Toroidal isolation transformer design - Page 1
Forum section: electronics / projects

author: sniper
author_rank: Newbie
timestamp: May 21, 2019, 07:45:11 pm
content: Hey folks,I'm in need of a few hundred VA isolation transformer for single phase 230/240 V 50 Hz AC. I'd be grateful for some insight on how to proceed.I happen to have a 40+, maybe even 60 years old auto-transformer which I decided to scrap for this project b/c at some point it has been dropped and 20+ turns of the silk-covered enamelled wire have been damaged. Upon disassembly - to my surprise - I found out that the core itself is actually a stack of 128 annular laminations (d=60 mm, D=120 mm, h=0.5 mm, H=68 mm assembled) rather than being wound from a single piece of silicon steel tape.I wonder if the reason for that is related to the core being relatively tall compared to the average tape-wound one.It is obviously hugely expensive to punch the discs out from that awful kind of steel, also of course incurring significant loss of the raw material in the process.The original transformer had 444+48 turns at the 220 V and 240 V terminals. I also find it interesting that the turns per volt ratio is considerably lower in the longer winding than the shorter one.Does that make sense to you?Unfortunately, the power rating is unknown, I'm assuming that it's rated for about 330 to 500 VA depending on the steel grade... probably closer to the former, considering its age. The laminations are in good shape, it's fairly difficult to plunge through the insulation and/or oxide using the multimeter probes. Barely any rust at all....and the actual question:how do I lay out the windingsto minimize leakage capacitance while keeping leakage inductance at acceptable levels? Do I just wind both windings on top of each other and cover the primary with a broken ring of copper tape with a separate lead to neutral? How can I calculate or at least estimate the leakage capacitance between the windings in this arrangement? Is it worthwhile to use techniques like sector winding (in my case, 2*180 or 4*90 deg), progressive winding or bank winding? At this point I don't foresee much of a difference neither in parts nor in labor cost between these options, so I might as well go with the one that's best for my use case.The literature I've found on the subject was pretty sparse and inconclusive. Many scholarly articles I've read about these topics had a very narrow FOV, mostly discussing transformers for HF (SMPS) or HV.Thanks,Gabor

author: T3sl4co1l
author_rank: Super Contributor
timestamp: May 21, 2019, 09:37:11 pm
content: You can't do both. Low leakage gives high capacitance, and vice versa. The underlying explanation is you're shifting around the characteristic impedance of the transformer -- a transformer is a complex LC component with impedance and bandwidth.What you're really after, then, is a higher bandwidth transformer: both low leakage and capacitance.To that end, you need less winding length. A thicker core helps, and so the stack height being relatively high is not too bad (and the toroid being almost "square"). It's at least not a bad candidate.Now, notneedingmuch bandwidth in the first place, is of course a big help. You don't really need terribly low leakage at 50Hz. I wouldn't be surprised if allocating the windings to separate sectors would work out okay. The leakage of this configuration I think will be worse than for a bank-wound (E-I core) transformer. But that might still be okay?Note that, if you do arrange the windings by sectors, you need the core grounded, so it acts as a shield. Otherwise, obviously it's just a capacitor between the windings, and that's no help. (If you can't get a good connection to the core -- may be the case due to the stacked-rings design* -- you may want to wrap the core with insulation, and then with shielding tape that makes a slitted turn, and ground that.)*Yeah, that is a bit unusual; I wouldn't expect it to be much more expensive than an EI stack though. They recycle the waste, or maybe they even punched smaller rings out of the holes, who knows.You can still do a shield with overlaid windings, put a slitted foil layer between them and ground that. That doesn't do anything to reduce the capacitance from each winding to ground (the bandwidth is about the same), but greatly improves the winding-to-winding capacitance, which is a big advantage with respect to mains isolation and noise.Next best is to put height between the windings, but for a toroid that's pretty annoying. You'd need some kind of spacer, like a layer of thick fiber tape or something?Tim

author: bdunham7
author_rank: Super Contributor
timestamp: May 21, 2019, 10:25:22 pm
content: I think the design goal is a little different depending on the reason for the isolation--safety/medical or instrument?So-called medical grade units often have two layers with a grounded shield in between--and the goal is to have low leakage and the safety shield is there to minimize the effect of interwinding capacitance. Transformers for safety isolation--outdoor lighting and pools--use a similar setup. I've often thought that the shield is a double-edged sword in that any capacitive coupling of the output winding to ground will now result in there being a potential between the outputs and ground, not from mains leakage but from the output potential itself.Here is a medical-grade toroid. It appears to have layers. It is probably fairly efficient (low inductive leakage)https://www.ebay.com/itm/NEW-Toroidal-Medical-Isolation-Transformer-2-x-120VAC-Secondary-400VA/254194799302Instrument power supply transformers often take a different route that results in an inefficient but very low leakage transformer which, like 'sector winding' separates the windings on plastic bobbins. Here is one:https://www.jameco.com/z/SPW2304-12VA-PC-Split-Bobbin-Power-Transformer-8-Pin-115-230VAC_102593.html?CID=MERCHI don't know your application, but unless you are worried about efficiency, my approach would be to insulate the core totally from both windings and then do either two windings on opposite halves, separated by an insulating collar, or do four windings on four quarters. I really don't know how much efficiency would be gained by using 4 windings but I suspect not much.