Making permanent magnets with high voltage - Page 1
Forum section: electronics / beginners

author: Capernicus
author_rank: Frequent Contributor
timestamp: May 16, 2021, 05:20:48 pm
content: I've got this strange idea that if I have enough high voltage dc i can conduct it through an iron spanner till it glows red hot, just before its melting, then slowly taper down the volts and itll slowly cool down and ill end up with the spanner perfectly permanently magnetized. Is this true?

author: IanB
author_rank: Super Contributor
timestamp: May 16, 2021, 05:26:44 pm
content: Quote from: Capernicus on May 16, 2021, 05:20:48 pmNo.

author: Gyro
author_rank: Super Contributor
timestamp: May 16, 2021, 05:32:41 pm
content: If you want to turn it into a magnet, then line it up with the Earth's magnetic North - South poles and hit it repeatedly with a hammer. It won't be a very strong magnet, and assumes that the steel alloy is reasonably magnetisable.

author: Capernicus
author_rank: Frequent Contributor
timestamp: May 16, 2021, 05:48:20 pm
content: Quote from: IanB on May 16, 2021, 05:26:44 pmOk, just a funny pipe dream that is then.How do they make the custom shaped magnets inside say dc motors then? (strong ones.)

author: JohnPen
author_rank: Regular Contributor
timestamp: May 16, 2021, 05:58:17 pm
content: One technique performed in the distant past was to place an iron rod in a multi turn coil ( typically an old solenoid) and to power it from the mains via a fuse ( typically 5 amp fuse) and a knife switch. It certainly worked but needed careful handling to avoid being a casualty.It would probably magnetize a spanner quite well but you are on the way to creating a Rail Gun if you are not very careful.

author: TimFox
author_rank: Super Contributor
timestamp: May 16, 2021, 05:59:59 pm
content: Permanent magnets are manufactured by passing a current pulse (from some kind of capacitive discharge device) through a coil around the magnetic material. The geometry, hysteresis curve of the material, and charge passed through the coil determine the strength of the resulting magnet.The fused current in the reply above relies on the short blowing time allowing a net current pulse with a non-zero charge flowing through the coil. In high school, we quickly ran through the supply of old-fashioned fuse wire in the physics stockroom, and found that flimsy aluminum foil worked in the fuse-wire holder. (We didn't care about the magnets, just wanted to watch the fuse blow.)

author: Refrigerator
author_rank: Super Contributor
timestamp: May 16, 2021, 06:01:38 pm
content: Quote from: Capernicus on May 16, 2021, 05:48:20 pmAFAIK speaker magnets get zapped with a high power magnetic pulse to magnetize them.Pretty sure you can google the rest.

author: Capernicus
author_rank: Frequent Contributor
timestamp: May 16, 2021, 06:04:17 pm
content: Quote from: TimFox on May 16, 2021, 05:59:59 pmWouldnt it have to be sustained energy? A short pulse would hardly do anything by itself, unless it was a repeated pulse like say out of a marx generator.

author: IanB
author_rank: Super Contributor
timestamp: May 16, 2021, 06:05:39 pm
content: Quote from: Capernicus on May 16, 2021, 05:48:20 pmI have visited a magnet factory. If you take a magnet and put it in a really strong magnetic field it will be re-magnetized. All that is required is that the applied field is stronger than the inherent strength of the magnet.For example, on the test bench was a really, really strong DC electromagnet. We had magnets that were magnetized the wrong way and we needed to change the orientation of the poles. Put one of the existing magnets between the poles of the electromagnet, turn up the field, and presto, the magnet was re-magnetized the right way.

author: Capernicus
author_rank: Frequent Contributor
timestamp: May 16, 2021, 06:07:39 pm
content: Quote from: JohnPen on May 16, 2021, 05:58:17 pmYes Im not actually planning on doing it for real yet, hi-voltage is not something I like to deal with very much, Im just wondering, cause one day I might like to manufacture my own motors, possibly.

author: BeBuLamar
author_rank: Super Contributor
timestamp: May 16, 2021, 06:09:31 pm
content: I assume the iron spanner you're talking about is a wrench. If so to make it red hot you don't need high voltage. Something like a couple of volts should be sufficient to make it red hot.

author: IanB
author_rank: Super Contributor
timestamp: May 16, 2021, 06:13:13 pm
content: Quote from: Capernicus on May 16, 2021, 05:20:48 pmSome general problems with your logic:1. High voltage DC is not what you need to make things glow red hot. For that you need very high currents, and the voltage will be rather low.2. Making things red hot destroys their magnetism. To magnetize things, you need them to be cold so they hold the magnetism.3. To magnetize things you need a strong applied field, which usually means a coil with lots of turns on the outside of the thing you are magnetizing. Sending current through a thing won't help.

author: Manul
author_rank: Super Contributor
timestamp: May 16, 2021, 06:33:27 pm
content: Maybe high voltage association comes from the fact, that a lightning strike can magnetize ferromagnetic rocks and metals.

author: james_s
author_rank: Super Contributor
timestamp: May 16, 2021, 06:35:44 pm
content: Frankly this seems like another troll thread, either that or drug induced. It reminds me a bit of the emails I started getting from a former friend when he would get high on something or other and have some amazing idea.

author: Gyro
author_rank: Super Contributor
timestamp: May 16, 2021, 06:39:26 pm
content: Quote from: Refrigerator on May 16, 2021, 06:01:38 pmYes, speaker magnets are supplied in a non-magnetised state and then the whole assembly magnetised (the high current coil method) afterwards. This both makes assembly easier, and ensures maximum flux density at the voice coil gap.

author: Robert Smith Eco Warrior
author_rank: Regular Contributor
timestamp: May 16, 2021, 06:46:26 pm
content: Quote from: IanB on May 16, 2021, 05:26:44 pmI thought I would also chip in with my 2p worth....No

author: Kleinstein
author_rank: Super Contributor
timestamp: May 16, 2021, 07:11:15 pm
content: The magnetization only need a short pulse from a really strong electromagnet. This is usually done with capacitor discharge of some kind to provide the short pulse of high current. It still needs some voltage because of the fast rise time wanted and inductance.I very much doubt one could magnetize the magnets of a speaker after they are mounted. The more normal way would be to magnetize the permanent magnet part first and only tham assemble the parts. The other way around may be used for old style Alnico magnets, but these days are long past.For just regular steel one does not need a very strong field and just a permanent magnet may be enough to magentize the steel part.

author: amyk
author_rank: Super Contributor
timestamp: May 16, 2021, 08:00:00 pm
content: Quote from: IanB on May 16, 2021, 06:13:13 pmOn the other hand, if a material heated above its Curie temperature is subjected to a strong magnetic field, then allowed to cool while in that field, it will retain the magnetism. This effect is used inhttps://en.wikipedia.org/wiki/Magneto-opticaldata storage.

author: Capernicus
author_rank: Frequent Contributor
timestamp: May 16, 2021, 08:13:28 pm
content: Quote from: amyk on May 16, 2021, 08:00:00 pmiYes that was what I was thinking, annealing more permanence into it? except I thought that the dc itself was going to do it, but apparently I am mistaken.

author: Gyro
author_rank: Super Contributor
timestamp: May 16, 2021, 08:24:51 pm
content: The point is valid - in fact this effect has been used to map the periodic reversals of the Earth's magnetic field, using Magma emerging from the mid-Atlantic trench. As the Magma cools, it locks in the prevailing magnetic field orientation.One problem you have (assuming that you slowly reduced the current to allow the spanner steel to drop below its Curie point) is that applying current to the ends of the spanner doesn't give you north at one end and south at the other. It gives you a circular magnetic field around its diameter.

author: Gyro
author_rank: Super Contributor
timestamp: May 16, 2021, 08:31:48 pm
content: Quote from: Kleinstein on May 16, 2021, 07:11:15 pmIt was / is done well into the days of anisotropic ferrite magnets, not just Alnico. I read it in a Tannoy paper (pleasedon't ask me to find it now!).I've no idea how it's done with NdFeB magnet speakers (although the incentive to assemble them in a demagnetised state would be much greater).EDIT: This paper may be helpful...https://www.coolmagnetman.com/Magnetizing.pdf

author: TimFox
author_rank: Super Contributor
timestamp: May 16, 2021, 09:55:43 pm
content: Quote from: Capernicus on May 16, 2021, 06:04:17 pmIf I remember the physics correctly, the relevant parameter is the charge that passes through the coil. That is easy to define with a voltage and capacitor.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: May 16, 2021, 10:00:43 pm
content: Not sure what high voltage was supposed to do with it... magnetic fields correspond to currents, besides, spanners have low resistance, how would you expect to drop a high voltage across it other than by arcing to it? And we can find the results of that in everyday life: normal metals aren't substantially magnetized due to welding.Useful magnets are formulated specifically for retaining strong fields (high coercivity and remenance). Most steels are poor on both of these. (Physically) hard steels tend to be also magnetically hard, so you will find nuisance magnetization on screwdrivers and pliers and tweezers and such (and yes, this even includes stainless steels -- the hard ones tend to be magnetic, and tend to retain a charge). But not really anything useful.Ye Olde classic iron horseshoe magnets need to be handled carefully, as merely increasing the magnetic path length may be sufficient to cause loss of magnetization -- that is, they're normally stored with a "keeper" plate across the face, which keeps the internal flux density high. Removing it, increases loop reluctance, reducing flux density, apparently enough to cross the opposite coercivity limit and lose magnetization. (Plotting B vs. H, a hysteresis loop is seen, H (horizontal axis) being applied magnetization (amps/meter), with the loop width being coercivity, and B being flux density (tesla, V.s/m^2), the loop height being remenance. Usually one just sees plots of full +/- scale magnetization, but hysteresis applies for smaller loops as well -- it's not that failing to use the keeper causes complete demagnetization I think, it's just that it gets walked into a smaller corner of the loop.)Alnico magnets had this issue as well ("barrel" style speaker magnets, etc.). Alnico has quite high remenance (Bsat ~ 1.2T like most steels, or is it more like 1.5? and remenance not much below there), but not great coercivity. So it works great when placed in a fixture, but not so great when removed, all by itself.NdFeB (and to a lesser extent, SmCo) has even higher Brem, as well as all the coercivity -- it's a very permanent magnet, requiring very high currents to charge (100s kA/m). Accordingly, very little of it need be used -- speakers using it can be much smaller. Motors can even hide thin strips of it within rotors, making some really interesting (highly efficient) synchronous machines.Tim

author: Capernicus
author_rank: Frequent Contributor
timestamp: May 17, 2021, 07:01:34 am
content: Thanks for being patient with me, I was stuffing up ohms law again.I guess you could do it high voltage if you want, but its not necessary, thankyou. Depends on what is easier for you with your setup I guess.Does correct use of heat improve the situation or make it worse?And I was thinking of using clay and Iron filings for the magnets, does that sound like a good idea?

author: amishasingh
author_rank: Contributor
timestamp: May 17, 2021, 08:04:17 am
content: I thinks at one single point of time magnetism cannot be induced into the metal body. You need to have the continuous procedure for some span of time to induce the magnetism

author: Kleinstein
author_rank: Super Contributor
timestamp: May 17, 2021, 09:17:43 am
content: With metallic material like AlniCo or steel you need some time to bring the magnetic field inside. Eddiy current in the metal would slow this process down. There is also a little delay in the actial magnetization change direction, but his is usually rather short delay. Applying the field much longer only increases the magnetization marginally. It's usually better to use a higher field, if only for a few ms to maybe a few 100 ms for larger conductive parts.Higher temperature may help to reduce the coercitivity. However it also reduces the magnetic field the magenet could hold. So one would have to cool the material inside the field. So for most purposes this is not really helping - it is usually easier to use a stronger field for a short time than keep a still relatively high field for the long time of cooling.Especiall normal steel has rather low coecitivity, but a relatively high Curie temperature. So one would need to quite a bit of time, while an external field to to it cold is quite low.Heating the steel may also soften the steel - not just mechanical, but also magnetic.