Magnet charging polarity - Page 1
Forum section: electronics / beginners

author: ZeroResistance
author_rank: Frequent Contributor
timestamp: November 05, 2018, 05:29:00 pm
content: For recharging a permanant magnet for the setup as shown in the imageIf we give a coil current that generates a magnetic field in the coil as the same polarity as the magnet that is N on the top and S on the bottom, will the magnet be recharged to a field supporting its existing magnetism? Or will the polarity of the magnet be reversed, I mean it will get reversed charged forming a S pole on the top and N pole on the bottom.And in what case will the magnet get demagnetised?

author: SG-1
author_rank: Regular Contributor
timestamp: November 07, 2018, 02:18:20 am
content: If the polarity of your field is the same as the magnet it should reinforce the magnets field. Google how magnets are made for some ideas.Many years ago I would use a micro-ohm meter & a coil of wire to magnetize screwdrivers & such. It would pulse 100DC Amperes.Now I just use a powerful permanent magnet to magnetize driver bits. If that makes things worse the bit or screwdriver can be struck against something hard, the shock will remove most of the magnetism. Heat will also demagnetize. I have no idea how hot it has to get. I have run alnico magnets through a fluidizing bed where powder insulation is melted onto copper bars with no noticable effect on the magnet. Different materials would probably require different temperatures.Neodymium magnets are suggested for use to remagnatize old bar magnets. I would think it possible to change the polarity.

author: Zero999
author_rank: Super Contributor
timestamp: November 07, 2018, 10:31:37 am
content: As mentioned above, if the field generated by the coil is the same polarity as the bar magnet's field, it will reinforce it.If the current is reversed and the field generated by the coil opposes the bar magnet, the magnet will be ejected from the coil, if it's not securely fixed in place. If the magnet is fixed in place and can't move, then the field generated by the coil will cancel it out to some extent. The coil will need to generate a much stronger field, than that of the bar magnet, to permanently reverse its polarity.

author: ZeroResistance
author_rank: Frequent Contributor
timestamp: November 09, 2018, 08:09:58 am
content: Thanks for your replies,I would have thought that a strong North pole in the coil will induce a South pole in the magnet at the top end, thus reversing its polarity. However that doesn't seem to be the case from your answers.Many thanks!

author: xavier60
author_rank: Super Contributor
timestamp: November 09, 2018, 08:44:33 am
content: Quote from: ZeroResistance on November 09, 2018, 08:09:58 amIf the coil is wound over the magnet, then the North pole of the coil has to match the North end of the magnet to recharge it.I used to recharge magneto magnets by discharging a large capacitor into about 50 turns.

author: ZeroResistance
author_rank: Frequent Contributor
timestamp: November 09, 2018, 11:47:41 am
content: Quote from: xavier60 on November 09, 2018, 08:44:33 amInteresting, what voltage you used to change the Cap to? and what value of Cap?Did you recharge Alnico magnets?

author: xavier60
author_rank: Super Contributor
timestamp: November 09, 2018, 12:10:43 pm
content: It was 1000uF charged to 300v, so a lot of caution is needed. These are very old magnetos, I think the magnets predate Alnico type.http://www.vintage-motorcycle.com/image.php?image=346dsc07123.jpg&width=425

author: ZeroResistance
author_rank: Frequent Contributor
timestamp: November 09, 2018, 12:41:19 pm
content: Quote from: xavier60 on November 09, 2018, 12:10:43 pmWo Wo Wo, thats around 45 joules!!What kind of coercivity were you looking at for those magnetos, i guess Alnico is around 600 Oe

author: xavier60
author_rank: Super Contributor
timestamp: November 09, 2018, 01:09:07 pm
content: Quote from: ZeroResistance on November 09, 2018, 12:41:19 pmI don't understand much about how magnets work.These old magnets are some sort of steel alloy that lose their magnetism when the magnetic path is broken such as when the magneto is disassembled for repairs.Normally a big heavy magnet recharger is used,https://www.google.com.au/search?q=magneto+magnetizer&client=opera&hs=kio&source=lnms&tbm=isch&sa=X&ved=0ahUKEwjcwcGyr8feAhUPVH0KHXfJDC8Q_AUIDigB&biw=1280&bih=669

author: wbeaty
author_rank: Contributor
timestamp: February 05, 2019, 10:39:09 pm
content: > remagnetize bar magnetsUse a neo magnet to put an "S" pole in the middle of the bar magnet, and "N" at both ends. Paint it white, with both tips painted red (rather than the classic half-red-half-white bar magnet.)Show the students that you have a monopole bar magnet, where both ends repel the "N" of another magnet, and both ends attract the same compass tip!

author: Maxjunior123
author_rank: Newbie
timestamp: September 09, 2023, 11:48:17 pm
content: I have a question I am trying to make something called flip dots but I am having trouble with magnet and how to flip polarity are certal mineral in the magnet make them more easy to change polarity and if so what should I be looking for and what should I try. This is the flip dot I am trying to makehttps://youtu.be/dn72Fezs_jM?si=0wYAZ9BYuP9UuVk8

author: MrAl
author_rank: Super Contributor
timestamp: September 10, 2023, 02:08:29 am
content: Hello there,It is interesting that there are a lot of articles on magnets and magnetism and magnetic fields, yet the N and S poles are more rarely talked about. It's all about which end of the magnet the field lines emanate out from which end they go into. One is called N and that is because it 'seeks' the north pole of the earth, and the other is called S.To be sure you have it right here is one simple experiment you can perform at home.Be careful not to use too big of a battery or you could end up burning something or melting the wire or something else damaging.First, wind a coil of wire around some cylinder like a dowel or something. Pay attention to how you wind it as to where you start the winding and end the winding, and what way you wind it, clockwise or counterclockwise as viewed from the top of the dowel, and mark the top of the dowel and top of the winding.Take a brand new soft steel nail such as a 10 penny nail and place it inside your coil. Connect a small battery or other current source such that the positive terminal connects to the top of the coil, and negative to the bottom of the coil. That should magnetize the nail to some degree depending on the number of turns of the coil and the current that is developed in the coil.Next, take a fine sewing thread and tie it close to the middle of the nail and maybe a little dap of super glue to hold it there, making the thread around 2 or 3 feet long (or around 1 meter maybe).Then, hold the loose end of the thread so that the nail is suspended in mid air.Wait until the nail stops rotating and see which direction the head points to. If it heads due north, then the head must be the N pole, and if south, then the point of the nail must be the N pole.Now that you know the three things:the direction of the turns of the coil,the connections of the coil to the battery plus and minus,the N and S pole of the nail,you can then correlate that such that if you want to re-magnetize a magnet you just have to insert it into the coil or a newer larger coil in the same way as the nail, and make the connections the same, and if a new coil wind it the same.You can also do this with a steel sewing needle but then you have to float it on top of a water surface or something to get it to rotate, maybe after sticking it through a small piece of Styrofoam.I outline this procedure because there are so many articles that also get it wrong or leave out important details. By doing this you can ensure you get it right and you have all the details to understand all the polarities and how they relate to the N and S poles.I've also seen several articles and videos by professors who neglect to show the entire experiment setup before the video is made, so they end up showing us absolutely nothing. You have to know all the details for something like this. This also happens with videos of experiments to show how the Lorentz effect works, and they end up showing us nothing.

author: BILLPOD
author_rank: Frequent Contributor
timestamp: September 10, 2023, 01:54:34 pm
content: Google 'Right Hand Rule Of Magnetic Fields' and you will find many articles and videos that explains a lot.

author: CaptDon
author_rank: Super Contributor
timestamp: September 10, 2023, 02:21:48 pm
content: Years ago I worked with a very special type of three phase motor that had permanent magnets attached to the rotor (18 I think) and a three phase wound stator. It was driven by three push pull half bridges. When one or more of the half bridges would fail (usually with exploding MOSFETs) the magnet polarities would change in weird random ways and the motor would no longer turn. We could detect this failure in the motor by connecting 3 channels of a 4 channel scope to the stator and spinning the rotor by hand. If the magnets were blitzed the output waveform would be very wrong in waveshape and wave amplitude. We hand built a pulsed 'remagnetizer' that you placed against any two adjacent magnets and pulsed it. I was surprised at how easily the magnets polarity could be changed!!! There were 36 capacitors 100uf charged at 325 volts and when a bridge blew it dumped the total stored energy through one of the phases.

author: MrAl
author_rank: Super Contributor
timestamp: September 10, 2023, 03:13:12 pm
content: This was an old thread from 2019 somebody recently hijacked.

author: Vovk_Z
author_rank: Super Contributor
timestamp: September 10, 2023, 09:01:01 pm
content: Quote from: MrAl on September 10, 2023, 03:13:12 pmBut the good thing is that electro-magnetism hasn't changed much since 2019 so it is still an actual and an interesting topic to read.

author: MrAl
author_rank: Super Contributor
timestamp: September 11, 2023, 01:10:17 pm
content: Quote from: Vovk_Z on September 10, 2023, 09:01:01 pmYes, but someone broke in after 2 years and asked a completely different question. Would have been better to start a new thread i think.I don't mind talking about EM either.