Help magnets in hub motor

Doit

1 mW
Joined
Oct 24, 2018
Messages
10
Location
Naperville,IL USA
Hi all, while I was replacing the hall effect sensors I damaged few magnets inside the stator. couldn't find an alternative! I believe they about 2.2 mm thick * 13.9mm wide *28mm long. Got N50 30mm x 10mm x 2mm from aliexpress that didn't work it's very weak compared with original one. any suggestion will be much appreciated
https://ibb.co/ixAJYA
 
Doit said:
Hi all, while I was replacing the hall effect sensors I damaged few magnets inside the stator. couldn't find an alternative! I believe they about 2.2 mm thick * 13.9mm wide *28mm long. Got N50 30mm x 10mm x 2mm from aliexpress that didn't work it's very weak compared with original one. any suggestion will be much appreciated
https://ibb.co/ixAJYA

N50 are pretty close to the strongest magnets commercially available, and are too heat sensitive to be used in something like a big ebike motor without careful control of temps. From memory, an N52 started permanently demagnetizing at 80*C, but an N45 could go up to 120*C

Odds are you bought junk labeled with a false rating.

These guys are Aussie based, but say they will make custom sized and rated magnets:

https://www.frenergy.com.au/rare-earth-magnets.html

No idea what they charge though. I bought just off the shelf stuff from them years ago for a project I never started.
 
Are you sure your replacement magnets have their fields aligned the same as your old magnets?
 
Hi Doit,
I have some spare magnets (out of an scrap hub)
size approx 27 x 13.9 x 2.8mm
Have a re measure and I'll send you some if it helps.

Jonno
 
Jonno said:
Hi Doit,
I have some spare magnets (out of an scrap hub)
size approx 27 x 13.9 x 2.8mm
Have a re measure and I'll send you some if it helps.

Jonno


Just make sure they're in a large enough box not to get magnetized to the inside of a post office truck :wink:
 
Chalo said:
Are you sure your replacement magnets have their fields aligned the same as your old magnets?

This is possible but unlikely as they tend to repel each other when Sliding into place.
 
Doit said:
Hi all, while I was replacing the hall effect sensors I damaged few magnets inside the stator. couldn't find an alternative! I believe they about 2.2 mm thick * 13.9mm wide *28mm long. Got N50 30mm x 10mm x 2mm from aliexpress that didn't work it's very weak compared with original one. any suggestion will be much appreciated
https://ibb.co/ixAJYA

I can only suggest to make sure you use glue that has a high heat rating and has a very very strong bond and to make sure you 'slide' the magnets onto the rotor and do not allow them to 'snap' down into place as this will break them.
 
fechter said:
What kind of motor is it? Finding the exact right size is going to be difficult.

Right couldn't find the right size the closest I got didn't work at all. It's ebike hub motor 48V 1K W got it from ebay, voilamart is the brand name.
Thanks for asking
 
[/quote]

N50 are pretty close to the strongest magnets commercially available, and are too heat sensitive to be used in something like a big ebike motor without careful control of temps. From memory, an N52 started permanently demagnetizing at 80*C, but an N45 could go up to 120*C

Odds are you bought junk labeled with a false rating.

These guys are Aussie based, but say they will make custom sized and rated magnets:

https://www.frenergy.com.au/rare-earth-magnets.html

No idea what they charge though. I bought just off the shelf stuff from them years ago for a project I never started.
[/quote]

Sunder thank you so much for the info and the link it's pricey but looks like worth it. I'll see if the folks here might have some extra magnetics from junk ebike hub motor.
 
Chalo said:
Are you sure your replacement magnets have their fields aligned the same as your old magnets?

yes, I came across many articles saying that it's possible to upgrade hub motors by using strong magnets since the cheaper hubs don't use the best magnets for cost considerations, the only issue I'm facing is couldn't find the right size.
thanks for asking
 
Jonno said:
Hi Doit,
I have some spare magnets (out of an scrap hub)
size approx 27 x 13.9 x 2.8mm
Have a re measure and I'll send you some if it helps.

Jonno

Hi, Jonno thank you so much this could solve my problem. I'll send you PM
 
brumbrum said:
Jonno said:
Hi Doit,
I have some spare magnets (out of an scrap hub)
size approx 27 x 13.9 x 2.8mm
Have a re measure and I'll send you some if it helps.

Jonno


Just make sure they're in a large enough box not to get magnetized to the inside of a post office truck :wink:
lol
brumbrum said:
Doit said:
Hi all, while I was replacing the hall effect sensors I damaged few magnets inside the stator. couldn't find an alternative! I believe they about 2.2 mm thick * 13.9mm wide *28mm long. Got N50 30mm x 10mm x 2mm from aliexpress that didn't work it's very weak compared with original one. any suggestion will be much appreciated
https://ibb.co/ixAJYA

I can only suggest to make sure you use glue that has a high heat rating and has a very very strong bond and to make sure you 'slide' the magnets onto the rotor and do not allow them to 'snap' down into place as this will break them.

brumbrum thanks for the advice, that's how I damaged mine I'll make sure that won't' happen again.
 
Stronger magnets will give the motor a slower no-load speed. You need to try to match the strength of the other magnets unless you replace all of them. Gluing them back on is a challenge too, as they don't want to stay where they go. I used clamps.
 
fechter said:
Stronger magnets will give the motor a slower no-load speed. You need to try to match the strength of the other magnets unless you replace all of them. Gluing them back on is a challenge too, as they don't want to stay where they go. I used clamps.
thanks for the input, you right not easy to glue them back in places and could be very dangerous when they stick back to the stator and could cut someone fingers! but it meant a lot to me to open a hub motor i really enjoyed it since it's my first time and first ebike I built.
I hope someone has a junk or a useless hub motor and can sale me the magnetics.
 
Send a pm to jonno. he said he had some.
 
fechter said:
Stronger magnets will give the motor a slower no-load speed. You need to try to match the strength of the other magnets unless you replace all of them. Gluing them back on is a challenge too, as they don't want to stay where they go. I used clamps.

Why does stronger magnets increase the motor Kv if that's what is happening? It ought to just make for stronger attraction/repulsion and therefore more torque, but no speed change in my thinking.
 
I smashed the magnets in an outrunner a few years ago. I bought all new ones and had the same sort of issues...keeping the magnets in place until the epoxy set up was initially a giant pain. Multiple tries later, my solution was to use stainless steel hose clamps and tooth picks. Stainless has very low ferrous content so the magnets are not attracted to it very much.

At the time I used regular JB weld which has some amount of iron content in it. This created it's own oddities while trying to place magnets since the JB weld wanted to flow onto the magnets. The iron content is still very low and had no effect on the motor that I could determine. About a year ago, I discovered Marine JB weld. It's a good bit stronger and more resilient than the regular or quik stuff and it has no iron in it. None of the JB weld varieties are conductive that I can tell. My DMM can read up to 30 meg ohms...effectively infinitely high resistance on my meter in the liquid or solid state. Even just the "iron" had infinite resistance all by itself. Next time I'd use Marine JB weld.

Something I'd like to try is spraying the inside of the bell with electrical paint to create a thin insulating layer so the magnets are electrically isolated from each other. Skin effect over the magnets does create some amount of losses in a motor. This is also the purpose of using segmented magnets in a motor. I think eliminating any electrical path between the magnets could help to reduce the skin effect too. However, I have quite a few motors and none of them have electrically isolated magnets.

Anyway, I turned the hose clamps inside out. This puts the screw feed inside the "loop". Crank the screw outward to fill the entire bell ID. This also applies outward clamping force on the magnets. My magnets were 55mm long so I needed 2 hose clamps; one either end of the magnets. As you add another magnet turn the screw inwards just enough for another magnet to slide into place. I used regular (slow) JB weld since it took a little while to get all 14 magnets into place. I added a popsicle stick to bridge the gap for the reverse curve under the screw feeds

I coated the ID of the bell in epoxy and then put wax paper over it where the hose clamps would mash into the epoxy. That helped to keep the epoxy in place and NOT get all over the hose clamps, but it was not perfect. By the time I was done the hose clamps still had epoxy on them. Once the JB weld got pretty solid...enough to not ooze around, I pulled out the hose clamps and then removed all the excess.

I discovered a trick about JB weld...
Cold JB weld is quite thick and cures very slowly. It's also not very gooey and sticky. You can mix up a fresh, uncured batch, freeze it and hold it in your fingers. Hot JB weld gets quite runny, gooey and super sticky. When it cools off again, it cures rapidly.

What I did in 15 easy steps:
1. Make sure the ID of the bell is perfectly clean. Clean all the magnets. You want good bonding.
2. Coat the interior of the bell in room temperature JB weld. You want about a 1/16" thick layer. Enough to completely fill any gaps behind the magnets
3. Stick the bell and magnets in the freezer. You want the JB weld really thick.
4. You want to keep everything cold until all the magnets are in place. I did it in my cold garage which was 40-50F.
5. Use the hose clamps to hold the magnets in place. At first your magnets will be crooked and not aligned well. This is OK. Just get all the magnets in place under the hose clamps.
6. Your hands will warm up the magnets, bell and epoxy...stick it back in the freezer to keep things cold.
7. Once all the magnets are in place, space them evenly apart and vertical in the bell. I used round tooth picks as spacers. They needed to be pushed in place with a little effort, but were exactly the right fit between the magnets to hold them snugly.
8. Give the hose clamps another turn or two outwards to really get them tight against the magnets and to push out all the extra JB weld behind the magnets.
9. Now that everything is "locked in place", bring the bell in out of the cold. let it warm up to room temperature. The epoxy will get much softer. See if you can tighten the hose clamps on the magnets a bit more with out anything moving.
10. Set it in your oven on it's lowest setting. Get out some aluminum foil to catch the JB Weld drips. Put the bell in the oven for 20 minutes. You should see the extra JB Weld start dripping off the bell. This serves 3 purposes. Hot JB weld bonds better to things, the extra JB Weld should drip off the bell and heat gets the curing reaction into high gear.
11. Set the bell on the counter to cool off slowly. The epoxy ought to take maybe 5-10 minutes to get pretty solid. You can test it's hardness with a tooth pick.
12. Once you are confident the epoxy wont move anymore, but is not fully set up, remove the hose clamps.
13. With an exacto knife, tooth pics or some plastic implement, start removing the spacer tooth picks and any excess epoxy that is NOT under the magnets. Be careful to NOT move the magnets so they don't come loose! The JB weld will be super thick and some what crumbly. It will come out from between the magnets like thick putty and can be cut with with a razor blade.
14. Let the motor bell set for a couple of hours so the epoxy can finish curing.
15. Put your motor back together.

Note: Removing the excess epoxy is about maintaining balance in the bell and reducing weight. Without precision equipment that can tell you how the bell is out of balance, the best you can do is not leave extra weight in places that will create imbalances. IE: Remove all the extra epoxy so the bell is as light as possible and has no heavy spots in it.
 
ElectricGod said:
Why does stronger magnets increase the motor Kv if that's what is happening? It ought to just make for stronger attraction/repulsion and therefore more torque, but no speed change in my thinking.

Stronger magnets generate more BEMF at a given speed, so will equal the pack voltage at a lower RPM. Pretty much the same effect as adding more turns to the windings.

Going from ceramic magnets to NeFeB magnets made nearly a 2x difference on one I did.
 
fechter said:
ElectricGod said:
Why does stronger magnets increase the motor Kv if that's what is happening? It ought to just make for stronger attraction/repulsion and therefore more torque, but no speed change in my thinking.

Stronger magnets generate more BEMF at a given speed, so will equal the pack voltage at a lower RPM. Pretty much the same effect as adding more turns to the windings.

Going from ceramic magnets to NeFeB magnets made nearly a 2x difference on one I did.

Interesting! Have you run across any articles that talk about this effect in detail?

I bet going from N35 to N45 might add a little more kv's to a motor, but not like going from weak ceramics to strong magnets.
 
ElectricGod said:
Interesting! Have you run across any articles that talk about this effect in detail?

I bet going from N35 to N45 might add a little more kv's to a motor, but not like going from weak ceramics to strong magnets.
I'm sure there are articles out there, but I haven't looked. There must be a way to do the math to calculate exactly how much the magnet strength will affect kV. It will be a function of the magnetic flux in the gap.
 
fechter said:
ElectricGod said:
Interesting! Have you run across any articles that talk about this effect in detail?

I bet going from N35 to N45 might add a little more kv's to a motor, but not like going from weak ceramics to strong magnets.
I'm sure there are articles out there, but I haven't looked. There must be a way to do the math to calculate exactly how much the magnet strength will affect kV. It will be a function of the magnetic flux in the gap.

I suppose one could "sneak up" on an actual calculation. If you can accurately measure the strength of a set of magnets, then a given motor with whatever magnets it comes with will have a specific Kv. Remove those magnets and replace them with a weaker set that is of known strength. Check Kv again...which should now be higher. Do this 2 or 3 times more with known magnet strenghts and you will start seeing a pattern to the Kv increase or decrease. I bet it's a fairly linear change. Once a set of results are known, they can be applied to any motor and get pretty close estimations what that motor would do with XYZ magnets.

Loss of Kv...that's not really a bad problem. Do the losses in the motor increase? If I lose a few RPMs and get significantly more torque with no other caveats, I can always change my gearing a little to compensate. I rarely use hubs so adding a tooth to a motor sprocket is easy to do.

It's been my experience that the magnets are always the "weak link" when it comes to torque. You can dump more current through any phase and make the magnetic field stronger. Heat will eventually stop you, but the magnetic field in a phase can easily be stronger than the magnets it is supposed to push or pull on. The only way to re-balance at higher wattage is with stronger magnets. I've seen a lot of people throw ridiculous amounts of current at a motor to get another .2 nM out of magnets that are already at or above their limits. More current when the magnets can't pull any harder gains nothing but waste heat.

Replacing magnets is quite often about not buying a bigger motor. I understand why people want more power out of any given motor. I'm right there too! Let's pretend my current motor cost me $300 and the next step up costs $450. I'll need to abandon my current motor and then pay another $450. All of a sudden stronger magnets get to sounding really inexpensive. I'm willing to pay $140+ for stronger magnets if it gets me the torque of that $450 motor and no weight or size increase. A 4kw hub and 6kw hub are already quite heavy and then the 6kw is going to be significantly larger and heavier than the 4kw version. Stronger magnets, over watting and no weight gain in the 4kw to get what the 6kw does sure sounds appealing to me.
 
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