# Leakage inductance on toroids

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What defines LL on toroids? I mean, if I send a wire down the center of one, is it zero? Nothing can be zero...

If it were an air core toroid, then it would be the ratio of a few diameters, probably with a logarithm I'm guessing. And if it's a ferrite core with such and such permeability, then it's roughly that many times less leakage, right?

Additional question: can I calculate LL on, say, a dual C type core? What defines that? Ugly geometry-dependent math I'm guessing, but how about ballparks?

Reason I ask is, I tried this:

(That's 6 turns 1/4" Cu tubing secondary, in series with 20uF and 1uH, and 24T 8AWG primary.) Resonant frequency was way low, like, lower than I can go, which was 15kHz. It's supposed to be 35k, so LL is at least 5 times bigger (>5uH). What was it actually at, and can I estimate it for a toroid?

I contemplated wrapping a slab of copper sheet around the core (to short out the leakage flux), but after a few minutes operation the primary winding was too melted to try anything more. On a related subject, I now despise proximity effect. ;-)

Tim

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nduction1001.jpg

I like Litz wire.

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Got any 8AWG Litz to spare me?

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Hmm, I'm not sure the gauge, but I've seen some scrap pieces laying around how long a piece do you need?

George H.

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Um. I think he was being 'funny.' 8AWG Litz would be... an oxymoron.

Jon

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You could braid your own. I did.

Cheers, James Arthur

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It isn't really. They make Litz equivalent to just about any size wire. For audio frequency coils, wire rope works about as well as Litz. If you have to wind a coil with #10 equivalent for audio frequencies, wire rope is much cheaper than Litz. You get more copper per square inch of cross section with wire rope.

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Is the 1uH supposed to be the induction coil in your picture? I'd expect to see secondary leakage leakage terms, for the secondary structure shown, that were in the same vicinity as for the unloaded coil. It might be better to put the series resonant capacitor in series with the primary, so that induction coil loading has a reduced effect. The intention is constant current - yes?

Assuming the circuit is primary current limited, shouldn't the primary be constructed to carry this limit safely? ... or are you just forgetting intended duty cycle limitations and the temperature limits of the wire shown?

Don't forget to use insulation between the primary wire and the core. Nomex is handy.

I can spare a couple of feet of 5.8mm OD litz, if you need it. This is equivalent to somewhere in between 6 and 8 AWG. Larger gauge litz is often constructed of multiple lighter gauges. I think this is 5x33 of

28AWG.

RL

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Not entirely. As a matter of fact, I have a fairly heavy air-core choke, removed from a motor drive. This was an, Eaton Electric I think, 10HP or so, thyristor powered VFD. This choke was either snubbing something on the supply rail, or providing commutation. Anyway, it's something like 6" diameter, maybe ten turns of 8AWG-sized wire rope in a single loop (the turns are bunched together toroidially, rather than as a flat solenoid). If it's varnished together, it won't be much good to me as wire. I haven't checked. That'd be the perfect kind of stuff to use though- either that or copper strip.

I do have enough copper on hand to cut my own strip, no worries there:

Tim

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Yes.

Right now, the intention is series resonance. Whatever happens, happens. I'd expect something constant-currenty, since as loading goes up, Q goes down so tank voltage goes down. That sort of thing. That would also behave similar to the Lmatch network I had (but with a better power factor!).

That was just the thickest, quickest wire I had on hand. I'd go for copper strap on the real thing. Obviously I had no concern for the wire's thermal limit, because I exceeded it quite flagrantly. :-)

The circuit is NOT current-mode, if that's what you mean. I may change that; I'm able to now. (With the Lmatch network, current mode feedback just buzzed at the Lmatch + coupling C series resonant frequency... ewww! Now the only thing it can buzz at is the resonant frequency, which I want anyway. Maybe I can use current feedback instead of a PLL!)

Hmmm. Is paper and Elmer's glue okay? ;o)

Hey, paper can handle heat at least as good as epoxy!

I'll keep that in mind, thanks.

Tim

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Kraft paper is a conventional insulation material for lower temperature systems. That's approximately what brown paper bags are made of - unbleached kraft stock. Layering is easy and effective.

It's hygroscopic, so for practical continuous use, it's better accompanied by vacuum impregnation with varnish.

RL

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A series resonant circuit is current limited below resonance by C, above resonance by L. Your's is shunted by the primary magnetic link, making lower frequency excursions potentially hazardous to the primary switches and conductors involved.

RL

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Well, I just recalled that Litz wire is designed for __skin__ effect. And there is a depth equation I once saw that was 7.5/sqrt(f) in cm for copper. 8 gauge is .326cm in diameter or .163cm in radius. So, this solves to an f of about 2kHz. Okay. Maybe.

It still seems funny, too. ;) And I thought that was what you were reaching towards.

Jon

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Yes. Low frequencies are doubly prohibited by the gate drives, which use coupling transformers at the moment. Fortunately, unusually high duty cycle is impossible from the TL494 (and the gate drives test perfectly from 0 to 50% duty). I don't know if I can state matter-of- factly that an unusually low frequency is impossible; I suppose a blob of molten iron could short out the timing capacitor or something and leave it locked in a high state. Even so, the desat detectors will disable the TL494 under overload conditions, so the worst that could happen is a few miliseconds overload I suppose.

Tim

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Well, it would do two things: increase current density (over solid wire) and reduce eddy current losses (which I've noticed can be a big problem).

I've ran the numbers, too, and I'm satisfied that my 1/4" copper tubing (about 0.02" thick wall) is carrying a fairly good current density up to 30kHz. :-)

As for what I was really reaching towards... well, I haven't seen

*much* Litz any bigger than, oh, 20AWG or so. The comedic effect would then be that something equivalent to 8AWG would be somewhat like getting whacked with a novelty baseball bat four times thicker than the real thing.

Tim

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this:

Yup! Franz Litz composed great music....

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DIY using multiple strands of insulated #28 wire..

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So what defines LL on toroids? =A0I mean, if I send a wire down the center of a toroidal transformer, is LL zero? =A0Nothing can be zero... so what is it, why?

Tim

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The minimum leakage inductance is roughly equal to the inductance of the isolated coil structure in the absence of all other materials. It follows the same inductance sizing rules as any air-cored structure.

RL

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