Wind a transformer like a roll of tape? Pros and cons? - Page 1
Forum section: electronics / projects

author: cur8xgo
author_rank: Regular Contributor
timestamp: June 03, 2019, 11:35:19 pm
content: My project is currently using a transformer as follows:Pri: 6 turnsSec: 16 turnsCore: 0.014" laminated M6, 24 sq cm. core areaWire gauge: 2 awg solid for both windings (about 30 feet total). Heat shrink tubing for insulation.Operating: 350A in 125A out (roughly)Things are working well. However, the transformer could be improved. As it is, its operating at only around 0.25T, and the windings are much bigger than they need to be because they were hand wound and the insulation is very thick.But besides fixing those things, I think I could go down to a significantly smaller core, like an EI-162. And to make that happen, I'd replace the 2AWG solid circular with solid strip of the same area.I have an easy source for copper strip of that dimension, and it _should_ fit into an EI-162 maintaining the same number of turns and copper area. However, winding with strip would be much more efficient with almost zero wasted space.Think tape roll winding, not on-edge. So the entire winding in one plane, overlapping itself. Can tuck the end of the first wind under at 90 degrees so it can get out.My question is, what is the downside of this? I think the winding thickness overall will end up sticking out from the core around 0.9" with 16 turns. Right now 16 turns of 2 AWG solid secondary on TOP of the primary is sticking out much further than that.To insulate I could put a single layer of kapton tape on one side of the copper strip. Or if thats a bit too weak, something marginally thicker like paper or whatever is currently the rage.So it seems to me leakage inductance would go down, even though this method of winding pushes each turn further away from the core.Any other issues with this? Seems really win win.I know this is done with foil wound transformers. But I'm just a little hesitant, not sure this is apples to apples. My "foil" would be identical except maybe 0.056" thick. And the pri and sec would not be wound on top of each other, they would be side by side.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: June 04, 2019, 12:26:33 am
content: I take it, this is at 50/60Hz?You don't want to put down an entire winding all by itself, because the magnetic field can't escape from the pile of turns. Within a section, the field from turns underneath are shielded by the overlaying turns, while adding on top of the field from those turns, vastly increasing eddy current losses (or skin effect losses, or whatever; this is broadly termed "proximity effect"). With copper of this size, I think even at 50Hz, you'll have issues with this, and I don't recommend it.Wire-wound windings are normally done this way, i.e. in single sections, because the wire is smaller (up to, oh say, 10 AWG?). This is around where copper wire starts getting unfavorable due to proximity effect.Another mitigation for proximity effect is to simply not have windings so tall -- reduce the number of layers that are stacked upon each other. If you're limited to "wasteless" style stampings, there's probably not much room to do this; but if you have the option to use a wider (and less tall) winding area, that would help.A toroidal transformer may be attractive for this reason. The winding area is quite wide (the circumference of the core!). Rather difficult to use foil windings with! Multi-filar windings would be more attractive (i.e., using a ribbon of finer wires in parallel for the same total cross section).Multi-filar also helps with proximity effect (you're making flat Litz, in a sense), which is another option. You may get a better fill factor with smaller enameled wire, than whole round wire.Back to foil windings. What you can do, is wind both primary and secondary at the same time -- much as you would wind a capacitor with two plates and two dielectrics, this keeps the current balanced within the winding, so that currents aren't piling up. There's always one forward current (primary) beside an opposing current (secondary), alternating.In this case, you might wind the primary and two secondaries, getting 6 and 6+6 turns respectively, then add another 4 turns (2 each at the top and bottom) to bring the total up to the desired 6:16. The additional turns aren't paired, so do exhibit proximity effect, but are a small part of the total so it's an acceptable compromise. Other divisions are possible but this is a simple one, and wouldn't be too bad.In summary, that would be:Pri, sec: full width foil; ampacity attained by selecting foil thickness appropriately.Windup:A: 2 turns secB: 6 turns of triple layer: sec-pri-sec (call these B1, B2, B3)C: 2 turns secWiring:Secondary start = A startConnect A end to B1 startConnect B1 end to B3 startConnect B3 end to C startSecondary end = C endPrimary start = B2 startPrimary end = B2 endBTW, if you have to neck down the foil, to get it out of the bobbin, that's fine. At the start/end of a winding, you'll do a 45° fold, to make a 90° bend, to escape vertically from the bobbin (as you already noted). If this is too wide (the bobbin winding area width, and therefore foil width, may be wider than the slot in the bobbin cheek), the vertical lead can be cut narrower (increasing current density, but just for a short distance -- no big deal). Or actually, it could be folded in half, lengthwise, or thirds or whatever for that matter. Crush it down with pliers or hammer, it's going to get pretty thick pretty quickly...You'll end up with a bunch of foil stubs sticking out of the bobbin, which need to be wired together. Just resolve these however you can. Expect to take up some space in the process. You'll probably fold over a few against the bobbin and solder them (if they're folded or rolled, make sure solder gets into the layers), or for that matter, drill holes and make bolted connections.Or you can do some simplification right away, and, I think, assemble one single strip, with the other layers positioned on it exactly as needed. Start with a 10-turn length of secondary, and assemble the 6-turn lengths of primary and additional secondary, onto it, at the 2-turn mark. Now you only have six terminals to resolve. You need to get the lengths and positions just right, however!By the way, regarding insulation: do use heavy plastic, or vulcanized paper ("fish paper"). Don't want that foil edge chafing through a weak layer of film tape or something! I would guess 10 mil PET should be effective here, or maybe two layers of 3-5 mil. Or anything equivalent in strength but higher rated in temperature, as needed.There's actually kind of a downside to this construction, in that it may betoo good-- the characteristic impedance will be much lower than most mains-frequency transformers. If you were expecting some leakage inductance in the process (as might be beneficial in the below-mentioned example, converting MIG to stick -- the inductance improves arc stability), you'll get very little with such a tight windup, and may need to add an extra reactor (AC inductor) in series. In other words, the available short-circuit current will be very high, or the regulation will be very good. (These are also attributes that toroidal transformers share, and these are precisely the reasons why.)Somewhat more out-there ideas: is a transformer really the best solution? This sounds like a pretty unusual application, and maybe there is a better overall approach. The numbers roughly sound like, changing a 350A MIG welder to a 125A TIG/stick welder? Or, stepping up a UPS/inverter's primary (~24VAC) to some oddball voltage (UPS'd 1kW audio amp??).In short, just to say: there may be a better (easier, cheaper), less clever alternative, e.g. selling the 350A MIG welder and buying a 125A welder.Or you could design a switching supply which performs the transformation, preferably DC to DC, but AC to AC is also possible; but a switcher is considerably harder to pull off well, so I'm not going to recommend it without more background details.No idea, these numbers are really oddball -- hence the custom transformer -- but when this happens, it's often a case of a missed assumption, and the reason no one uses this low-level solution, is because there is a better high-level alternative.Tim

author: cur8xgo
author_rank: Regular Contributor
timestamp: June 04, 2019, 01:16:40 am
content: Thanks for the detailed answer Tim.The operating frequency right now is 625Hz. Its a full bridge switching DC into the transformer. The output of the transformer is rectified and used as DC.My perspective right now is that the prototype is working as designed (with some anomalies but nothing serious). The existing transformer, although hand wound somewhat uneven, is working and does not appear to be breaking a sweat during operation.Transformer cost is critical here, so I'm pretty sure a stamped wasteless core is the only way I can go. (also why I can't use a toroid) An EI-162 would in theory fit 16 turns of 0.056" thick copper strip with the primary at 6 turns right above it, and room left for a bobbin, the 45 degree bend,and inter-winding gaps and insulation. All while being much smaller than the working prototype core, and using about 1/3rd less copper. Lots of gains to be had both in cost, weight, size, and performance.If eddy currents will go up by stacking 16 turns on top of each other, it matters very critically, by how much. I think in this situation there is definitely a budget to afford that, so I need to quantify it. Any idea how I can do that besides just building the new transformer and having at it? Even just a ballpark. EDIT: I dont think this applies to conductor-to-conductor effects, especially at idle.->At the moment the transformer draws 18Arms at idle, so I have to think proximity losses aren't that significant at least at the current prototype config.Do they go up proportionally with turns or exponentially? I mean, the copper area/resistance wont change (EDIT: but the geometry will, significantly, and thats very important for this effect), just the number of turns on top of each other. So should be able to ballpark this right?The savings here are so big and the construction of the transformer would be so significantly simplified that its a road I have to go down unless there is a clear dead end. At the moment the prototype has 3 layers of windings on top of each other (1 pri, 2 sec).Your idea of using wider windings is something I could look into. If I use the entire core length and wind the pri and sec together like you suggested, maybe we end up with the same core, but an much improved proximity effect situation. Its not out of the question to jump to an EI-175 or bigger, allowing even wider windings and making pri/sec simultaneous winding more do-able. The cost of the core for these stamped laminations is not too prohibitive yet, I dont think it will go up very fast from EI-162 to jump to the next size.I'm going to have to stare at your summary of winding arrangements..I think I need a diagram. I will look up winding strategies like that so I can know what you are saying.Also something to consider are losses and expense of joining several foil stubs together. With the single winding of thick strip, I can weld it to wherever it goes, and possibly make the stub long enough to reach where it needs to reach in the chassis in an inexpensive and joint-free way. But with many thin foil strips, I need to come up with a reliable mechanism to join them all without creating significant drops. Even a 0.2V drop is significant here on the primary side. I've worked hard to cut out 0.0015 ohm drops from the various pathways, especially on the primary side.This application doesn't need the leakage inductance, thankfully, at least, in my analysis. It might benefit in inductance after the bridge but at the moment thats not showing up as a must have.This is a project from scratch, not running on mains.I'm seeing some good resources on calculating proximity effect but I'm going to have to learn a bit to be able to take a stab at it. Its not easy for me to do experiments without disassembling the prototype core and I'd like to avoid that if possible, its an epoxied-together situation that works.Its too bad I can't wind the strip on-edge as a normal helix. Would get the same savings. I just don't think I can do that without special equipment. Whereas with strip wound in one plane I can do that by hand for prototyping and just as simply for making more than one.

author: cur8xgo
author_rank: Regular Contributor
timestamp: June 04, 2019, 01:28:36 am
content: I'm seeing lots of good papers on calculating proximity effect..just not the easiest readinghttps://ecee.colorado.edu/~ecen5797/course_material/Lecture35slides.pdfhttp://citeseerx.ist.psu.edu/viewdoc/download?doi=10.1.1.916.5488&rep=rep1&type=pdf

author: cur8xgo
author_rank: Regular Contributor
timestamp: June 04, 2019, 03:06:38 am
content: Wikipedia shows the "dowell" method to calculate Rac/Rdc for proximity effect in rectangular wires. I think that I may be able to ballpark with this... maybehttps://en.wikipedia.org/wiki/Proximity_effect_(electromagnetism)Why is "b = width of winding window" in there?Hmm Dowells curves. Seems like this is the ticket. Should at least be able to compare current design to proposed design and ballpark increase in losses.

author: cur8xgo
author_rank: Regular Contributor
timestamp: June 04, 2019, 03:41:19 am
content: Okay I'm going to take a stab at this.The idea being now compared to future. With the assumption now is "ok" (although actually, I don't know how much proximity loss it has)Now = 2AWG solid, lets just say 3 layers (1 pri 2 sec actually).Skin depth for 2AWG @ 625 Hz = 0.094" (diameter is 0.258" so...ouch)https://www.allaboutcircuits.com/tools/skin-depth-calculator/So first multiply by 0.83 correction for circular wire, 0.258 * .83 = 0.214". Ratio of full/skin is then 0.214"/0.094" = 2.277Then using Dowells curves, to figure the Rac/Rdc loss ratio:for layer 1, 3 times the lossesfor layer 2, 10.5 times the lossesfor layer 3, 45 times the losses (!!)I think this explains some voltage drop and warmth in the prototype.Okay so now lets compare to a theoretical single-plane (roll of tape) strip wound version.Skin depth still 0.094"BUT...the strip I want to use is 0.056" thick.So thickness ratio is now 0.056"/0.094" = 0.595 (versus 2.277 before).Note that at this thickness and width (not mentioned), the cross sectional area of the strip is the same as the 2AWG wire, so resistance and therefore Rdc doesnt change between the scenarios.Now using Dowells curves for the losses:drum roll pleasefor layer 1: ~1 (cant tell)for layer 2: 1.25for layer 3: 1.5for layer 4: 1.6for layer 5: 2for layer 6: 2.5for layer 7: 3 (?)for layer 8: 3.5for layer 9: 4for layer 10: 5for layer 11: 5.5for layer 12: 7for layer 13: 8for layer 14: 9for layer 15: 9.5for layer 16: 10 (?)Okay so now how to add the cumulative effect from these. The paper says there are another set of Dowell's curve that do that intrinsically. Let me look.Okay I think the wikipedia article graphic is actually the "combined" curve:So probably should have just started with that for all this.For a total number of layers of 16, with skin depth ratio of 0.595, looks like we are at an Rdc/Rac of.... 6(?) for the whole winding.And going back, for the 3 layers of 2awg, with a skin depth ratio of 2.277, we are at an Rdc/Rac of....20(?) for the whole winding.So apples to apples here is by nature of the conductor having the same resistance per unit length. It looks like even winding 16 layers in a single plane is still superior by a factor of a little over 3 compared to 3 layers of 2AWG (current design).And as a further enhancement, winding with foil will have a much higher winding efficiency, and on a smaller core, and will not be wound over each other. This will reduce the total length of the conductor by about 1/3rd. So that 6 becomes a 4. So 5 times less than the Rdc/Rac of the 2AWG current version.Of course all of this is firmly in the category as "wishful thinking" as I literally did all this for the first time in this post and could easily have missed some arithmetic or just done this wrong.EDIT: Trying to understand why this is true. (assuming I did it right). That skin effect ratio really hits the proximity effect hard apparently. Why?I guess this:"Horizontal axis corresponds to theratio between the thickness of thecopper layer and the skin depth. 0.1corresponds to the copper layer thicknessequal to 1/10 of the skin depth,where proximity losses are completelysuppressed.10 corresponds to thecopper layer 10 times thicker than theskin depth, where proximity losses arein full strength. (If round wire is used,the diameter for the calculation of theratio should be multiplied by 0.83)."I suppose now I could go try to understand how skin depth and proximity effect are related but Im pretty happy with a 4 time improvement in performance so until the shoe drops and I realize I did all this wrong I'll just not do that.

author: Someone
author_rank: Super Contributor
timestamp: June 04, 2019, 05:03:32 am
content: Quote from: cur8xgo on June 04, 2019, 03:41:19 amSkin depth, one sided measure. Diameter, overall measure. And its only an estimate...Before getting all tied up on trying to calculate this get the basics in place, a good starting point is here:http://ridleyengineering.com/images/phocadownload/13%20proximity%20loss.pdf

author: Kleinstein
author_rank: Super Contributor
timestamp: June 04, 2019, 06:07:28 am
content: There is not much wrong wit using the tape like winding. It adds to the winding capacitance - this could be a problem at high frequency, but usually is not at 60 Hz or 600 Hz.Why use the same gauge for both windings ? normally it is a similar amount of copper for both windings, so a thinner wire for more turns.In theory the optimum winding uses thicker wires more to the outside, reducing the current density invers proportional to the length per turn.It usually is attractive to really make use of the possible magnetization. So going from 0.25 T to 1 T would only need 1/4 the size of the core and correspondingly shorter turns. The downside is slightly higher no load loss, and less effective cooling, but this is usually more than compensated by less loss.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: June 04, 2019, 07:01:14 am
content: Quote from: cur8xgo on June 04, 2019, 01:16:40 amOh...Well, that triggers all the SMPS flags I had set. Let's go there!Why 625Hz? 0.25T may well be too much, for that thickness of lamination, actually. IIRC, you want more like 5 mil thickness, or even less, at that frequency.Why low frequency, for DC-DC? Why not high frequency?Why a transformer at all -- how much isolation is required? If none at all, why not a boost/buck?It would seem the concerns about DC-DC convertersmustbe broached anyway, so the frequency would seem to be arbitrary, and, if nothing else, pushing it over 10kHz will make it a lot less unpleasant to sit beside, if nothing else.The most important thing I think, is this: if you subdivide the converter into equivalent, parallelizable units, you will get much better performance (power density and freedom from parasitics; and hopefully, choice of commercial transformers too?). Then it's simply a matter of stacking, whatever, ten modules say, and you're done.I would guess a transformer this size is going to be around $1000 custom, probably more for one-offs, maybe a little less in quantity. How much (qty, cost) is available for budget?Tim

author: T3sl4co1l
author_rank: Super Contributor
timestamp: June 04, 2019, 07:18:56 am
content: Quote from: cur8xgo on June 04, 2019, 03:41:19 amNice. So that's about what I expected from my intuition, though at 10x the frequency from what I was expecting. So doing that (sheet, single winding sections) at line frequency is still 100% okay. But as it turns out, you're doing this at a high enough frequency where it's a concern, so that's cool.And like I was saying with the layered (interleaved) construction, that gives m ~= 1, and very low R_AC/R_DC for this case. It seems, even up to higher frequencies (a few kHz). That would basically be the crossover point (sub-kHz vs. few kHz) where you'd want to go from lone winding sections to interleaved construction.I think you're using the curves about right.Also, the relevance of width as well as height (thickness of the sheet) is that, even for very thin foils (t << delta), the accumulation of current in a section forces current flow to the edges of the sheet. You get the same physics as skin effect again, but it's concerned with the confinement of current to the edge, instead of just the surface. This depends on the sheet resistance (effectively, thickness is factored out as overall sheet resistance), instead of the resistivity. So it's cool that the formula includes width as well as height, as a rectangular conductor. Handy!The only solution in this direction is to use thin and narrow strips, rotating through permutations of them, essentially, making flattened Litz braid. IIRC, West Coast Magnetics has a patent on such a construction?Tim

author: cur8xgo
author_rank: Regular Contributor
timestamp: June 04, 2019, 04:33:56 pm
content: Quote from: Kleinstein on June 04, 2019, 06:07:28 amI don't have a reason for using the same gauge for both windings. Its a blind spot I haven't examined yet. The only rationale I have at the moment is that the working prototype also uses the same gauge for both windings and so to keep things equivalent I'd do the same with an improved transformer design, if not just to maintain or improve on the DCR.If the flux density goes up significantly, I think there will be some pretty crazy core hysteresis loss once things go up near the knee, at 625Hz. I submitted for a quote on a similar design and the engineer there suggested the core would melt at 1.6T at 600Hz, and that 0.5T was the "absolute max" for M6 at that frequency (1 min on 1 min off duty with forced air). That seems a little extreme to me but I have not done the calcs so I will use it as a guideline.So my proposed improved design will operate at 0.5T and see what happens. If my calcs are correct this puts the new design in a cost, size, and weight area that looks great for this application. Basically an EI-162 stamped core with about 18 feet of 0.056" copper strip to make both windings.

author: MagicSmoker
author_rank: Super Contributor
timestamp: June 04, 2019, 04:57:35 pm
content: Yeah, neither proximity loss (which increases with the number of layers) nor distributed capacitance are really relevant at <1kHz. However, M6 seems a bit thick for operation at 625Hz, and the engineer advising you that losses will go through the roof if you come anywhere close to its saturation limit is precisely right. You might find a more favorable induction limit vs. core loss vs. cost with the thinner laminations like M3 and M4 even though they are higher in cost.Kind of a weird frequency which is way too low for ferrite, but a bit too high for most electrical steels, even with really thin laminations.

author: cur8xgo
author_rank: Regular Contributor
timestamp: June 04, 2019, 05:46:44 pm
content: I selected operating frequency in an early stage of prototyping, and its survived until now, although could probably be revisited to get some not-insignificant gains.At that time, I believe the factors were:-make it high enough to reduce idle losses (magnetizing current)-make it low enough to theoretically leave me some breathing room with frequency dependent effects which I didn't understand well (skin, proximity)-make it "reasonable" for M6 laminations..this is very hand wavy I admitNow, further down the road, I have learned:Power input bus transients occur at twice the operating frequency, and I have been unable to snub them entirely into a capacitor. They avalanche through the fets (and presumably into the bus cap) at 45V for about 5 us per switch event. Luckily, I selected some fets that are fully avalanche rated and they are heavily heat sinked with forced air already, so they don't seem to be breaking a sweat. I'm not sure how much energy is being snubbed by the fets and how much by the bus cap. But if it was all into the fets it would be about 98W per H-bridge leg (625 * 2 * 45V * 350A * 5us).That said, increasing frequency will increase the power dissipated by snubbing the bus transients more often. And although nothing seems to be running hot, and losses should be proportional to frequency, I would prefer to avoid touching frequency unless I have no choice. But if my thinking is right here, maybe I could double it? I would really have to put thermistors on the fets and know exactly where things stand temperature wise before I could do that though.And there are other switch-event-frequency related energy absorbing events:-primary snubber-bottom mosfet gate dv/dt transient-possibly the full bridge output cap ripple-mosfet linear region dissipationSo, 625Hz was originally somewhat arbitrary, but turned out I got a little lucky and my intuition was also a little right, I think.Full-bridge was selected based on advice given in many books and online for this power level. And, because it seemed like something I could get my head around as being new to SMPS. Although now, I am very tempted to experiment with a boost design at some point, at least in a simulator, to see what would be what.Isolation, unfortunately, is probably required in this application. There may be a twist on this device that could allow it to use an autotransformer or non-isolated topology. But I don't think I should jump into that since things appear to be going well with this full bridge isolated design.Now, since the prototype works, my pragmatic choice would seem to be turning my somewhat-realistic prototype transformer into a much improved, producable transformer.To do that:EI-162 M6 0.014" lams with a core area of about 3.1" for a flux density of about 0.5T (twice now)..about $24 in lams (about 125 pairs per core) from Tempel in small qty0.056" thick copper strip (because that thickness is easily available to me and coincidentally fits into an EI-162 with the sames turns as prototype) about $25 retail in very small qty for 18 feet (both windings)Custom bobbin: lets just say $5Varnish, paper/kapton, hardware, etc..etc.. lets say $10An hour to wind and assemble? $15?Haha so $80 for a 3kw strip wound custom transformer! Hmmm that can't POSSIBLY be true can it?To prototype it the lams are the expensive part..looking at a minimum order of $124 from Tempel, but I can will call it so no shipping. So we are looking at around $200 to try this out.As far as literally having someone else make these...I've gotten not too unreasonable quotes from Pacific Transformer..about $220 in single quantity for something similar (not identical though) to this. Thats much higher than the budget for this though. I need this transformer to be under $100 or I will feel like I haven't done my job.This just in: its occured to me that my original excel spreadsheet showed me 0.25T at the LOADED input voltage..not the unloaded input voltage. And, I have been talking about 0.5T for the new design at the UNLOADED input voltage. So that means I could probably further reduce the core size to get to where I thought I was going, and use even less core and less copper, than the EI-162 idea. But..to be conservative I will probably just ignore this to leave some slack, since I still don't know what will happen with the smaller core with the same number of turns. Nice to know there is some breathing room though.