May 8, 2017

EMRAX Motor 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 was donated to MIT's EVT, passed along to CRC, where it was briefly involved with a big battlebot.  This model (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 out of reach of normal silicon MOSFETs or IGBT's of sufficient current and voltage rating.  So it's a great motor, but essentially useless 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.  Not sure why exactly.  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!