yes I see what you mean, on that subject , it seems to me that 30 strands of 0.1mm all couped up together is going to present a big layer effect problem.
The reason we don't get a layer effect in a bundle of wires is because the bundle is twisted so that (hopefully) each strand spends as much time inside the bundle as it spends outside the bundle.
My understanding is that the German word "litzendraht" means woven wire. A lot of wire marketed as "litz" wire is, very strictly speaking, not litz wire. It is twisted bundles of wire, grouped together with other bundles, and then twisted again, not really "woven". But, this is less expensive than weaving, and is almost as good. See:
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especially their tutorial:
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Professor Sullivan at Dartmouth has done a lot of work on this topic. He has a paper on economical choices of "litz" wire.
It is always said that the individual strands of wire have to be insulated from each other to make "litz" wire, but as Prof. Sullivan points out, the strands making up the bundle will have some oxide on the outside of each strand which will to a great extent, serve as "insulation", one strand from another. So plain old stranded wire will be better than solid wire in many cases.
Also, stranded wire where the individual strands are insulated, but where the construction is not "bundles of bundles" will still have a beneficial effect.
Finally, a way to minimize layer effect is to interleave layers of primary and secondary. In another post, I'll show what this can do.