Noob plays with transformers - Page 1
Forum section: electronics / projects

author: rebelrider.mike
author_rank: Contributor
timestamp: October 12, 2018, 03:29:42 pm
content: Hi folks. It's been a while since I've been on this forum, but I've gotten back into playing with electricity lately, and to do some of the stuff I want to do, I'll need some transformers.For my very first transformer build, I found an autotransformer from a broken UPS. Being a noob, I cut the wire out rather than dismantling the core first and just unwinding it. I still have the wire, but it's now in 20 foot sections. At least I still have the intact spool and core.Now, when I started this, I knew nothing of transformers or the math behind them, so I did the best I could. To start with, I figured Watts, Volts, and Amps on both sides of the transformer. Watts being the same on both sides. I never did figure out exactly how to estimate the maximum power a core could handle. I found an equation, core area (sq cm) squared = Watts. I still don't think that's quite right, but it's all I could find, so I used it. I came up with about 122W.With 122W, at 122V, that's 1A on the primary side. I used 22 AWG wire for that side as it can handle 1.2A.With 122W, at 9.7V, that's 12.7A on the secondary side. That's way more than I want, so I chose 18 AWG wire, as it can handle 3.2A. More than enough to power what I need.While figuring all this out, I managed to get the core sheets apart so I could rewind the spool.The number of windings on the primary side was not straightforward to me at all. I finally found an equation for 50/60Hz transformers: 42 / core area (sq cm) = Turns per Volt. This seemed promising as 42 is the answer to life, the universe, and everything. I learned later that this was a very simplified version with several assumptions in it. But my understanding is that you have to have a minimum number of turns per Volt, or the magnetic field will become saturated. This would cause current to start flowing excessively between the Live and Neutral wires, and perhaps trip the circuit breaker. I think that's how it works, anyway.So, with the equation, I had 42 / 11.1 sq cm = 3.78 turns per Volt. At 122V, the primary would need 462 turns. I did my best to estimate the length of wire I'd need, and came up with 244 feet, so I bought 250 feet. Turns out, I was still about 2 feet short, so I need to come up with a better way to estimate wire length. But it all worked out, and I ended up with a nice neat primary coil. (Which I later wrapped in tape before adding the secondary coil.)For the secondary winding, I guess I could have used the ratio of Volts, but I already had turns per Volt so I used that. For 9.7V I would need 37 turns. I didn't take inefficiency into account (because I don't know how), and so I ended up with 8.7V once I got the transformer put back together. But it was easy enough to add 4 more turns, and I got 9.6V. Close enough!Also, I got to play with my new pocket oscilloscope. I've never had one before, and am still learning how it works, but I did get a nice sine wave, plus several numerical values that my multimeter can't do.After confirming the transformer would actually work, I decided to make some improvements. I got some nuts, bolts, and washers at the hardware store, and a few L brackets to tighten up the core, and give it a way to be mountable. Squeezing the sheets together with the bolts reduced the buzz to the point where I can barely hear it. I also found a really nice power cable with a ground to replace the old lamp cable I was using before.I had a lot of secondary wire left over, so I used the rest of it to make extra taps in order to have different Voltages to choose from. I wrapped the outside with tape, and squeezed a couple pieces of cardboard between the coil and the outside parts of the core. Just to make sure the wires will never touch the metal.In addition to the 9.7V I wanted, I also tried to get 12, and 24V. I ended up with 12.8V and 24.7V, which is fine. By using different combinations of taps, I also got 3.1V, 15.1V, and 12.1V. I don't know what I'll use these for, if anything, but there they are.These are all open Voltages, so they drop a little under load, but not by much.Since building this transformer, I've continued to search, read, and watch videos. I know more now than I did, but not as much as I'd like to. I found a more involved equation for turns per Volt (T/V = 1/(4.44 x 0.0001 x B x CA x f) which tells me I probably had more windings than I needed. I've also heard a rumor that the core doesn't actually have a limit for Watts. It's all dependent on the windings. I don't know if that's true or not. And there's still more websites I haven't read yet. I'll apply all this to my next transformer.

author: timh2870
author_rank: Newbie
timestamp: October 12, 2018, 03:41:37 pm
content: Without doing any math, there's a pretty simple way to find out if you have enough turns on the primary; put a lightbulb in series with your coil. If it lights, then add more turns. Reactive power should cancel out, and all you're left with is the resistive losses of your primary winding and core losses in the iron.

author: exe
author_rank: Unknown
timestamp: October 12, 2018, 04:06:47 pm
content: Just in case, for transformers output voltage is specified under nominal input voltage and full load. Without load output voltage is higher. Sometimes it specified as "regulation", that is ratio (in percents) saying how much voltage is higher when there is no load. Bigger transformers have better regulation. Small ones may be very bad at it.

author: rebelrider.mike
author_rank: Contributor
timestamp: October 16, 2018, 02:25:35 am
content: For my next transformer, I decided I want to build a 1:1 isolation transformer as a safety device while I work on various electrical projects. I found this guy at the local Habitat Store:It's an autotransformer, so no good as is for isolation. I'll have to rewind it. As I was taking it apart, I found that it has previously been exposed to moisture. The paper wrap is fine, as is the winding, so it probably just sat out in a barn or something for a while. But the core sheets are a mess, and will have to be cleaned up.I was able to remove all the winding, and will be able to reuse it. The spool is in perfect shape too.Looking at the wire, I discovered that it is aluminum and not copper. It only looks copper because of the insulation. It is 1.3mm, which works out to about 16 AWG. That would normally support 5.2A current, but since it's aluminum, I'll figure it could handle only 3.2A. I'll do some math later to confirm this.Anyway, to find the number of primary windings, I'll use the more complicated equation I found. That is Turns/Volt = 1 / (4.44 x 0.0001 x CA x B x f.f is the frequency of the change in current. In this case 60HzB is the maximum flux density measured in Webers/sq m. Or sometimes Gauss. In this case, I'm using the average value for iron/silicon cores. About 1.2 Weber/sq m.CA is the area of the core. In this case, 4.4cm x 5.1cm = 22.44 sq cm0.0001 is a conversion factor to make Weber / sq m work with the sq cm of the core.4.44 is a value derived from calculus that I don't understand. Let's just call it a constant that is good for a transformer using sine waves.1 / (4.44 x 0.0001 x 22.44sq cm x 1.2 Weber/sq m x 60Hz) = 1.39 T/VMy household Voltage seems to vary from 121V to 123V, so I'll go down the middle and pick 122V for the primary side of the transformer.122V x 1.39 T/V = 170 Turns.Normally I'd be shopping for wire at this point and would have to estimate the length. For my last transformer I used the core circumference per turn rather than the spool circumference. I think that is why I came up short when I bought that wire. This time I'll use the spool circumference of 20.6cm. I'll also add 10% extra.170 Turns x 20.6cm/Turn x 1.1 = 3854cm / 30.48 cm/ft = 126ft.Another mistake I made last time was not to take into account that the spool will be larger around with the primary winding on it. This would be difficult to figure if I were buying both winding wires at once, but in this case I'm cheating and reusing the old wire. And I've already got the primary wound, so I can measure the new spool circumference: 27.6cm. So for the secondary winding, also 170 Turns, and adding 10% extra:170 Turns x 27.6cm/Turn x 1.1 = 5163cm / 30.48 cm/ft = 169ft.Again, I'm cheating with already having the old wire, so I'm not actually measuring the length. After the primary winding, I've still got lots of wire left over, so I'm not worried about having enough. It took a little more effort to get the used wire nice and tight and straight on the spool, but I think I did pretty good.The covers were dented and had some rust under flaking paint, so I got them cleaned up and did a little body work. I found some old appliance enamel paint and gave them a new coat.I found some new insulated connectors to use on both sides, and they fit the existing holes perfectly. I even painted one green for ground. The ground will be on the primary side. The secondary, in order to remain isolated, will not have a ground.

author: planet12
author_rank: Regular Contributor
timestamp: October 16, 2018, 03:48:13 am
content: Firstly, good on you for getting stuck into transformers, black magic to most. I've now taught myself to design and make transformers from standard 50Hz iron-core through to >100KHz ferrite ones for SMPS use - understanding the magic is awesome.A great resource, if you haven't seen them before:http://ludens.cl/Electron/Magnet.htmlandhttp://ludens.cl/Electron/trafos/trafos.htmlAdditionally, a warning: be very careful reusing magnet wire; the insulation is very thin and has a habit of getting fragile with age, making shorted turns / isolation failures more likely.

author: station240
author_rank: Unknown
timestamp: October 16, 2018, 06:09:26 am
content: You can actually buy the yellow tape used on most modern transformers, just search for "polyester tape transformer" and you'll find it.The tape used for motors looks better though (comes in white).Can also used Kapton tape, also easy to find.Also this table I swiped out of an old book, who got it from Magnetics Inc anywayCode:Table 6.1Wire and Winding Data: Sizes, Areas(a), Resistance, and Current Capacities(b) forSynthetic Film Insulated Wire(c,d)Wire Wire Area (max)(circular mils)Size Current (mA)(AWG) Heavy Triple Quad Ohms/1000ft Capacity8 18,010 18,360 18,960 0.6281 16,5109 14,350 14,670 15,200 0.7925 13,09010 11,470 11,750 12,230 0.9987 10,38011 9,158 9,390 9,821 1.261 8,22612 7,310 7,517 7,885 1.588 6,52913 5,852 6,022 6,336 2.001 5,18414 4,679 4,830 5,112 2.524 4,10915 3,758 3,894 4,147 3.181 3,26016 3,003 3,114 3,329 4.020 2,58117 2,421 2,520 2,704 5.054 2,05218 1,936 2,025 2,190 6.386 1,62419 1,560 1,632 1,781 8.046 1,28920 1,246 1,310 1,436 10.13 1,02421 1,005 1,063 1,170 12.77 812.322 807 853 949 16.20 640.123 650 692 778 20.30 510.824 524 562 635 25.67 404.025 424 458 520 32.37 320.426 342 369 424 41.02 252.827 272 296 342 51.44 201.628 219 240 276 65.31 158.829 180 199 231 81.21 127.730 144 161 188 103.7 100.031 117 132 154 130.9 79.2132 96.0 110 128 162.0 64.0033 77.4 90.2 104 205.7 50.4134 60.8 70.6 82.8 261.3 39.6935 49.0 57.8 67.2 330.7 31.3636 39.7 47.6 54.8 414.8 25.0037 32.5 38.4 44.9 512.1 20.2538 26.0 31.4 36.0 648.2 16.0039 20.2 31.4 36.0 846.6 12.2540 16.0 19.4 22.1 1,079.6 9.6141 13.0 16.0 - 1,323.0 7.8542 10.2 13.0 - 1,659.0 6.2543 8.4 10.2 - 2,143.0 4.8444 7.3 9.0 - 2,593.0 4.00

author: rebelrider.mike
author_rank: Contributor
timestamp: October 17, 2018, 03:59:22 am
content: I had originally planned to buy new wire, but when I figured the length I'd need, I discovered I don't have the $ for it right now. I will probably rewind it with new wire sometime, as I'd like to have new copper wire, but also a larger gauge so I can use more current.The home made transformers I've seen seem to work just fine with masking tape, but the yellow polyester stuff would look much sharper. I've got that on my wishlist too.The table of wire gauges, is that for copper or aluminum?Well, cleaning the iron plates is progressing, but it's awfully tedious. I've got 97 Es to do, and 97 Is. I've gotten all but 30 of each done so far. It is pretty satisfying to see shiny rust-free plates though. Here's a couple of pictures from earlier when I still had most to do:Later I'll wipe them down with some acetone, and put new varnish on them.The covers are looking pretty good. They're dry now, and the connectors are installed.I'm not sure how I want to label them yet. Sharpie, or maybe something a little fancier. Not sure what.I've been thinking about the possibility of shorts also. One thing I've got in mind is to install a current limiter (light bulb) in the circuit. Another thing would be to see what the resistance is across the two ends of the winding wire. I found the equation to find resistance from resistivity:R = p x L / A Resistance (Ohms) = resistivity (Ohm-meters) x Length (meters) / Area (sq meters)Area is of course, pi x radius squared. And the resistivity of aluminum is 0.0000000265 Ohm-meters.So the aluminum wire, diameter 1.15mm / 1000 mm/m = 0.00115m / 2 = 0.000575m (radius).Area = 3.14 x 0.000575m x 0.000575m = 0.000000330625 sq mResistance = 0.0000000265 Ohm-meters x 1m / 0.000000330625 sq m = 0.025525869 Ohms.Since I used a length of 1m, this could also read as 0.025525869 Ohms/m. (Because algebra.) A more useful number would be 0.00025525869 Ohms/cm, or just round it to 0.000255 Ohms/cm.Earlier I estimated that I've got 3854cm of wire on my primary winding, so 3854cm x 0.000255 Ohm/cm = 0.98 Ohms. My multimeter isn't able to measure past the tenths place for Ohms, but I do get a reading of 1.0 Ohm. So that's about right on! If the resistance was significantly lower, I'd suspect a short. For now though, I'm just jazzed that my math worked out!Still looking for a way to figure maximum current that can safely be run through aluminum wire. I'm pretty sure it has to do with how much temperature the insulation can handle, and what the temperature increase is on a particular wire from a particular number of Amps. I think I'm getting close to something after reading this article on transformer design:https://en.wikibooks.org/wiki/Electronics/Transformer_DesignAnother thing I might do is start with the transformer's original 1000VA rating and work backward from there. I've done enough thinking for today though.

author: bsfeechannel
author_rank: Super Contributor
timestamp: October 17, 2018, 05:18:57 am
content: From a noob to another. Never reuse transformer wire. You never know the condition of the insulation. A shorted turn in a transformer is simply catastrophic. You should insulate each winding layer with the appropriate tape. The primary and the secondary must be specially insulated from each other by a thicker layer of tape.Sometimes it's better to leave the rust on the plates than to have the trouble to polish and varnish them. Rust is a poor conductor anyway.

author: BravoV
author_rank: Super Contributor
timestamp: October 17, 2018, 05:20:51 am
content: Thanks for sharing, interesting thread.Questions :- How did you detach those core sheets without bending or chipping it ?- Those iron sheets look so new after you cleaned it, how did you do that ?

author: BravoV
author_rank: Super Contributor
timestamp: October 17, 2018, 05:25:34 am
content: Quote from: timh2870 on October 12, 2018, 03:41:37 pmAdd more turns until the bulb stops light up ? Elaborate further please.

author: xavier60
author_rank: Super Contributor
timestamp: October 17, 2018, 05:59:52 am
content: Cleaning the core laminations will increase eddy current losses, by how much, I don't know.

author: GeorgeOfTheJungle
author_rank: Super Contributor
timestamp: October 17, 2018, 08:31:43 am
content: Quote from: BravoV on October 17, 2018, 05:25:34 amWhen the core enters saturation the primary turns into a short circuit. The core saturates beyond a certain ampere-turns figure which is proportional to 1/turns2=> to stay below that figure you just have got to keep adding turns.

author: station240
author_rank: Unknown
timestamp: October 17, 2018, 08:49:02 am
content: Quote from: rebelrider.mike on October 17, 2018, 03:59:22 amYes, the table is for copper.Couple of links for you on what to re-apply to the E I pieceshttps://forum.allaboutcircuits.com/threads/what-is-the-clear-plastic-coating-i-see-on-laminations-of-electrical-transformers.134030/https://en.wikipedia.org/wiki/Electrical_steelOrdinary varnish isn't an ideal option, but easy to get.Perhaps you can find an electric motor rewinder for sourcing the special lamination paint, or cheap end of roll copper.

author: xavier60
author_rank: Super Contributor
timestamp: October 17, 2018, 09:52:50 am
content: Quote from: rebelrider.mike on October 16, 2018, 02:25:35 amHow do you insulate the primary from the secondary windings and provide adequate creepage distance?

author: kkkwj
author_rank: Newbie
timestamp: October 18, 2018, 01:41:21 am
content: Wow, what an interesting thread! Thank you for posting it!

author: ArthurDent
author_rank: Super Contributor
timestamp: October 18, 2018, 05:56:02 am
content: Years ago, when I was a kid, old radios that were thrown out were tube type with power transformers and there was a rating tag on the back so you knew how many watts the transformer was good for. Almost all these radios had tubes with 6.3 volt filaments except for the rectifiers which were generally 5.0 volt filaments. (The really old TVs had much larger transformers as did some industrial test equipment.) If I took all the ‘E’s and ‘I’s out of the transformers then unwound the low voltage secondary filament windings and counted the turns I could calculate the turns per volt needed when I rewound my windings on the existing primary. The high voltage windings could be unwound without counting because I already had the turns per volts from the low voltage windings I removed so I didn’t need to count a few hundred more turns of wire.I would leave the insulating stiff paper on top of the primary winding and also save any other insulating paper to reuse. I would anchor the start of my new winding with sturdy fabric tape and evenly wind the required number of turns for my new secondary, generally putting insulating paper between layers, just in case. The wire size I needed could be calculated from knowing how much total current the tube filaments used and that could be found from looking up all the tubes used in a tube manual. Using a wire gauge I could measure the wire size and look up the circular mils for that wire size in a wire table then calculate the number of circular mils per amp. With this information I could use the wire table to calculate the wire size I would need for my winding. Knowing the wattage of the transformer from the rating tag on the radio would tell me if the transformer was large enough for my needs. For instance, if the tag said 50 watts and I wound my secondary carefully I should be able to wind a 25 volt 2 amp winding on the existing primary.There was a 16 volume set of ‘Popular Mechanics Do-It-Yourself Encyclopedia’ from the 1960s or 1980s I had that went into detail on winding transformers that I used back then. There is one reference I found on line that probably goes into more detail than you need at:https://www.electronicdesign.com/power/build-your-own-transformerbut is pretty good. As to the rust on the ‘E’s and ‘I’s you had, I would have just brushed the loose flakes off because as others have mentioned, polishing them could increase eddy currents and heating. Generally the better transformers alternate the ‘E’s and ‘I’s rather than put them in clumps. Some transformers do put all the ‘E’s in one side then lay the ‘I’s across the top and weld where they join but I believe that makes for a higher watts-loss design even though it is cheaper to assemble that way.

author: rebelrider.mike
author_rank: Contributor
timestamp: October 19, 2018, 04:56:09 am
content: Wow, lots of responses, thanks for the interest folks!So getting the core sheets off of the spool. The Is popped out pretty easy. I did manage to tear up a couple of the Es. I've hammered them back into shape as best I could, but I don't know if they'll be reusable. As for cleaning, I've been using a scotch bright wheel on a grinder. Takes the rust and old insulation right off without damaging the iron underneath.As for trying to guess the number of primary turns without math. Once the basic math is learned, it's super easy to run the calculation and get very close to the actual number of turns I'll need. In my humble opinion, way faster than trial and error. Using a light bulb in series is a very good idea though. Keeps a dead short from happening if there are not enough turns on the primary, or if there is a short in the winding. Without some kind of short, the light bulb will act as an ordinary resistor, and not much will happen. If a short occurs, rather than melting something, tripping the breaker, or some other annoyance, the light bulb will simply light up as a short would provide sufficient current to light it up. It's also a convenient signal that something's wrong.Isolating the secondary from the primary winding is the difference between a "regular" transformer and an autotransformer. The windings share the core, but they are not physically connected to each other. I have no idea what creepage distance is.As far as re-insulating the plates, I'm not sure what to use yet. I was thinking either some kind of urethane, or maybe trying to make a black oxide layer. I will be alternating the Is and Es as I put it back together. I only have 40 plates left to do out of 194. So I'm going to have to figure this out soon!And thanks everyone for the links. I've read them all. In fact, the "Build Your Own Transformer" article from ElectronicDesign.com is one I'm already familiar with.I'm pretty convinced now that the power rating of a transformer is only limited by the maximum current that can be run through the wires. And that is more a limitation of the insulation getting too hot than anything else. So yesterday, I did a little experiment. I took a meter of the aluminum winding wire and ran a current through it with a CV CC power supply. One of those really low cost ones that can be found on Amazon:https://smile.amazon.com/KNACRO-Converter-Regulator-Constant-Voltmeter/dp/B01N7CG71M?keywords=CV+CC+power+supply&qid=1539923210&sr=8-38&ref=sr_1_38Anyway, I slowly turned up the Amps to see when the wire would get hot. I got all the way up to 4.5A before giving up. My little power supply was getting very hot (it has a limit of 5A, and I didn't want to push it) and the wire was just barely maybe getting warmish. I think it may get a little warmer all wound up tight in a transformer, but I'm pretty sure I can get 540W out of it at 120V and be ok. The transformer was originally rated at 1000VA with the same wire. Also, I don't plan to use anywhere near that much power.Another thing to keep in mind is that the transformer was originally wound for both 110V and 220V, and I'm only winding it for 120V. So I should have plenty of room for larger gauge wire if I decide I need more Amps in the future.I'm curious about the different winding methods people use. Most DIY transformers I've seen are the same as what I'm doing. Wind the primary closest to the core and wind the secondary on top of the primary. But I've also seen other people wind the primary on the bottom half of the spool, and the secondary on the top half. I've also seen people wind half the primary, then wind the secondary, and then wind the other half of the primary. Like a sandwich. I wonder what the pros and cons are to doing it each way?

author: bsfeechannel
author_rank: Super Contributor
timestamp: October 19, 2018, 07:25:30 am
content: Quote from: rebelrider.mike on October 19, 2018, 04:56:09 amNormally you start with the winding that has the thinnest wires. In a step-down xformer the primary wire is the thinnest. A split bobbin provides better insulation between primary and secondary and lower capacitive coupling. The "sandwich" winding (technically called interleaving) is normally used in output transformers for tube amps. That way you reduce the leakage inductance, which is responsible for limiting the high frequency response of the transformer.

author: The Electrician
author_rank: Frequent Contributor
timestamp: October 19, 2018, 08:19:07 am
content: You probably shouldn't have polished the laminations. Transformer steel is usually coated with a very thin layer of magnesium silicate, called Carlite.See:http://nikomag-europe.com/index.php/markets/grain-oriented-electrical-steelHere's another good read:https://en.wikipedia.org/wiki/Electrical_steelThe C5 coating is Carlite. The minimum coating (C0) is also called steam oxide; they run the steel through a furnace with exposure to steam.Also see:https://www.quora.com/How-does-CRGO-steel-reduce-core-loss-in-transformersYou might be better off if you took all the laminations outside, sprayed them with water, and left them for a day or so until they get a little rusty.Varnishing them will give you a coating that will be so thick that you will not have a good stacking factor--you probably won't be able to get all the laminations back in.

author: BradC
author_rank: Super Contributor
timestamp: October 19, 2018, 08:29:47 am
content: Quote from: xavier60 on October 17, 2018, 05:59:52 amAs clean as those are, quite a *lot*. As indicated by The Electrician in the post above, the insulating layer on a core is extremely thin. It's also extremely important. Transformers do not like shorted turns.I'd wager you'll never get an insulating coating thin enough to get them all back in and not have a shorted turn with any form of applied synthetic coating. If you manage to get a decent oxide coating on there that'll do the job, however I will be seriously impressed. So please don't take this as a "toss it and start over". Just keep us posted on what you are trying and how it's working out. It's all good learning.

author: rebelrider.mike
author_rank: Contributor
timestamp: October 19, 2018, 06:19:39 pm
content: In addition to the rust, I found a thin layer of dull gray coating on the sheets. Some of the sheets were relatively intact, but most were in pretty bad shape, with the coating peeling off and I felt that a new insulative coating on a clean surface would be best. Perhaps this was a mistake, but what's done is done. If I can't fit all the sheets back in, I'll have to compensate by reducing the area of the core in the calculation. So the number of turns needed may increase. Better that than to not have adequate insulation between the sheets.Sounds like an oxide is the way to go. Black oxide tends to be thinner, tougher, and more stable than the other iron oxides. It's also less permeable to oxygen and so protects the metal better from future oxidation. I'll look into the best way to produce a good oxide layer on the sheets. Hot distilled vinegar comes to mind. I've never actually tried it before, so I've got some reading to do.

author: GeorgeOfTheJungle
author_rank: Super Contributor
timestamp: October 19, 2018, 08:28:30 pm
content: Yep, clean them well with diluted muriatic/hydrochloric acid, and it will take no more than half an hour outside under the sun to have them coated in an even, beautiful rust. A steam cleaner may work quite well too.

author: Ian.M
author_rank: Super Contributor
timestamp: October 19, 2018, 09:41:35 pm
content: Hot vinegar is usually used to strip blueing and other oxide coatings.DIY zinc Parkerising in phosphoric acid solution 'loaded' with some zinc, is probably the easiest option, or you could flash rust them as George suggests then boil them in water to convert the rust to black oxide. Either coating will need stabilising by drying them thoroughly with moderate heat then wiping with a drying oil or well thinned varnish to fill pores in the surface, then wiping off as much as possible of the excess before it dries if you want it to be durable and scratch resistant.Beware of light blueing as it can act as a semiconductor so may not have a high enough resistance.

author: rebelrider.mike
author_rank: Contributor
timestamp: November 06, 2018, 10:30:28 am
content: Finally got all the plates cleaned down to bare metal!I spent a while looking for ways to get black oxide on the plate, and I think this video pretty much sums up the details pretty well:I've already started degreasing with acetone. It's what I normally use on metal that I'm about to paint.Not sure if etching is needed, as I've just run them all on a scotch-brite wheel, but it's easy enough, and I've got the distilled vinegar.I've got the salt, but I'll have to go get some hydrogen peroxide. Unless I can find some around the house...And it turns out I've already got a little electric burner, so I won't be doing this in the kitchen, LOL.I think I will use deionized water for boiling. I've got pretty hard water, and deionized is cheap, even for me.

author: bson
author_rank: Unknown
timestamp: November 08, 2018, 08:20:46 pm
content: Quote from: rebelrider.mike on October 17, 2018, 03:59:22 amHow about a lettering stencil and black nail polish? Or a stencil and a sharpie for that matter.

author: rebelrider.mike
author_rank: Contributor
timestamp: January 11, 2019, 10:37:44 am
content: Hi folks! It's been a long while, but I haven't given up yet. I've been re-rusting my plates, and though it's not as easy as YouTube suggested, it's still going well.I got all the plates cleaned with acetone, etched in white vinegar, and the rusting process has been kick-started with hydrogen peroxide and salt.It's not as uniform as I had hoped, maybe because I didn't get all the coating off like I thought I did. But they're still rusting quite nicely, and boiling them is definitely converting the orange rust into black rust.I've found that now the rusting process has begun, I only need to let them set out for a few hours, and the bare spots start to rust on its own, and can be re-boiled. Here's a few that are part way through the blackening process:At this point, it's taking longer because I don't have the time to sit and watch the "soup" boil more than once every couple days. But more than half the plates are just about done.

author: Ian.M
author_rank: Super Contributor
timestamp: January 11, 2019, 12:00:41 pm
content: You are getting there, so you might as well carry on, but IMHO zinc parkerising would have been easier as its a one step hot bath, rather than the rust, convert, and repeat ad-nauseum black oxide process you are using.

author: rebelrider.mike
author_rank: Contributor
timestamp: January 12, 2019, 04:49:52 am
content: First 11 plates are done! I think they turned out well:They got a light sanding and a thin coat of oil. Only 183 left!

author: rebelrider.mike
author_rank: Contributor
timestamp: November 06, 2019, 03:38:39 pm
content: Wow, my last post on this thread was in January! Where has the time gone? Oh yeah, other projects.Anyway, I guess I never told you all that I finally got the plates done. That was a while ago. But I've been stuck trying to decide whether to reuse the aluminum wire, or buy new copper wire. The aluminum has the advantage of being free, but I'd have a much lower power rating on the transformer when finished. New copper would give me a much higher maximum power, but I have to buy it. With limited funds, and multiple projects competing for money, it was actually something I had to think about for a while.Finally decided on new copper. Not only that, but also a new tool to help me keep track of the number of turns. But first I had to get the old wire back off.I picked 16 AWG which will give me a maximum power of 360W. Good thing I didn't go any bigger, as the coils just barely fit. Here's the counting machine I got with the spool adapted to it.The counter machine is supposed to let you do winding faster with the little handle on the side, but 16 AWG wire is really stiff, so I wound up (no pun intended) just turning the spool by hand, and using the counter to keep track. It's actually very accurate.167 turns later, the first coil is done.I used the original insulator between the coils, and did another 167 turns on the second coil. It also has a center tap at turn 84, because the box it goes in has three holes in one side, so may as well. Gotta remember though that the center tap will be 60V, and at 3A, that means it's only good to 180W, as opposed to the 360W using the end taps.I did a few maths before and after I started. Knowing the circumference of the spool, I estimated that I'd use about 124 feet of wire. That includes about 10% extra for good luck. Not knowing the circumference of the finished primary coil, I took my best guess that 167 feet of wire would be enough. Adding more extra, I figured a 300 foot spool would cover it. Seems everyone sells copper wire by the pound instead of the foot. So I got 2.5 lbs. of wire estimated at 315 feet. It all worked out, as i have a fair amount left over.Later, I did more calcs and figured that at 124 feet, my primary coil should have about 0.5 Ohms from end to end. That is what I got on my multi-meter. Measuring the resistance of the secondary coil, I got 0.6 Ohms. That roughly translated back to around 150 feet. Or about 120% of the length of the primary. So I over estimated how much wire I'd need by quite a lot, but better to have more than enough than less.

author: rebelrider.mike
author_rank: Contributor
timestamp: November 14, 2019, 09:11:22 pm
content: Well, I'm no further trying to predict how much power an unknown transformer core can handle. This transformer was rated at 1000W. One of the tricks I picked up was to square the surface area of the tongue cross section. In this case 22.44 sqcm x 22.44 sqcm = 503W. Half of the original rating. The other trick I found was to weigh the transformer. They are supposedly able to handle about 40W/kg. Mine weighed in at 4 kg, so 160W.Moving along though, I put the plates back on the spool, and wired in a 12A braker, a mains Watt meter, and a 250W lightbulb for the load.I also checked beforehand the various continuities and resistances to make sure nothing weird was happening. Everything was in spec.So here are the results:It was loud, but didn't trip the breaker.121V in vs. 119V out, without load.Under load, the input was 119V, 3.58A, 426W.Output was 115.5V, 2A, 231W.With Watts in / Watts out * 100 to measure efficiency, I've got a whopping 54.2% Impressive, no? No.Here's what I think is wrong. In my enthusiasm to coat the plates with oxide, I think I made the coating too thick. It's certainly effective. The bare plates are almost no resistance, and the coated plates are thousands of Ohms. But not all the plates fit back in. I think I'm going to have to thin down the coating on all the plates until they do. Not enough iron mass in the core means the flux magnetic thing is wrong and it won't absorb enough energy from the mains current. I think that's why there's about 2.5A being used even without load.Time to take it apart and do some sanding...

author: soldar
author_rank: Super Contributor
timestamp: November 14, 2019, 10:23:15 pm
content: Quote from: rebelrider.mike on November 14, 2019, 09:11:22 pmI know you are having fun and I don't want to spoil it but "an unknown transformer core" can "handle" no definite amount of power because the core does not "handle" power. I understand what you are trying to get to but you are looking at the problem from the wrong angle.The entire transformer handles power and you cannot really put a theoretical limit on what a core can handle and it depends also on what you mean by "handle".A separate primary and secondary are very different from an auto-transformer and the power they can handle will be different.But, the way I look at this problem is the other way around. It is not "how much power can the core handle?" because I look at it more as the conductors being the ones that handle the power so I ask more "how much power can the conductors I can fit in that core handle?"When designing transformers I would start out with a core and bobbin chosen almost by eye and then design the coils. If the coils did not fit in the bobbin I would switch to the next standard size up and start over again.As you already know, Al wire will handle less power than copper wire. If you could get a wire with much less resistance than copper the same core could handle much more power.So, you have a core, you need to determine how many turns you need, then what diameter wire will fit with those turns and the copper resistance will be what mostly determines the power it can handle.So, you have a core and bobbin and want to build a 1:1 transformer. You can either measure the characteristics of the core or just assume a safe, low value.If you give us the dimensions of core and bobbin I can take a stab at it.If you can measure the properties of the core that would be even better. Assemble the core with, say 100 or 200 turns and measure inductance. at mains frequency by putting in series with a light bulb.You want to determine how many turns you need to have a suitably small magnetizing current. Once you determine that you just put that number of turns with wire as thick as will fit.

author: Cliff Matthews
author_rank: Unknown
timestamp: November 15, 2019, 01:26:53 am
content: Careful with bolt isolation. I had some500VA hot bolt issuesand got it sorted-out with Kapton tape. Good luck!

author: exe
author_rank: Unknown
timestamp: November 15, 2019, 08:08:29 am
content: I think what limits power handling is rise of temperature. As long as a tranny remains within the temperature limit it should be fine. Afaik Rod Elliot wrote that he could get more from trannies by forced cooling:https://sound-au.com/xfmr2.htm?> Fan cooling can increase the effective VA rating of a transformer significantly, but does not improve regulation. Large power distribution transformers are almost always oil cooled, and they are now starting to use vegetable oils because they are less inclined to catch on fire, and pose minimal environmental impact should there be a coolant leak or other major fault.Having said that, by looking at the size of your tranny, it should have a better regulation, more than 80, imho (you can see some figures for similar-sized trannies to get a rough idea how much it can take). So, this makes me thinking you pushed it well beyond what it was designed for.Also, due to high mass the temperature may raise quite slowly. This may give a false impression that it can sustain current load. Don't get fooled by this, in a real device the tranny is unlikely to be in open air, so cooling will be much worse. Also, I suggest doing performance testing for at least half an hour with closely monitoring the temperature. Trannies are good at handling short overloads, but it in the long run they may tend to overheat if constantly abused.

author: The Electrician
author_rank: Frequent Contributor
timestamp: November 15, 2019, 11:31:41 am
content: The power "handling" capability of a silicon steel transformer such as you are working with is given by the area product WaAc.See this reference:https://coefs.uncc.edu/mnoras/files/2013/03/Transformer-and-Inductor-Design-Handbook_Chapter_5.pdfand:https://www.mag-inc.com/getattachment/Design/Design-Guides/Transformer-Design-with-Magnetics-Ferrite-Cores/TransformerDesignWithFerrites2013.pdf?lang=en-US

author: Zero999
author_rank: Super Contributor
timestamp: November 17, 2019, 03:03:46 pm
content: Quote from: rebelrider.mike on November 14, 2019, 09:11:22 pmWell if you've gut the core size, it might be saturating slightly which will increase losses. If the voltage drop is an issue, then wind extra turns on the secondary, but accept the off-load voltage will be a little on the high side. The output voltage of transformers is normally specified at full load, so the off-load voltage will be significantly higher, but you might want to go for 5% higher than desired voltage off-load and 95% of the desired voltage on-load.One thing to note is that auto-transformers can be much smaller than isolating transformers, given the same power rating. An auto-transformer with a 2:1 ratio will have half the copper and iron of an isolation transformer of the same power rating. If your transformer is rated to 1kVA, then it'll be rated to no more than 500VA, when re-wound as an isolating transformer. Indeed, it's possible to convert an isolating transformer into a 2:1 auto-transformer, with double the power rating, just by connecting the windings in series. Think about a 1:1 120V:120V transformer, with the secondary winding connected in series with the primary winding. Assuming the phasing is correct, the voltage on the secondary will be double the primary voltage. This is how auto-transformers work.

author: soldar
author_rank: Super Contributor
timestamp: November 17, 2019, 04:11:47 pm
content: You want to make a 1:1 isolation transformer with a core and bobbin you already have. 120V, 60 Hz I assume. Here is what I would do:First of all measure the characteristics of the core by making a coil with a number of turns, say 100, and putting a 60 Hz current through it. Measure voltage and current and determine L value for 100 turns. L is proportional to the square of the number of turns so 200 turns would have 4 times higher L. That gives us the characteristics of the core we need.Now measure the window of the bobbin, the space where the wire will go. Of the total area let us subtract 10% as safety margin, for insulation tape, etc.The secondary will have, say, 10% more turns than the primary (and several taps). I assume primary and secondary will have same diameter wire. So, of the total bobbin window area Wa, the primary gets 43% and the secondary 47%.Now you have an exact window area to work with. How many turns of wire you can fit depends on the wire diameter. Bigger wire, fewer turns. Assume a diameter and work out the number of turns you can fit in the primary allocated area. Work out the inductance and the resistance. Then the magnetizing current and power dissipation in the primary. If not enough inductance then you need more turns of thinner wire.This way you are using all the bobbin space to the best advantage.

author: GeorgeOfTheJungle
author_rank: Super Contributor
timestamp: November 17, 2019, 04:53:31 pm
content: Quote from: rebelrider.mike on November 14, 2019, 09:11:22 pmYep. That means the core is entering saturation. 120 times a second. Not good! You have to either put back the irons, or add turns to the primary. The no load losses ought to be max. 5% (less is better) of the transformer rated power. Connect it unloaded to a variac and decrease input voltage until you're at less than that 5% figure, then do the math: if for example rated Vin is 120 and you've got to set the variac to, say, 80V, then the number of turns you need in the primary is 120/80 => 1.5x the turns it's got now.

author: soldar
author_rank: Super Contributor
timestamp: November 17, 2019, 06:44:12 pm
content: Don't design it for exactly 120 V but give it at least 10% margin. One of the first transformers I designed, ages ago, was so tight that it would easily saturate and heat up. When the standard was raised from 220 to 230 V that was the end of it.Note also that load current is irrelevant to saturation. Only the magnetizing current counts and the magnetization does not change from no load to great load.So, basically, you need to design a primary that under no load has enough inductance to keep the current well inside the working park of the curve and not reach saturation. If that condition is met then the core will not saturate under load.For a one time project like this my advice is to give wide margins in every step. You might not get the absolute last drop of power out of the thing but you can be sure it will work reliably and safely at the first try.I have had to re-wind more than one transformer because I was too optimistic in the amount of copper wire I could fit in the bobbin.

author: rebelrider.mike
author_rank: Contributor
timestamp: November 25, 2019, 09:24:34 am
content: Wow, thanks for all the replies! I guess I'm not getting email notifications, because I just now saw them.I really appreciate the explanation between the isolation and auto transformers. That makes sense.Some of your responses have a lot of info that I'll have to go back an re-read a couple times to make sure I get it all straight.So the core's ability to handle the power going through it is really only dependent on how many turns you can physically fit inside... I think I get that.Looking at the equation I've been using: T/V = 1 / 4.44 x 0.0001 x CA x B x f the turns per Volt will go down as the core size goes up since it's 1/CA (and all the other stuff). So either more turns are needed, or there needs to be more metal in the core. Seems the problem with CA (the core cross section area) is that it seems to assume almost no distance in between the plates for insulation. I wonder which is worse: eddy-currents from lack of insulation, or not enough core material? I've already decided to thin out the insulation I put on the plates so I can fit more in.I figured there would be some Voltage drop anyway, and even if it were to drop to 115V, I think that anything I'm using if for would be fine. I like the tip on how to estimate how much wire will actually fit in the transformer, based on the cavity size.I don't have a Variac, but I do have that little transformer that has lots of taps. Anyway, you all gave me lots to think about! Hopefully I'll return with good news.