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Old May 05, 2015, 09:44 AM
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how do YOU chose magnets?

It's hard to find info on what's optimal.
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Old May 05, 2015, 11:25 AM
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You won't find info on what is optimal because every situation is different. Here's my design procedure. (I'm assuming here that you are talking about an outrunner and you are talking about using individual sintered neo magnets).

I start with basic rules of thumb - 1) air gap in the 0.5 to 1.0 mm range, 2) magnet thickness (in the direction of magnetization) to air gap thickness ratio of about 6 to 8, 3) full pitch, 4) arcs (not rectangular), and 5) Ni-Cu-Ni coating.

All these assumptions will be adjusted as the design progresses, so you can't stop there.

I don't pay attention to max energy product - BHmax - because it really isn't all that useful in motor design. More important are remanence (Br) and intrinsic coercivity (Hci). After you have the general shape from all the rules of thumb above you'll want to pick a magnet that gives you approximately 0.8-0.9 T flux density in the air gap but also doesn't saturate stator steel. So I would estimate what the hottest my magnets will get and then look at the BH curves for that temperature and use my estimated permeance coefficient to pick a magnet grade that has a remanence that gives me an air gap flux density in the 0.8 - 0.9 T range. (Are you following this? If not you need to read Hanselmann's book as somebody else suggested. Ignore the math if you need to - just the concepts will get you a long way.) Intrinsic coercivity is harder to estimate so I usually just start with a mid-range Hci (say 17 - 20 KOe) and then adjust later.

All that is basic rule of thumb stuff. After I have my best estimates at magnet size and shape, then I can run that through that into a magnetic circuit analysis for the motor and make adjustments to fine-tune the design. Once I've done that I'll move on to FEA to really fine-tune the design for certain things that can't be done very well with magnetic circuits. An example of what I mean by "fine-tune": I'll run a study to look at performance for various magnet pitches from 180 electrical degrees (full pitch) down to 120 electrical degrees (2/3 pitch) and see how the pitch affects performance. Most of motor design is making small changes like this in order to make gradual improvement to the overall design.

I'll say again that it is important that you don't stop at the rules of thumb above. You really need to take it through the magnetic circuit analysis for sure. FEA is helpful for many things but isn't always necessary (especially if you don't know what you are doing).
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Old May 05, 2015, 03:16 PM
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no way I'll be able to do that analysis. overwhelming. I'm pretty new to even electric motors and the basics of how they work. Do you think it's a safer bet to just copy what the stator was paired with or considering I'm trying to get as much torque as possible, just generalizing, do you think it would be a good idea to go for 80percent fill and curved stronger magnets. As the motor is now it has about 70percent fill
I'm sure, as you say, there are tons of variables but there's no way I'll be able to figure all that out in the short time till I order these.
any advice you have is appreciated.. please keep it in layman's terms as much as possible if you can.
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Old May 05, 2015, 04:05 PM
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Hi,
highest torque can you get by using a wedge shape magnetsystem or a Hallbach system . And you need thips open slot stator where you can slide the prefabricated coil from outside in the slot. every second teeth has a coil , that is named separet phas section SPS winding. With 12 slot stator you need 14 magnetpole and with 24 Slot a 26 magnetpole system .
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Old May 06, 2015, 12:20 AM
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thanks for that writing learningrc. I've been rereading it and wondering what you mean by "full pitch". You use the term twice and the second time I think you're meaning the maybe simply angling the magnets in the rotor...but I doubt that as I've never seen it.. sounds like a good idea though. I'd like to quickly, if possible, skip through that book you recommended to to see if I could find answers. I just got two books from the library two days ago and they're more generally about electricity. One's by Faraday "experimental researches in electricity" published 1855 and the other's "the electron" PRINTED in 1917. I like old books. Trying to sharpen my noodles as they've been let go dry for too many years now. Asking questions on here will have to be part-time.

thanks for the pictures Christian. the first picture is hard to see but you say every other tooth is wound and the other is bare? that is very LRK of you! Very cool. They probably aren't spotting you in the grocery store and whispering but.. I haven't looked it up much but recently linked to this while looking up hallbach
http://www.aerodesign.de/peter/2001/...index_eng.html

According to this great read the magnet fill can be up to 100 percent in the hopes of getting more torque:

http://www.instructables.com/id/Make...otor/?ALLSTEPS

The only nod he gives towards a downside to a large magnet fill:
"Oddly enough, very high fill percentages actually have a slightly negative effect towards motor performance, because the magnets become so close together they "leak" to each other. The effect is minimally noticeable for low speed hub motors, however."

Doing 100 percent fill with curved magnets in a hallbach array sounds great to me. Fat and loaded with badass curved magnets I'd glue in holding things in place simply with super strong clothes pins and duct tape. hopefully. but... Can you tell me if you think the downside to doing this are too much? Would my tacon 160 stators not be up for the challenge you think. Would I have to adjust the esc timing setting or something because of a larger back-emf?


Maybe more important though, the stator windings on one side will be pressed right against a steel wall that supports a large skirt bearing and the other side of the stator the windings will be close to a wall that will support the rotor and be spinning and also steel. Do you think these magnetic metals will negatively effect the motor's performance ? And if there'd be an appreciable effects do you think some thin MU metal foil would solve any risks?

these magnets are my second choice though (I'm hoping someone says a hallbach array is a good idea with 100percent fill and curved magnets). The pictured magnets would get almost 80percent fill where the original rotor had9mm magnets so only 67 percent fill!! doesn't sound good sounds like it could be better.
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Old May 06, 2015, 08:04 AM
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Magnet pitch is basically another name for magnet arc angle. (What you are calling "magnet fill" I believe.) Consider a motor with only 2 magnets. If it were full pitch, each magnet would have an arc of 180 degrees. A 4 pole motor with full pitch magnets would still have an arc angle of 180 electrical degrees, but only 90 mechanical degrees.

The quote about the magnets leaking is basically correct. You can and probably wil get flux leakage from one magnet to the other if they are full pitch.

I would advise against pressing the stator windings up against a steel wall. This is for safety reasons, not for magnetic reasons. The UL requirement for spacing between insulated magnet wire windings and dead metal parts is 1.6 mm (if your voltage is less than 125 V). I would suggest having a spacing of at least that between the windings and the steel wall. Or find some suitable insulator (e.g., a nomex sheet) to put between them.
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Old May 06, 2015, 03:09 PM
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I'm not deterred and am liking doing a full pitch. I'm guessing it will not as efficient running with lower loads but at WOT it would produce more torque and in the needed situation it would become more efficient.

If the only reason to not press the stator windings directly against a steel wall is purely safety related to shorting that's fine with me as I plan to further sink the stator in thermally conductive epoxy.
I'm hoping that the thermal epoxy will be able to transfer heat to the steel case well enough as I plan to have the motor sealed to air and debris.
Maybe some thin MU metal sheet would stop some eddy currents though and help keep the heat down?
A hallbach array would be another small hurdle in magnet placement.

If I'm doing what would be typically a 14 magnet rotor as a hallbach array and those 14 magnets would roughly be 34x12x3....I assume I want to replace each of these 14 with 5 magnets, and the dimensions of these five would be roughly 7x12x3?
I'd glue them outside of the rotor against each other and then glue them in.
after reading that changing the fill from what the stator teeth were intended for it could even make the motor not work!!!

With a circular hallbach arrangement I imagine I could only effectively half fill the rotor.
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Last edited by Hummina; May 06, 2015 at 08:21 PM.
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Old May 06, 2015, 11:40 PM
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Quote:
If the only reason to not press the stator windings directly against a steel wall is purely safety related to shorting that's fine with me as I plan to further sink the stator in thermally conductive epoxy.
Safety reasons are usually the BEST reason not to do something. Again, I would advise putting an air gap between the windings at the steel, even if you are using an epoxy.
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Old May 07, 2015, 07:17 AM
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Quote:
Originally Posted by Hummina View Post
It's hard to find info on what's optimal.
You can find quite a bit of info on these Neodymium magnets on the K&J pages:

http://www.kjmagnetics.com/

Look at the Neo Mag Info, Specs, and FAQ links from the navigation block on their pages.

Jack
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Old May 08, 2015, 03:38 AM
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kjmagnetics comparison of a hallbach array with other magnets is disappointing and leads one to believe a simple good back iron is by far the best. they describe them as not as strong.. they don't show a hallbach with a back iron which might be better. my other post here has a better lead. I'm trying to believe.
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Old May 08, 2015, 07:29 AM
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they describe them as not as strong..
Where? The only thing I could is a blog post about them that said they were strong but there were certain cases where they aren't the best.
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Old May 08, 2015, 10:36 AM
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Here

https://www.kjmagnetics.com/blog.asp?p=halbach-arrays
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Old May 08, 2015, 10:46 AM
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Yeah, that's exactly what I was talking about. They are great for some uses but they aren't the best configuration for EVERY application. They say there are stronger choices if what you are concerned with is the pull force to a piece of steel. Then they gave an example of what they were talking about. They then say "Remember, these arrays were originally devised to make a really strong, alternating magnetic field on one side to steer a beam of electrons, not to find a stronger way to stick to a steel surface!"

Most importantly, they didn't say anything about a motors.
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Old May 08, 2015, 11:09 AM
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But if u look at the link in the other thread I posted he gets great results
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Old May 08, 2015, 11:42 AM
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Hi,
at www.magnetmotor.de the strongest ,highest torque dens motor had a Halbach magnet system and looks like the pic in my answer #4 . With a Halbach System you can have a bigger airgap . It is also possible to build a motor with semi high statorteeth to get somthing betwheen sloted and slotless motordesign ,but with the ability to bridge bigger airgap torque will close to normal stator designes with hammerhead statorteeth.
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