Repolarizing ceramic magnets.

[tcmtech]

This is probably a simple one but I have yet to ever try it on purpose. Can I change the polarity orientation of ceramic type permanent magnets by brute force electromagnetic power?

Reason being I have a few big servo motors that are presently 8 pole designs but they use four ceramic magnets set up with both poles on the same side to make the eight fields.

I want to change these to four pole motors by changing the double pole side by side polarization setup they currently have on each magnet to be of a single front and back polarity configuration.

These are large Getty's servo motors that list for around $2500 - $3000 new so I would rather not kill one on a whim.

I have the capacity to generate a multi KW magnetic field per pole in the motors themselves for the new polarity configuration. I just dont know if it will properly reshape the ceramic magnets polarity layout by doing so.

 

[dknguyen]

You'd have to heat them up past their curie temperature, expose them to a very strong magnetic field at that temperature to realign the magnetic domains and let them cool while being exposed to that field to lock in that alignment.

 

[Sceadwian]

Umm, I hate to be obvious here, but flip the polarity of each pair physically... I'm not sure about the physical construction but couldn't you take two motors and swap odd poles to get the desired results?

Home made electromagnet creation has been discussed here before, the heat required and the electromagnetic power required to be applied in a very specialized manner is not in the realm of sanity able to be done on a small scale.

If you think you can do better, you're wrong. In order to slam the temperature during pulsed magnetic field to freeze it will likely cause thermal fracturing of the material. It's much like heat treating metal, it has to be heated to X degrees for X period of time, and cooled slowing to X degrees while the magnetic field is being applied.

 

[unclejed613]

i could be wrong here, but i think ceramic magnets are truly permanent. they are made from what is called a "hard" ferrite, which refers to the magnetic properties, not the material hardness. hard ferrites have a very high remanence. i have yet to see a ferrite magnet lose it's field strength like i've seen metallic magnets do. as has been previously mentioned, you would need to heat it beyond it's curie point, and then cool the magnet to below it's curie point with a field applied. the heating process alone can fracture the already brittle ferrite material.

ferrites are magnetized during the bonding and curing process, so even the above method may not be very efective.

 

[tcmtech]

Umm, I hate to be obvious here, but flip the polarity of each pair physically... I'm not sure about the physical construction but couldn't you take two motors and swap odd poles to get the desired results?

I thought about that being it is very obvious but how do I free up the magnets from the motor case without damaging them? I am uncertain of the thermal limits of epoxy that they used to hold them in place and how hot I would have to get it before it would let go. Also the possibility of cracking a magnet from thermal expansion concerns me as well.

Home made electromagnet creation has been discussed here before, the heat required and the electromagnetic power required to be applied in a very specialized manner is not in the realm of sanity able to be done on a small scale.

Sanity never was part of my realm and size of scale is relative to what toys you have access too!

For me creating the electromagnet is easy. I just have offset the brushes so that the rotor is in a magnetic stall and put many hundreds of amps DC through each relative rotor pole using one of my big welder power supplies. 70 volts at 700+ amps should easily put the rotor into full magnetic saturation.

This is a Getty's servo motor that weighs around 100 pounds and is purposely built to take peak loads of several hundreds of amps for short durations without damage.
The specs sheets say that at around 350 amps they will demagnetize the magnets so I was wondering what happens at double that current?

My curiosity was if that much power is put against a ceramic magnet that is already half way polarized the right way could it reverse the other half of the polarity as well?

Perhaps I will have to do some experiments with some old speaker magnets first and see if I can force one of them to change polarity and if so what strength can I get out of it afterwords.

 

[Hero999]

Obviously the epoxy won't be able to stand the curie temperature but are you talking about exposing the magnets to a huge field in order to repolarise them by brute force?

I don't know if that will work.

What about simply heating the assembly until the epoxy breaks down and regluing the magnets in place?

 

[Sceadwian]

I should have realized they were potted, a lot of motors are. The only method I can think of that has any possibility is chemically dissolving the epoxy, there is a pretty wide variety of epoxies though and the chemicals may be difficult to obtain. I'm not much of a chemist so I'm not sure what to suggest, and any chemical is gonna be nasty and messy.
Thermal expansion of the magnets is going to cause them to crack long before the epoxy breaks down. Unfortunatly the motor doesn't look like it was meant to be modified =)

I've tried to do this myself, albiet on much smaller magnets, never could find anything that touched the epoxy, and broke the magnets.

 

[tcmtech]

I did some testing this morning and found out some interesting information.
Apparently its rather easy to re polarize ceramic magnets!

I have had no trouble rearranging the poles and number of poles on some old and farily large ceramic speaker magnets, 6" OD 3/4" thick, just using my big N42 neodymiums so apparently the brute force method works rather well!
I dont have a Gauss meter but the hanging screws test says they are noticeably stronger too!

Given that a good electromagnet can produce magnetic fields far greater than a N42 neodymium I am now rather confident I can re polarize any ceramic magnet if I have enough power.

Once again some hands on play time shows theory needs a little follow up work.

Try it and see for yourself. Force the like poles of a ceramic magnet and a neodymium together for a few seconds. Where they touch will reverse the ceramic magnets polarity in that spot. If you use a pair of big neodymiums and move them around while pressing them together on the ceramic magnet from front and back you can make the whole ceramic magnet change its polarity from front to back.

By using several neodymium magnets you can put multiple alternating poles around the sides as well and change the ceramic magnet from an axial polarization to an alternating radial polarization.

 

[Sceadwian]

Very cool stuff tcm, I'll have to remember that, does make sense after the fact though since the magnets are getting their strength from the retentivity of the material. The strength of neodymium magnets nowdays is nutty.

 

[tcmtech]

It takes a little practice to get the neodymiums to line up right and put the poles where you actually want them. If they are off to far from each other or ones backward the first thing they do is scramble the original polarity and you get strong spots and weak spots of mixed polarity's on each side of the magnet which makes for a basically weak and nearly useless ceramic magnet.

I will have to set up a big electromagnet now and see what I can do with that. Its well worth experimenting with though!

I am rather glad I didnt take the theoretical advice that just assumed nothing would happen.

 

[Hero999]

Yes, I agree, don't just listen to people here who are often wrong, myself included.

 

[Sceadwian]

It's not theoretical advice; it's just someone's personal opinion from their knowledge and experience, no one ever stated otherwise. Theory is tested. DK's suggestion was based on them being rare earth magnets not ceramic, the rest of us were just basing our responses on conjecture. If you weren't abnormally intelligent you wouldn't even have the understanding enough to understand what was going on.


Just a note, big electromagnets are... well.. BIG Don't get carried away as ceramics based on their makeup have a finite ability to store the field you impress upon them. Small coils pulsed repetitively would probably do better on a retentive material than a really big electro magnet might, if the applied pulsed field were that much stronger.

 

[tcmtech]

If you weren't abnormally intelligent you wouldn't even have the understanding enough to understand what was going on.

Its not very often that 'abnormal' and 'intelligent' get used in the same sentence when referring to me!


I have been pondering on the power limits as well being the problem with fairly large permanent magnets and big electromagnets is that the physical parts need to be well secured or stuff goes flying or gets broken easily. I have worked around the big scrap yard magnet cranes enough to know how high powered electromagnets can send any type of magnet flying with considerable force.

I can generate insane magnetic flux densities with highly pulsed CD type air core coils but I am aware that at some point the ceramic magnets physically wont respond to any additional input. What I am reading is that a slower longer duration of applied power is nessisary to get more of the internal magnetic structure to re align. The new pole locations sort of have to soak in for several seconds of sorts.

Plus as long past experience has taught me unsecured magnets in close proximity to high powered pulsed electromagnetic fields make surprisingly effective scatter guns if not properly secured! Good thing my neodymiums have rounded edges otherwise I could be needing a few band aids by now.

The simple testing I did earlier seems to indicate that getting a newly located pole to have any power does require some degree of precision as to where and how the stronger magnetic field is applied and then removed. Otherwise when I would move to a new spot to make anther pole it would sort of erase or weaken the first one if I didn't move the magnets strait in and out or if I overlap the two pole locations.

I have more reading and experimenting to do yet but I have a fairly good idea that simply overpowering the old poles with a much much higher magnetic field for a short period is all that I will need to do to re orientate the magnetic structure of the ceramic magnets themselves. The rude crude brute force method looks to be a viable option so far. It may not be the ideal way to re polarize a magnet but it should get me close enough for what I am doing.

 

[Sceadwian]

That being said, aim for a warm room when you're doing it =)

 

[unclejed613]

like i said, i could be wrong about it...

i have never had any experience of a ceramic magnet losing field strength, let alone being able to reverse it, so i stand corrected... a lot of the information i saw indicated that ceramic magnets are polarized during the formation process.

 

[Hero999]

I just tried this with some flexible sticky magnetic tape and some rare earth magnets and it worked perfectly.

The magnetic tape is magnetised with four alternating poles across the strip. I cut a piece off, sandwiched it between the two powerful magnets and it became re-polarised with one pole either side of the strip.

So all you need to do is expose the magnet to a much stronger field and it will re-magnetise accordingly and there's no need for any heating.

Come to thing of it, heating isn't required for magnetic recording tape or discs so why should it be required for any other material?

 

[tcmtech]

I cant say I have ever had any ceramic magnets loose noticeable power either but then its not something I have ever paid much attention to as well.

So far with the bit of experimenting I have done there seems to be an obvious relation between the size of the ceramic magnet and the size and strength of the other magnet as to how effective the re polarizing becomes.
My little pencil eraser sized neodymiums can't put a dent in the old ceramic speaker magnets but the 1/2" x 1" and larger N42 neodymiums seem to be able to shift the pole locations around with fair results. That tends to make me think that a powerful enough electromagnet should be able to easily re polarize any size of ceramic magnet and possibly neodymiums too.

I never thought about the magnetic recoding media but that would have been an obvious indication that magnetic material can be re polarized at room temperatures or colder.

 

[Hero999]

I think you shouldn't worry about weakening the magnets.

Going from what little I know about magnetics, all you should need to do is apply a strong enough field to push the material to the other side of its hysteresis loop and the field will suddenly reverse and be just as strong as before, but in the reverse direction. I suppose I could be wrong, but it makes sense to me so I'll stick with that unless I'm proven wrong.

 

[Sceadwian]

Come to thing of it, heating isn't required for magnetic recording tape or discs so why should it be required for any other material?
Because not all materials derive their magnetic properties in the same manner.

Try to re-polarize those super strong neo magnets. It's not the same mechanics involved but it's much like a basic semiconductor diode. You can hit it with .2 volts for a billion years and it'll never conduct. give it a few extra tenths of volts and it'll go to being willing (if not able) to conduct hundreds of amps.

It's only become possible to buy neo magnets of sufficient strength in recent years, at least reasonably. I'm going to call these years the tcmtech years. Where the acquisition of technology places that technology in the hands of people that might accidentally do something great, rather than the same technology only 5 years ago being something that was only available in a laboratory.

 

[tcmtech]

As soon as I come up with a solid and high powered electromagnet design that can push several KW's I am going to attack a few of my neodymiums and see what it takes to re polarize them. (Or shoot one through a wall! )

Granted their Currie point is low enough that if I have to I will just heat them to slightly over that temp and the put the power to them as they cool down and drop back below it.

My intentions are to make a few more induction motor to PM alternator conversions with re polarized neodymiums if I can. Being able to reorientate the pole locations on common stock sizes and shapes will make it much easier.