[HugoW]

Hi,

(question below)

I am an avid RC car enthousiast and I tinker on (real) cars a lot, too. I am toying with the idea of converting my MX5 to EV for a long time. To be honest (and clear in andvance) for me the technical challange and the journey are more important than the result. This might be weird, but true.

A little while ago, I put an airplane brushless outrunner in an RC car. This worked to a certain extent, but it missed the finesse of the sensored motor. So, I mounted sensors, with an amplifier circuit, to the motor. It took a bit of tinkering, but I made it work. This got me more interested in making my own motors and (arduino-based) controllers. And although not outperforming the store bought stuff (my race car has no diy electronics in it (yet)), I made a brushless inrunner, sensored, and a mosfet based controller. Jeee... I blew up both in quite a lot of smoke, too, BTW, but that is besides the point. This made me think. The motor I made is considerably bigger than would fit the RC car, for the simple reason it was easier to make at that 'scale'. I could make it smaller, a next version, to fit the RC car. But could I scale it up even more and use it for the MX5? My dream would be two 50kW (peak) motors each driving one rear wheel, making the differential function a software part on the arduino.

So for the question, what are the up and down sides to using such a sensored brushless inrunner for a real car? From what I googled / found on youtube, the Tesla model 3 motor has permanent magnets, but not so many, and other 3 phase motors don't have any at all. What am I missing?

Cheers,

Hugo

 

[brian_]

So for the question, what are the up and down sides to using such a sensored brushless inrunner for a real car?

A "brushless DC" motor is just an AC motor, not using sinusoidal power. All EVs use sinusoidal power; square-wave or trapezoidal is just for small stuff. "Inrunner" (powered stator on the outside, PM rotor on the inside) is the normal configuration for motors; although there are few specialized EV motors which are outrunners, most outrunners are in model/RC aircraft or bike hubs. EV motors generally use rotor position sensors.

From what I googled / found on youtube, the Tesla model 3 motor has permanent magnets, but not so many, and other 3 phase motors don't have any at all. What am I missing?

Actually, the vast majority of production EVs have permanent magnet 3-phase synchronous AC motors. Tesla was late to switch from induction to PM, but they are switching all models (yes, starting with the Model 3, but now the Model S and all coming models are PM). The best-known EV motor is the one in the Nissan Leaf, and it is a PM AC motor.

The Tesla Model 3 motor is a completely conventional interior permanent magnet synchronous motor, with normal size and number of magnets. Due to some bull spouted by Elon Musk, some Tesla fanatics became convinced first that it was going to a switched reluctance motor, then that it was a PM-assisted synchronous reluctance motor... but that's not true. More than one Model 3 motor has been torn down and examined, confirming the conventional IPM construction. It's a perfectly good IPM motor, though.

The only other popular choice for EV motors is 3-phase induction. That was popular a decade ago for limited-volume conversions (such as those by Azure Dynamics), that's what Tesla used before the Model 3 (and still has in some positions of some models), and there are some other recent vehicles using induction as well (such as some Audi models).

DIY conversions traditionally used brushed DC motors because that's what could be salvaged from old forklift trucks. Companies selling motors for conversions started with brushed DC motors (such as from NetGain), then 3-phase induction motors became popular (such as from HPEVS), and now the most popular choices are to salvage a PM AC motor from a wrecked production EV or to buy a PM AC motor (including from the same NetGain, who now sell the HyPer 9 in addition to their old stuff).


So, overall there are lots of up sides and few downsides to sensored brushless inrunners for real cars, so that's what most of them use. On the other hand, no one builds these motors themselves.

 

[HugoW]

Hi Brain,

thanks for the quick and clear reply, sorry to have missed it. I must change my setting as I didn't get an answer-notification. I meanwhile did some searching based on your input. I have decided (sort-of) to go with axial flux BLDC motors, one magnet-disk and two coil-packs each, and still one per rear wheel. I am now trying to find logics in the black art of winding coils; turns, wire gauge, voltages, currents, magnet sizes, etc. Will probably come down to an educated guess for the first proto...

BTW, I realize it is silly to build my own motors, but it is what I want to do. The obvious route for this car would be a Model 3 drive unit (motor and diff), but that is not the object of the exercise.

Cheers,

Hugo

 

[Frank]

IIRC, someone over on endless-sphere built his own motor. I'll post a link later if I can find it.

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[Frank]

Actually, there is a fair amount of discussion over there. I'd recommend looking around the motor technology sub forum.

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[Frank]

This is the specific thread I was thinking of: APL's DIY axial-flux motor - Endless Sphere but there's tons of info over there.

 

[brian_]

... I realize it is silly to build my own motors, but it is what I want to do. The obvious route for this car would be a Model 3 drive unit (motor and diff), but that is not the object of the exercise.

It's not silly if it achieves your goals.

 

[brian_]

I have decided (sort-of) to go with axial flux BLDC motors, one magnet-disk and two coil-packs each...

I'm not challenging your choice, but I think it's definitely worth looking at the YASA (Yokeless And Segmented Armature) design, which inverts that to put a rotor with magnets on each side of a single stator (and the stator only needs small SMC cores instead of a laminated structure); even if you used conventional laminated cores the yoke part is still omitted. The design with a stator on each side of a single rotor certainly works - some Motenergy models use this design, although I've never heard of a motor at automotive scale being built this way. All variations have their advantages and challenges.

 

[HugoW]

Thanks, guys, great stuff. I goes to show that when English isn't your first language, not knowing what words to google is a handicap. Your info is not only usefull on the technical side, reading your posts and links helps my vocabulary and makes me find even more interesting stuff!

Cheers,

Hugo