Step 8: Mechanics and Materials

Now we're getting to the mechanical design. Let's lay the ground rules for what you might need or have access to.

Rotor Material

Your magnets would like to be contained in a material which offers low resistance (reluctance) to the magnetic field, and also does not magnetize itself permanently in the presence of the magnets. Many high performance alloys of nickel, cobalt, iron, and trace metals have been invented to optimize the magnetic properties of a motor. They're expensive, require specialized heat treatment, and even specific machining processes to conform to the geometry of a magnetic machine.

We're not going to bother with that. The most common rotor material for hobbyists is just plain steel tubing. It does a good enough job, and the best part, it's cheap and readily available.  I will list sources of steel tubing in the Resources section, but as a general rule, the tubing you purchase should be:

low carbon or "mild" steel. High carbon and heavily alloyed steels have significantly worse magnetic properties.

� seamless or at minimum DOM style tubing. This is the majority of steel tubing, but keep an eye out regardless. DOM tubing has a more uniform wall thickness and no ugly weld seam to affect the roundless. It is generally made to tight tolerances. Avoid cast iron pipe.

� plain finished. A precision ground or machined and polished finish will not do you any good unless it's already precisely the diameter you need.

� Oversize (OD larger than your rotor's outside diameter) AND undersize (ID smaller than your magnet mounting surface) so you can machine it to suit and not worry about hitting the limits of your materials' physical manifestation.

Endcap Material

Since the only thing which has to support a magnetic field in your motor is the rotor, the endcap and other structural elements only have to be mechanically sound. That means you have way more choices here. Generally, it's some kind of nonferrous (not steel) metal.

� Aluminum is the number one choice. It's light, strong, easy to machine, and common. Not exactly cheap in "big", however.

� Plastics! Engineering polymers such as nylon, polycarbonate, acetal, and polyethylene in high density and high  molecular weight varieties all exhibit high strength and lightness. Plus, plastic machines like... well, plastic. Easy to shape, especially if you are new to machining.

Some plastics let your motor have the magical see-through effect. The BWD Scooter uses Lexan (polycarbonate) side plates so you can see the robot in disguise.

� If you are into that stuff, you could conceivably craft endplates out of fiberglass or carbon fiber panels. The ultimate in light weight and stiffness, but be aware of the fact that you have to attach it to the can somehow. This will be addressed shortly.

Center Shaft Material

The most important trait of the shaft is that it can't bend. I'll address shaft design shortly, but you should expect to make the shaft from some kind of metal. Larger hub motors use steel, smaller ones may be aluminum. I used an aluminum shaft on Razer's motor for weight savings and ease of machinability.

� Aluminum should be limited to the aircraft alloys: 6061, 2024, 7075, and similar. These offer higher strength than other aluminum grades.

� You can get away with a mild steel shaft such as the low 10xx alloys (e.g. 1018, 1020), but if you are already using steel, moving up to a medium carbon or alloy steel shaft wouldn't hurt. Very low alloys (1006 and similar) do not machine well - they are actually too soft to finish machine finely.


Let's be honest: a motor is a precise alignment of opposing magnetic fields. Invariably you will need access to machine tools to make them. Unless you are very crafty with your shop drill press and Dremel and can make things conceptric to within 5 thousands of an inch (0.005", or around 0.1 millimeters!)  constructing the endcaps and rotor (and shaft, and stator mount...) will require access to...

� A metal lathe. Not a wood lathe, where you hold the tool yourself, but a metal lathe. If you have made it this far, I assume you know how to operate such a machine already, because giving machining lessons over Instructables is slightly troublesome. 

You will need the ability to precisely bore an inner diameter. Boring bars, or something which can function as them, are a must.

� A milling machine, or at minimum, a drill press with X-Y table and fixturing & indexing abilities. This can be a full size Bridgeport or similar, or a miniature hobby mill like those found at Harbor Freight. Basic tooling should be available. You should have a spindle drill chuck to precisely drill holes with at the least.

� Some kind of vise. Handy to have for pushing in bearings and cans, and also for holding the stator while you wind it! Extra leverage will only help in winding.

� Measuring calipers, micrometers, dividers, etc. Because several parts need to fit closely with one another, you must have metrology tools. I get by with a single digital caliper.
<p>this is one of the absolute best instructables I've ever read : mainly because it is actually instructive !!</p><p>great work</p><p>for my part I'm interested in the topic because I'd like to make an electric skateboard with hub motors : options are cheap Chinese ones I'm afraid to waste money on, custom made ones that are quite expensive (250$ apiece+shipping) and now I think it might be worth going the DIY way if I can get my hands on an acceptable stator...</p><p>in fact I think the best would probably be to make 2 with lower power to make them easier to wind and especially to heat less...</p><p>thanks again for the inspiration</p>
<p>Hey there I just recently found this instructable and first I want to say, nice work! I did want to <br>point out that in step 2 Kv and Km are inversely proportional. I.E. Km has units <br>[V/(rad/s)] and Kv should be [(rad/s)/V], which means when you multiply <br>Kv by your battery voltage you get a rough estimate for top speed. What you calculated by substituting your equations is that the back-emf constant Ke is equal to the torque constant Kt, which is true. You refer to both of those as Km.</p>
<p>End-Turn also contributes to Torque. Anything that increases magnetic flux desity in the cores also increases the torque.<br></p>
<p>Hi</p><p>I wanna make an hub motor inside an longboard wheel.</p><p>How should I build it?</p>
<p>great write up, thank you!</p>
<p>great job</p><p>can i control this motor using arduino ? and if i can do that how could i do it ?</p>
<p>Hi, great job.</p><p>what if regenerative braking concept. in the sense do we get back emf in BLDC motor.</p><p>if yes what if you harness the power from it</p>
<p>I am not good in electricity, but you explain very well, thanks!</p>
<p>I learned something A=heat that's why ev are high voltage.</p>
<p>This looks so cool, but after I read the why not to build list it got a little stale. </p>
<p>how large a motor would I have to build to power a bike light consisting of approx 50 led lights?</p>
<p>Could you just clarify for me the &quot;AC&quot; part of this whole thing.</p><p>Does the controller simply emulate the commutator of a DC motor, energizing the coils in such a way as to create a rotating magnetic field or are you saying it also converts the supplied DC to a true alternating current which it distributes to the coils in such a way as to produce a rotating AC field?</p><p>Thanks</p><p>Doug</p>
hi. i'm from tunisia. good job man !. can you contact me on jakefouly@gmail.com i need some help. thank you
<p>hello guys. i am new member. hope to could find good things for better future for succeses. tx all</p>
<p>Issues with your formula:</p><p>You pick Mevey's 2.30 equation:</p><p><strong>T = 2 * N * B * Y * i * D/2</strong></p><p>Where N is the number of turns per pahse, and '2' is the number of active phases. Then you redefine N as the number of turns per tooth, and define m number of teeth per phase. So (previous definition of N) = (new definition of N) * m. The result equation:</p><p><strong>T = 2 * </strong><strong>m * </strong><strong>N * B * Y * i * D/2</strong></p><p>Not four but two!!!</p><p>Next, in Mevey's 2.30 equation D is the diameter of the coil centers. Not stator's outer diameter, but diameter of the coil centers which is obviously smaller than stator's outer diameter.</p><p>P.S. Thanks for the article anyway. It is really motivating!</p>
<p>the most in depth view about brushless motors awesome saving this page for future guidence cheers</p>
<p>Hi, </p><p>good math derivation, T = 4 * m * N * B0 * (t / (t + g)) * L * R * i, <br>but this is exactly the double the torque you calculate using what reported on <br>LRK Motor Analysis Worksheet<br><a href="http://www.femm.info/examples/lrk40/lrk-bldc.pdf" rel="nofollow">http://www.femm.info/examples/lrk40/lrk-bldc.pdf<br></a>T = 4 * (rr+rs)/(rs-rr) * Br * N * L * t * ( I - I0)</p><p>where this formula is calculated taking into account that just two phase current are active on a trapezioidal drive, and that the current to be used should be just the active current (total current less free running current).</p><p>Which formula is the correct one???</p>
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
<p>Or better, using correct parenthesis position:<br>T = 4 * m * N * B0 * (t / (t + g)) * L * R * i<br>but furthermore:<br>- for &quot;i&quot; you have use the useful portion of total current, i.e. &quot;i&quot; - free running i zero (taking into account the eddy current and friction losses);<br>- for &quot;R&quot; you have to intend the medium radius between stator and rotor (where the magnetic force act) which is in the middle of (t+g)=magnet thickness + air gap zone.</p>
Hi mate I'm making a hub motor for a skateboard, 100kv 15turns in wye dlark with 6 strands 0.34mm wire (this is all I can fit)<br>80mm diameter wheel, 50mm diameter motor, 40.7mm x30mm stator 12t 14p, with 40SH magnets 30x7x3, air gap between 0.5-0.7 depending on tolorance.<br>Will be using 29v batterys 8ah 30c.<br><br>I want to get up a hill of about 10-15% grade at about 20km/h<br>70-90kg. How much power do I need, is my wire cross section ok?<br>Iv been struggling with the math, and worried about the new winding not having the current capabilities I need, but it's hard to fit the copper inside of the stator, but maybe I'm just not good at it!<br>I'm also using hall sensors or optical sensors soon to get better start up as I seem to get a lot of cogging! From even a small push!<br><br>Each skateboard is using 2 motors on the rear.<br><br>I really need help!<br>My email is jacob.bloy(at)gmail.com<br><br>My build page.<br>http://endless-sphere.com/forums/viewtopic.php?f=31&amp;t=65636&amp;sid=32c74875705d1d55d0801eeae1381c11
<p>So I have a question about the equation to measure the theoretical torque. T = 4 * N * B * L * R * i in my case would be 10 turns per phase, 52 for the neodymium magnet and assuming you measure things using the imperial system the length of the stator would be 19.8 inches and the stator radius 3 inches. Putting through 42 amps would theoretically give me 5189184.0 torque. Now this can't be accurate because at 1500 RPM that would give me 1.4821e+6 horse-powers just to put things into perspective, which is a insane amount of horsepower.<br>t = 4 * 10 * 52 * 19.8 * 3 * 42 - Where did I go wrong in the equation?</p>
<p>If we rewire series connection of coils to parallel i.a.w. <a href="http://www.thebackshed.com/windmill/FPRewire.asp" rel="nofollow">http://www.thebackshed.com/windmill/FPRewire.asp</a> without changing position of hall sensors, does it influence on steering algorithm? How to include this wiring difference in one equation(e.g. torque equation)?</p>
<p>Excellent instructable! Thanks for taking the time to document and share your work. I need clarification on (at least) one topic. In the section discussing Magnet Length the author states:</p><p>&quot;Optimally, the magnet length is equal to the stator length (<em>L</em>).&quot;</p><p>In the same section, magnet width is mentioned. Would someone clarify for me the magnet dimensions that should be used for a given stator? Specifically, what is the stator length (L)? A diagram would be especially useful.</p><p>Thank you.</p>
<p>Jah mahn, danke mahn, huge, this is monster info bro.</p><p>Thanks you!</p>
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<p>You mention that you used 2 x 22AWG wires instead of 1 x 18AWG wire because it was hard to wrap and bend for 25turns. You said &quot;Use the wire gauge table to compare diameters!&quot;, now 22 AWG wire is 0.644mm in diameter and 18AWG wire is 1.024mm in diameter. So 2 x 0.644mm is 1.288mm and thats well over the diameter of the 18AWG wire. Now 24AWG wire is 0.511mm in diameter, and 2 x 0.511mm is 1.022mm which is a lot closer to 18AWG. I don't want to be annoying i'm just confused. If we compare the surface areas though 18AWG wire is 0.823mm^2 and the closest pair that would measure similar is a pair of 21AWG wires, at 0.411mm^2 x 2 = 0.822mm^2, BUT neither of those are the wire you said you were using. Should the diameter of the multiple strands not add up to close to the diameter of the single wire? Thanks for any help, just confused.</p>
<p>Hi, I thought I'd jump in here and clarify a few things for you: When doing anything with electrical wire, especially when said wire is going to be carrying a significant percentage of its maximum safe current, you should be aware that the determining factor in current capacity is the cross-sectional area of the conductor, not the outside diameter. Since the area increases faster than the diameter or circumference, a wire of half the diameter will have one fourth the cross-section and thus one fourth the current capacity. A wire 1.024mm in diameter has a section of 0.82 mm^2, where a wire of 0.511mm diameter has a section of only 0.20mm^2. A 0.644mm wire has a section of 0.32, which means a pair are up to 0.64, close to the original 0.82. If I were doing this, I'd use three strands of 22, for a section of 0.96, better than the 18 gauge.</p><p>TL;DR don't go by diameter, go by cross section. They don't equate directly.</p>
<p>Thanks for the formulas to make my motor, but your math was off by .01 on 3.66/(4*4*10.9*0.03 *0.035)=19.98.</p><p>it is really 19.99 (19.986)</p>
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<p>Being one of the last electrical and electronic engineering graduates from my school, before they dropped the &quot;electrical&quot; part, electric machines have always been a favourite subject of mine. This 'ible is one of the best I've ever read. Excellent work. </p><p>Incidentally, you can get tyres made by the guys who can retread forklift truck wheels. They vulcanise the tyre onto your own hub. </p>
<p>Being one of the last electrical and electronic engineering graduates from my school, before they dropped the &quot;electrical&quot; part, electric machines have always been a favourite subject of mine. This 'ible is one of the best I've ever read. Excellent work. </p>
<p>Oh my, that's a lot of work and thanks for putting it all up here.</p><p>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: </p><p><a href="http://youtu.be/RV9fvg3C_fo" rel="nofollow">http://youtu.be/RV9fvg3C_fo</a></p><p>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.</p>
very interesting very ( ty iv bine tring to find info on moters like this )( o and I Quote &quot; Their large outrunner motors are inexpensive enough to consider cannibalizing for stators. &quot; <br>well LOL!!! ) thank you for this it was very help full. :)
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
No not plastic. The material has to have a high permeability to concentrate the magnetic fields and at the same time reduce Eddy Currents.
I wonder if a motorcycle stator from the magneto would make a nice stator for a brushless motor? Used they are not too expensive.
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.
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. <br><br>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&quot; - 6&quot; - 12&quot;) stators.
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 :-(
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.
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, &quot; It builds dexterity!&quot;
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
very nice and educative. learned a lot from this.
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. <br> <br>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? <br> <br>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. <br> <br>https://www.instructables.com/id/6-AXIS-ROBOTIC-ARM/ <br> <br>check it out. <br> <br>and vote for me <br>
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.

About This Instructable




Bio: lol robots
More by teamtestbot:How to Build your Everything Really Really Fast Chibikart: Rapid-Prototyping a Subminiature Electric Go-Kart Using Digital Fabrication and Hobby Components The New and Improved Brushless Electric Scooter Power System Guide 
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