Help with magnetic vice from microwave transformers - Page 1
Forum section: electronics / beginners

author: Spiritual Queef
author_rank: Newbie
timestamp: August 29, 2022, 12:39:10 pm
content: Hi there,I am looking to start a small project making an electromagnetic vice using microwave transformers.Similar to this oneThe main worries I have are that I have 5 transformers and apart from two, they have different resistances in their windings. Two at 3.1, then 2.3, 2.0, 1.2.In many of the videos and instructables I have come across there has not been any mention or as far as I can tell, any consideration given to the differencesIn the resistance. However, in one or two it has been mentioned and suggested to only couple transformers with the same value of resistance in the windings.So my question is, is it actually an issue that I need to take into account? And if so, how can I accommodate for the differences?To be fair, I am not very good with electronics, I can quite comfortably wire up a simple low voltage DC circuit and could follow a schematic to create a bit more of a complicated circuit. But I have very little understanding of the functions of components much further beyond that of pre 2000 automotive electrical.So I do understand that there is a way to mitigate the potential issues between the different resistive values if its even necessary, I just wouldn't know where to start apart from adding resistors to things to make adjustments and yet still being clueless about it all.I would appreciate any advice an assistance that anyone could offer. And I am happy to answer any questions you may have.Thanks.

author: CaptDon
author_rank: Super Contributor
timestamp: August 29, 2022, 01:14:35 pm
content: First off, the magnetism is contained within the core and not externally, so the transformer won't hold anything. Second, if the transformer is energized how are you going to deal with the over 2000 volts on the secondary?? Third, M.O.T. are terrible for most any project, they run at saturation and get extremely hot and have magnetic shunts that need to be removed for most attempted projects. I do have a M.O.T. as part of a high voltage supply for a ham radio amplifier, but even with the shunt removed and some extra turns of wire added to the primary to get the transformer out of saturation it still has piss poor regulation due to the high D.C. resistance of the secondary.

author: janoc
author_rank: Super Contributor
timestamp: August 29, 2022, 03:16:55 pm
content: Quote from: CaptDon on August 29, 2022, 01:14:35 pmLook at the video. The transformer is cut open, the high voltage winding is removed and basically the guy uses two of those cut-up cores with their primaries as a large electromagnet. And powers them from 12V only.So as far as efficiency and safety are concerned I wouldn't be too worried. This is one of the few more sensible projects that I have seen a MOT used for.Quote from: Spiritual Queef on August 29, 2022, 12:39:10 pmIt would be best to use identical transformers for this because otherwise one side of the resulting electromagnet/vise will have different holding strength even if you connect the coils in series, like in the video. However, the difference between 2.3 and 2.0 ohms is probably negligible for all practical reasons. I likely wouldn't use 3.1 and 1.2 ohm ones - one side of the magnet would be visibly weaker. It may still not matter as long as it still sufficiently strong to hold you work down but YMMV. At the very least I would test the magnets before potting the entire thing to make sure it does what you want.Quote from: Spiritual Queef on August 29, 2022, 12:39:10 pmThere is literally no electronics whatsoever in this project. As long as you can wire two coils in series to a correctly sized power supply you should be good.OTOH, I am not sure I would want a vise like this. It works only for iron and steel workpieces (normal vise works for everything and doesn't need power). Another thing is that while it is energized the chips from the drilling/machining will be stuck to your workpiece instead of blown away as normal. Not great for accurate work.It will also magnetize both your work, your bits/tools, machine, metal chips ... Likely not something you want because those sharp chips will be sticking to everything even after you turn the magnet off - and good luck degaussing e.g. a drill stand!And in case the power supply fails because it didn't survive a heavy inductive load it may not have been designed for, your work will likely start spinning or fly out - that could be pretty dangerous. I would probably use an extra C-clamp at least to be safe. But in that case I could use normal vise or two clamps and save me the trouble with the magnetic vise ...

author: mushroom
author_rank: Regular Contributor
timestamp: August 29, 2022, 05:00:11 pm
content: Mag chucks/vices are commonly used on grinding machines, not on drill presses or milling machines...Years ago, I built one from a car air conditioner compressor solenoid. There's a center hole that makes it more suitable for a drill press. It worked... But I very rarely used it on the drillpress, and it never went on the Bridgeport (except while machining its top surface).BTW, I would never trust a youtuber that uses a VFD without a properly wired enclosure !

author: T3sl4co1l
author_rank: Super Contributor
timestamp: August 29, 2022, 05:05:50 pm
content: Pick windings with similar number of turns, size, and resistance. In that order. Resistance isn't very important, just means one gets warmer than the other.Should be able to count turns from the sides and at least estimate the total, they're usually packed in tight layers.Equal turns means equal magnetization given equal currents, and then you can wire them in series to ensure that.The main downside I'd think here, is the cores are rather bulky, and remember most of the holding force is between poles, not on a given pole face. The same way that iron filings (or indeed, swarf!) attract between poles, especially their edges/corners, and hardly at all in the middle of a face. So you don't get much if any holding force for parts smaller than the spacing.The core pieces are also rather tall, unless you're milling them down (basically cutting down the height where the secondary + magnetic shunts were), which is just fine to do.Do put a TVS diode or MOV across the winding, so you don't melt the switch with arcing on turn-off! Or power it from AC with a FWB, the FWB will carry the inductive current even after turn-off, no TVS needed!Tim

author: janoc
author_rank: Super Contributor
timestamp: August 29, 2022, 10:21:55 pm
content: Quote from: mushroom on August 29, 2022, 05:00:11 pmWell, I was referring to the video posted by the OP where the guy has used it on a drill press, so

author: Ed.Kloonk
author_rank: Super Contributor
timestamp: August 30, 2022, 12:31:18 am
content: Quote from: janoc on August 29, 2022, 10:21:55 pmThere is a thing known as a magnetic drill. You take the drill up into the girders and it sticks to what ever you're drilling, via a switch. The newer ones have smarts in them that detect if the magnetic conduction is insufficient and wont allow the drill to start. (Galvanised metal being a notoriously hard to stick the drill to).Seen a few mishaps with home-made magnetism and metal. Be careful.

author: CaptDon
author_rank: Super Contributor
timestamp: August 31, 2022, 12:22:09 am
content: Sorry folks, when I made my post I didn't get the youtube video link. I thought the O.P. was trying to use the transformer without the modifications. Now I see the video and what they were doing. We had one of those 'magnetic base' drills at Lord Corporation for drilling vertically aligned holes into flat metal work tables that were over 2" thick and then tapping those holes for bolting down test fixtures to the table. Cheers mates!!

author: SmallCog
author_rank: Frequent Contributor
timestamp: August 31, 2022, 11:37:13 am
content: Could you just use a magnetic door lock?https://www.ebay.com.au/itm/255019920284?mkcid=16&mkevt=1&mkrid=705-154756-20017-0&ssspo=jp7GilJXTgO&sssrc=2349624&ssuid=fd5nzGNrSmq&var=&widget_ver=artemis&media=COPYNot saying a magnetic vice is a good idea, just might be an easier way to accomplish it…

author: Spiritual Queef
author_rank: Newbie
timestamp: September 01, 2022, 02:11:07 pm
content: Thank you everyone for your input.First off, I just wanted to say that I am very aware of the restrictions and limitations of a magnetic vice/chuck made from microwave transformersAnd for my limited needs they are all within acceptable margins.Plus I have solutions to work around most of those restrictions and limitations to make it a viable option.As for the idea of using magnetic door locks, yes I would say they would work just fine and maybe even better than the transformersHowever, to get a table the size I will be making I would probably need about 8 of them which would mean spending over $200 for themAnd the whole point of using the the transformers is that they cost me nothing to acquire in order to accomplish the goal I had, as I have limited funds.My goal is to make a table for my drill press with a foot pedal momentary switch to engage/disengage the magnet to make drilling multipleholes in a single piece of work more manageable as clamping and repositioning and clamping ect for every hole is a more significant task for methan it is for most others due to my disability, so anything that I can make less labour intensive means that I can put more time and energy into the piece that I am working on at the time.I am also aware that it is not likely to be a permanent long term set-up as it will likely wear out sooner rather than later, but my hope for coming here was that I might be able to get some advice on how I can make that closer to later than to sooner.So the main things i need to know are, with the differences in resistance of the windings, is it possible to add a resistor of particular values to each winding so that they will all be the same? Would that work or would that not have an affect on the windings themselves once they are wired in series?Would wiring them in parallel be an option, and would the resistor idea work if wired In parallel?Also, Thank you for the suggestion for the TVS, I had a vague idea that something of that nature may be needed, but I have no understanding of where or when something like that is necessary.So what type/value diode would I need for this project? And where would I wire it in? Would I wire it across the positive and negative on each end of the series circuit, before and after the first and last transformers?If they were wired in parallel, would you just wire the TVS in the first position in the parallel circuit?I was going to do some tests to see if I should use a 12v or a 24v supply to power it depending on which gave greater magnetic force with the least amount of heat, but I am not sure of the current and figured I would get that answer when I did the testing, but I would be open to suggestions.I don't plan for this to ever be a magnet for use over long periods, maybe a minute, two at the most, hence the idea for a momentary switch so I could never leave it on by mistake.I have also considered adding in a thermistor to monitor things for excessive temperatures.I would not wire it as a cut-off circuit as that could be dangerous to happen in the middle of drilling something, but rather wire it to an indicator light so I can disengage it if it gets to hot.I do understand that this might seem like a lot of effort to go to for something that might not even last for too long. But I'm the sort of person that will try to give something like this the best chance for success that I can and I will try to make it to the best of my abilities in that off chance that it does last a while so that I can get the full functionality out of it while I have it working for me, rather than get frustrated with not having put my all into it when I find it needed something that I chose not to do just because of the possibility that it might not work out how I hoped.So please instead of listing all the reason why "you" would not do it, would you be willing to help "me" accomplish it. Because I am going to go ahead with it anyway so I would be extremely grateful for any assistance that anyone could give me.Thank you everyone

author: T3sl4co1l
author_rank: Super Contributor
timestamp: September 01, 2022, 06:11:00 pm
content: Don't mind the haters, just do whatever makes sense for you.BTW, consider adding an aluminum or stainless top plate to it. Much more robust than a raw resin surface. If thin, this plate can just glue on top, I suppose (in which case, stainless is preferred -- stronger, more scratch resistant?). Else, it should have cutouts for the pole faces, basically just covering what would otherwise be open resin surface.It doesn't need to be filled with resin either, you could put everything face-down instead and fill with just enough resin to cover and secure it all. Hmm, which would be greatly improved by adding flanges or angle stock around the edges, so the sheet doesn't get knocked out of place as there's much less supporting it this way.A resin-free build might even use flanges bent on said sheet, or angle bolted/welded to it, and screws drilled and tapped into the pole pieces. (Threads can sort of be made into the edge of the steel laminate stack -- use extra clamping to prevent it from splaying apart -- but preferably just do it on a flat side instead.)Of course, the more fabrication you do, the flimsier the finished product may be; probably you'll figure out a compromise here.Oh yeah, doesn't even need to be resin, really; it looks best, of course, but I mean, we don't even really care about good insulation, the filler could be concrete for all that matters. Which, if you opt for that, do add enough height so the pole pieces are adequately surrounded, and try to tack on some rebar (wire/mesh, coat hanger, screws..) so it doesn't fall apart after being bumped around a little bit.A low-aggregate mix, or say just cement and sand, is probably best. The top surface can still be sealed with varnish or epoxy or whatever, and milled flat, if you keep the concrete just behind the surface (so the cover plate idea does double duty here).Series ensures equal currents, regardless of resistance. Parallel, the currents divide based on resistance, and some ballast might be desired (resistance in series with the lower one, to make up the difference). Again, what really matters is amp*turns. A higher resistance is fine if it also has more turns. More efficient!I'm guessing these will be 240V windings, so, rated for 5-7A or so? Expect similar rating at DC, and then, use just whatever voltage is needed to get there. So, for ballpark 2 ohms each, that's 10-14V, which would be fine at 12V in parallel (12A total).Now, that is the maximum rating based on ampacity in intermittent use -- they aren't designed for continuous operation, and notice 12V * 11A = 144W is a pretty generous level for copper that's just floating there in space [or resin]. Likely you won't need nearly so much current, and probably somewhere around 20-40W is safe even in continuous duty (say if you forget to turn it off!).You can switch them in series if you find you don't need the extra force. Indeed, if you want to go back and forth, you can use a DPDT switch like so:http://www.incbtech.com/index.php/80-mechatronic/1186-series-parallel-switch-for-motor-control-mec051ejust replace the batteries with the coils, and the (M) with the DC power supply.TVS: put one across each coil if series/parallel switched, or just one across the total (both fixed in parallel/series). Or you can do one each even if it's fixed, it doesn't matter.1.5KE18CA or similar part is probably fine. The "18" in the number is the nominal breakdown voltage; pick the next highest value than your power supply voltage (so, 15 or 18 is fine for a 12V supply). It doesn't even need to be close, like, 50 or 100V is still fine to help protect the switch here. Just matters that it's above the supply's output, otherwise, well, it tries to clamp the supply all by itself, which will create problems.And, the -CA means bidirectional, so you don't have to care which way you put it in. Does the same job either direction.You may want to draw up a wiring diagram just to double check everything. Hm, don't forget to add a lightbulb, that'll be nice.Oh, can also switch the AC input to the supply, instead of its output -- the supply will handle the clamping by itself. Turn-on/off is just slower, which, like, by a few hundred milliseconds, you won't know the difference. There is maybe some hazard of the supply not handling turn-off into an inductor like this -- but, you'll probably end up accidentally switching off the AC while the DC is still flowing, some day, and, well, then you'll know if it handles it or not.Most likely it will be fine, and, it's likely not a wear item or anything, so okay to just do it that way all of the time.Tim