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Make Your Own Miniature Electric Hub Motor

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In-wheel electric drive motors represent an effective method of providing propulsion to vehicles which otherwise were not designed to have driven wheels.

That is, they're great for EV hacking and conversion. They're compact and modular, require no support of rotating axles from the parent vehicle, and can be designed around the vehicle to be propelled. Pure DC electric hub motors, in fact, were used in some of the first electric (and hybrid electric) cars.

They are also not as complex and mystical as one might think. The advent of my project RazEr, a stock Razor scooter with a custom built electric conversion, has raised many questions from amateur EV builder looking to construct their own brushless hub motors. Until now, I have not had a single collective resource to point anyone towards, nor have I been confident enough to understand what I actually built to write about it for other hackers.

Hence, I will attempt to show that a brushless DC permanent magnet hub motor is actually relatively easy to design and build for the hobbyist, resource access considerations aside. I will first exposit some of the details of brushless DC motor theory as applied to hub motors. I will provide some thoughts and pointers about the mechanical construction of the motor itself and how to source major components. Finally, I will briefly glean over ways to control your newfound source of motion. The arrangement of this Instructable is designed for a readthrough first - because it relays theory and advice more than specific instructions on how to create one particular motor.

This is intended as a basic primer on DC brushless hub motors. Many assumptions, shortcuts, and "R/C Hobby Industry Rules of Thumb and Hand Waves" will be used. The information is purposefully not academic in nature unless there is no way to avoid it. The intention is not to design a motor that maintains above 95% efficiency across a thousand-RPM powerband, nor win the next electric flight competition, nor design a prime mover that will run at constant power for the next 10 years in an industrial process. Motor theoreticians avert thine eyes.

I will assume some familiarity with basic electromagnetics concepts in order to explain the motor physics.

Below is an exploded parts diagram of a prototype motor that I am in the process of designing and building. Let's clear up some of the vocabulary and nomenclature immediately. The can (or casing) hold a circular arrangement of magnets (electrically called poles) and is supported on one or both ends by endcaps. This whole rotating assembly is the rotor. Internally, the stator is a specially shaped piece of laminated iron pieces (the stack) which holds windings (or coils) made of turns of magnet wire on its projections (teeth). It is stiffly mounted to the shaft (a nonrotating axle) which also seats the bearings for the rotor assembly.




 
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juliajohn3 months ago

I really love your
write-ups guys continue the good work.

http://www.zapelectricianbrisbane.com.au/

steveastrouk5 months ago

Being one of the last electrical and electronic engineering graduates from my school, before they dropped the "electrical" part, electric machines have always been a favourite subject of mine. This 'ible is one of the best I've ever read. Excellent work.

Incidentally, you can get tyres made by the guys who can retread forklift truck wheels. They vulcanise the tyre onto your own hub.

steveastrouk5 months ago

Being one of the last electrical and electronic engineering graduates from my school, before they dropped the "electrical" part, electric machines have always been a favourite subject of mine. This 'ible is one of the best I've ever read. Excellent work.

Melia Esperas6 months ago

Oh my, that's a lot of work and thanks for putting it all up here.

I was looking for a motor I could pass my leg though instead of using a ring gear and a small motor to rotate it off to one side. The open motors would be used to rotate segments of a leg roughly depicted here:

http://youtu.be/RV9fvg3C_fo

I'm still working out how many segments and at what angle and speed each segment should rotate at for the maximum comfort of the rider while still providing a good, natural leg motion. Seems making the motor would be beyond my capabilities and I'll have to settle on the ring gear driving by a motor or the like.

very interesting very ( ty iv bine tring to find info on moters like this )( o and I Quote " Their large outrunner motors are inexpensive enough to consider cannibalizing for stators. "
well LOL!!! ) thank you for this it was very help full. :)
ckizer11 year ago
Can the stator core be plastic? Does it need to still be magnetic at all? I dont' understand why you wouldn't build it out of something completely non-magnetic
valveman ckizer111 months ago
No not plastic. The material has to have a high permeability to concentrate the magnetic fields and at the same time reduce Eddy Currents.
valveman11 months ago
I wonder if a motorcycle stator from the magneto would make a nice stator for a brushless motor? Used they are not too expensive.
colt0112 months ago
Is it possible to melt down many cores in a foundry and then cast my own core? The core I need is huge and would cost a lot of money to have it machined and cast by a specialized group. Especially when I will need at least 3.
teamtestbot (author)  colt0112 months ago
Stators are not cast. If you look at one, you'd notice they are many thin and fine layers. Each of those actually are insulated from each other.

A cast stator would basically be a big magnetic brake and would be extremely inefficient and heat up quickly due to eddy currents. I think you should look into motorcycle alternators and washing machines for large-ish (5" - 6" - 12") stators.
ckizer11 year ago
Where can we buy one of these motors (not the wheel) as a kit to put together and learn? It's easy to get the wire, but not the pieces the wires get wrapped around :-(
lgabor1 year ago
it looks to me that the torque should be proportional to the square of the radius. At constant magnet induction and current density the force per unit circumference length would be constant so the torque would be proportional to the radius and the length of the circumference, in turn proportional to the radius , hence the radius square.
Ottoclav1 year ago
Im an electrician, and house wiring is done in 14, 12 and 10 gauges mostly. Winding a motor in 18 gauge must be a chore! But im sure chris farley would say, " It builds dexterity!"
asdqweasd1 year ago
Torque should definitely depend on current. So, it should be T = 4 * m * N * B0 * (t / t + g) * L * R * i instead of T = 4 * m * N * B0 * (t / t + g) * L * R
That's a lot to read but I read it anyway I can't make one of these. Because I don't have the tools nor the supplys to build it but awesome job
antoniraj1 year ago
very nice and educative. learned a lot from this.
ElvenChild2 years ago
So there is probably something stupidly wrong with what I am about to write, but I am tired and can not get this idea out of my head, so on with it.

What is to stop you from taking a standard dc motor, like the ones used in toy scooters, and reinforcing the !#$@% out of it, namely in the (casing? or is it a shell?) itself and the axel, welding a rim to the (reinforced) casing of the motor and using that as a hub motor with the motors axel acting like the axel of a bike wheel, with everything revolving around it?

Would the motor just plain not have enough torque?, or is there some other blatantly obvious issue that I can't think of?
here is a mild example of your concept and a solotiuon using a standard dc brushed motor as an axle or pivot point. and having to add a gear reduction to it to get it to move.

http://www.instructables.com/id/6-AXIS-ROBOTIC-ARM/

check it out.

and vote for me
You would not have enough torque, even on scooters with small diameter wheels the motor is usually geared down at least one to five, on a bike you will need a gearing of at least four times that! I hope this helps.
Sort of like this, or this.
image.pngImage 2.png
67spyder1 year ago
On this page you have a picture of some small car hub motors. Can you tell me where the came from?
san0905912 years ago
how much cost for four wheeler hub motor?.
notingkool2 years ago
i have a question (yes, i read the entire instructables). If the thickness of my magnetic field isn't determinant can i build the stator as thin as i want? or there is some equation that relates the power (Pe) with the area of stator across each coil?
because when i study electronics there was a equation for transformers that related that.
i'm an electronic technician, but i never design a brushless motor (sorry for my english man, i do my best)
Thanks.
Ganhaar2 years ago
Has anyone investigated the use of motorbike / scooter magneto stators for converting the bldc motors? The smaller / basic bikes without alternators appear to have magnetos with stators that look very similar to what is needed for the centre of a bloc motor. There seems to be heaps of cheap 18 pole and 8 pole magneto stators and a few 12 pole stators. From the bike sites, it seems these fail fairly often on bikes, so there would be heaps of old stators being thrown away.
teamtestbot (author)  Ganhaar2 years ago
Absolutely - this has been done by a few people, most notably:

http://wattsdottime.blogspot.com/ (both as motor and as a generator)
http://amymakesstuff.com/2011/06/07/pf-hub-motor-complete-mostly/

The only downsides to the bike stators are their large bore and narrow teeth - can't stuff as many windings on it as you otherwise would be able to, and the teeth potentially will saturate earlier. But, those are all parameters you can design around.
karlpinturr3 years ago
Here's a (very simplistic) thought - just how important is it that these "electrical steels"/"transformer steels" contain silicon as an alloy?

What I'm thinking is laminating very thin steel sheets with very thin sheets of silicon to mimic the effects you outline... Or maybe just laminate the steel with a silicon adhesive...

There'd probably be a minimum size beyond which you'd lose too much efficiency, but would larger, slower-turning, motors be feasible?
Where would you get these "very thin sheets of silicon"?
Sorry, my bad. I put the idea down as it came, without checking up, so I don't even know if they exist... - that's partly why I mentioned the adhesive.

Thinking about it now, anything thin enough would probably not be available in small-enough quantities to be affordable for the DIY'er (yet, anyway).

So, we're back to the adhesive - and you'd probably need 100% pure silicon (like aquarium sealant is, I think).

 Maybe an idea for someone else to tinker with?
You must be thinking of silicone, a group of polymers with a high silicon content but entirely different physical properties from the element.

It's a common mistake.

I believe the idea is to make the sheets of an iron-silicon alloy as the silicon decreases the electrical conduction within each sheet while the sheets are coated with some kind of coating (the perfect sort of thing to coat something with) to prevent conduction between the sheets. you could use silicone, but I think saran wrap or wax paper would work just as well if you didn't have any lacquer, shellac, or spray paint.

But if you did use extremely thin sheets of steel and insulated them from each other I'm sure it would still work much, much better than a block of steel.
You can get something called "shim stock" in thicknesses down to 1 or 2 thousandths of an inch (0.02 to 0.04 mm) at an auto parts store or from a machinists supply.
That's about half the thickness of a piece of paper. they're pretty flimsy so if you went this route you'd probably want to put thicker sheets on either side to maintain the shape.

Now all you have to do is make the poles/spokes/teeth. We're talking about hundreds of sheets for one motor. You can cut the stuff with scissors but we're talking about hundreds of sheets for one motor! I don't know how many times you'd have to sharpen them.
If you have access to a laser cutter you're set.
Otherwise I'm thinking a whole bunch of squares with a hole in the center. Two big end blocks and a bolt down the center. Then to the lathe and the end mill or better yet a wire EDM machine.

BTW shim stock is also available in brass, copper, plastic and possibly aluminum. None of these will do. It has to be steel. Iron would be better but I've never heard of iron shim stock.
The insulating layers between the Steel/Iron sheets is to reduce losses from eddy currents. The key is to get as high a density of iron in the area as possible with as little possibility for electrical conduction as possible. I am wondering if a fine grain iron powder mixed with a two part epoxy resin could be used.

From my experience the epoxy makes a very good insulating layer around basicly everything it touches and with a fine to small grain iron powder could get a fairly decent density of Iron in the mixture.

Using this aproach you could actually cast the stator with a small mould and a basic vacume rig. Advanced, but not impossible for a DIY person.

I am curious to hear any criticisms of the idea.
I tried this, started by making a sample piece of the stuff. Magnets stuck to it like steel, no electrical conductance, machined well. Seemed perfect. Made a mould for a rotor, cast it, made ali hub wheel castings wound it all up then the big test day came. Not a cracker of movement. No cogging. Back to the test piece of epoxy iron, wound up a coil like the old nail test. Yes should have done this first. Not a hint of an electric field. Turns out the iron epoxy is magnetic in a sense that a magnet sticks to it perfectly, but does not conduct a magnetic field. Im sure there's a more technical term and description, but no it doesn't work.
Did a bit more research and there are some uni's doing research on sintered rotors for high speed electric motors where eddy currents become a bigger loss. They have developed some cast materials but these seem to be mostly rare earth sintered materials that are much harder to fabricate individually but might be an option in a decade or three if the idea catches on and the rotors start to get mass produced.
I really like your idea, sadly i don't have the experience to know how it would fare. Any thoughts on using a high iron content ceramic? Molding might be more difficult, though. Either way, your idea puts this closer to the realm of the DIYer
You're right - I was making that mistake, thanks.

And you're right about the "hundreds of sheets" - another reason (or hundreds of them!)  the average back-yard'er would have a horrible time...

Still, it was an idea that might have been worth pursuing, and at least I've learnt from the discussion.

Thanks again.
biolethal2 years ago
I have a couple of these: http://www.powerditto.de/18N20PKuniinrunner.html
could I rewind these (or leave them in their current state) and use them in an electric vehicle?
keithbrook2 years ago
That was a tour-de-force on direct drive motors.

Good man!!

It's stuff like this that needs to be cherished as the dwindling supply of smart people heads towards zero.

Thank you.

Or

'Blagodarya Mnogo', as we say in Bulgaria.
Great Job; Two thumbs up for this instructable. I feel a bit sorry for I have only the words of thanks and nothing else to give. You really deserve an award for such a comprehensive study that you have done on BLDC motors. I salute you and wish you all the best of luck throughout your future endeavors.
Respectfully Yours; Naeem from Pakistan
alan brown2 years ago
I know this might be heresy, but I love the motor but have no time to build one.
Does anybody know where I can get a miniature electric hub like this one for sale?
Thanks!
madomonaem2 years ago
like
bykeon2 years ago
Your instructable was very well written and the highly technical portions were simplified so even I could understand it. I do however have one suggestion that might assist in mounting bearings, True, the surfaces must have closer tolerances and do not handle coaxial loads due to the inner and outer race design. In the machine tool trade it is common, when possible to leave the shaft in a freezer overnight, or failing that spraying with a pure CO2 or nitrogen, while leaving the bearing under a heat lamp, again overnight. The shaft will contract of course, while the bearing will expand, allowing very gentle taps to a bearing set tool to seat the bearing. Often bearing to shaft tolerences to a 30,000 rpm spindle is .02mm of the bearing specs in an Okuma spindle. When mounting the bearing and shaft to the outer bearing, the heat lamp to the casting is used to assist the shaft / bearing placement into the casting. Because of the high speeds NGK bearing grease is used in precision amounts (30 cc for example) and is heat resistant. I'm not sure how this will affect normal grease as it is not specifically designed for high speed high temperature enviroments, This negates any possible coaxial "bumps" to the bearings. Hope this may help in future.
bykeon2 years ago
This is easily my next construction project. After a year of look-ee loo, this is the one that peaks my interest enough to post a big thank you. Good instructions and easy to follow. Thanks!
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