 50 KHz ?

  11. This comment has been removed by the author.

  12. Dear Ben,First of all i have to thank for all of your hard work.Iam working on a e-motor bike project with EMRAX 208.Can you help with selection of motor(shaft and cooling type) and controller?.

    1. Hi, did you continue with your e-bike project?

  13. Hi, by any chance have you run the motor at high power or RPM? How was the cooling performance?

  14. Hi, Can anyone help me with the depreciation rate of Emrax 228 HV LC?