Litz wire

Clear back in the dark ages, with a 200W off-line SMPS running at

20kHz, I found that winding dissipation in a single strand winding was significant clear up to the 9th harmonic! The melted bobbins drew my attention to calculate that little detail :-(

So I switched to Litz and cool transformers. ...Jim Thompson

--
| James E.Thompson, CTO                            |    mens     |
| Analog Innovations, Inc.                         |     et      |
| Analog/Mixed-Signal ASIC's and Discrete Systems  |    manus    |
| Phoenix, Arizona  85048    Skype: Contacts Only  |             |
| Voice:(480)460-2350  Fax: Available upon request |  Brass Rat  |
| E-mail Icon at http://www.analog-innovations.com |    1962     |
             
I love to cook with wine.     Sometimes I even put it in the food.

It also depends on what he stated. If the run is short enough it makes no difference. Lower frequencies at lower currents also have less issues with the outward EM field push that we all know as "skin effect".

AND the mag field around each individual conductor in a bunch, whether Litz woven or not, will be different than that of the solid, so the EM field effect claimed to also affect non-woven bundles is not as pronounced as you might think.

The wires in the centers of non woven bundles do NOT exhibit the same problem that a solid does. It is several parallel fields, not a single conduction field. Yet another reason why the effect is still realized sans the weave. It is not 100% efficient, from that perspective, but it DOES STILL work.

It is a fact. Get used to it.

com...

need that

0A at

want to

d looks

t is, but

hat's

be good for

=A0I

l of

em from

thout

o

of

Yeah in the video they were using it to hook up various experiments.

George H.

=A0 =A0 =A0 =A0 Mike- Hide quoted text -

Sure here's a link to the Earths fied NMR.

To outer wooden forms are about 2.5 feet in diameter and hold the gradient coils. The NMR coil, wound with litz wire, is in the center of the thing. This is not 'my baby' so I only know the broad details of operation. The two tricks are turning off the polarizing B field 'in the right way'.. proper choice of diodes and R's and the use of noise canceling coils.

George H.

I completely agree Jim. I just went on a rant when qrk said, " If you're under 1MHz, you're mainly fighting proximity effect, not skin effect." I don't believe that, and I think a properly sized bundle of twisted (insulated wire) will have a large effect over a solid wire with the same circular mils. (as Dogmargoodboat said, 3x) It would be interesting to find what percentage change a twisted litz would have over a properly braided litz. I'll bet it's under 20%. What do you think? Mike

So.... what a hoot. You accidentally agreed with me?

I'd rather say you agree with me, and in that case, you sure are smart. Mike

It matters if it is not complete, and it touching other conductors along the way (and it will) causes problems. If not detrimental parasitics, in fact.

need

10A at

want

strands

strand

strands

driver,

them

=A0Without

so

skin

the

almost-parallel=20

It is not braided in the sense that hair is braided, it is twisted in = increasing=20 levels much like rope is. Say 3 strands twisted, in groups of 7, in = supergroups=20 of 3, in hypergroups of 7. Though this clearly would not be optimal.

The idea of the braiding is to try to equalize the effective resistance of all the strands by giving them all the same exposure to the outer layer of the bundle. A nice inclusive democratic idea, but all bat's wings and newt's eyes from the POV of electromagnetics--and even circuits.

Here's the circuits argument. Suppose you take two series strings of resistors, each of which alternates 1k and 10k, like this:

*----1k---10k---1k---10k---1k---10k---* | | 0--*---10k---1k---10k---1k---10k---1k----*--0

Total resistance: 1650 ohms (3.3k per string). Now suppose you put all the 10k resistors in one string and all the 100 ohm resistors in the other--like letting a couple of strands be the outside conductor the whole way. Then you have this:

*---10k---10k--10k---10k--10k---10k---* | | 0--*----1k---1k----1k---1k----1k---1k----*--0

Which is 6K // 600 ohms, or 545 ohms. Which is better?

The electromagnetic argument for Litz wire is also murky, because it relies on that dopey single-straight-wire-in-free-space derivation we all know and love. It's mathematically simple, but it has very little to do with the actual physical environment inside a transformer, where there's all that other copper running with similar dI/dt right nearby, plus the huge collective effect of the other turns and the core.

We pretty well know what the dB/dt is inside the winding. For a single conductor, the B field at the surface is transverse, and is all due to the current in that one wire. That field configuration gives rise to zero eddy current in the wire, just a radial gradient of the current density, and it makes very little difference to the fields outside the wire.

In a winding, however, there is a huge component of B perpendicular to the wire surface, due to the fields of all the other wires and the huge effect of the core. That will drive circulating currents inside the diameter of the wires (like turbulence in a pipe). There is thus the opportunity for a lot of eddy current loss in the copper if it isn't split up into very small transverse pieces (as in a laminated core).

The net effect is that parallel multifilar windings ought to be at least as good as Litz wire in transformers.

I've never seen a proper derivation of the eddy current loss in a transformer winding, but I'm sure it's been done. It sure doesn't have much to do with classical skin effect, though.

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs
Principal
ElectroOptical Innovations
55 Orchard Rd
Briarcliff Manor NY 10510
845-480-2058
hobbs at electrooptical dot net
http://electrooptical.net

It's been almost 25 years since I last worried about transformer windings. But I can recall at least one 5V 100A jobby where the output "winding" was 2" wide copper foil. (45º fold to make 90º connection tabs at the ends.) ...Jim Thompson

--
| James E.Thompson, CTO                            |    mens     |
| Analog Innovations, Inc.                         |     et      |
| Analog/Mixed-Signal ASIC's and Discrete Systems  |    manus    |
| Phoenix, Arizona  85048    Skype: Contacts Only  |             |
| Voice:(480)460-2350  Fax: Available upon request |  Brass Rat  |
| E-mail Icon at http://www.analog-innovations.com |    1962     |
             
I love to cook with wine.     Sometimes I even put it in the food.

This is pretty good:

John

com...

need that

0A at

want to

d looks

t is, but

hat's

be good for

=A0I

l of

em from

thout

o

Wow.. lotsa different configurations there. Here's the magnet lab video for anyone interested.

Enjoy!

George H.

As a gross rule of thumb, proximity effect dominates under 1MHz. Above

1MHz, skin effect will come in to play. Most of my work designing power pulse transformers is in the 50kHz to 800kHz range. Checks with an impedance analyzer (HP 4195 and 4194) have shown that the equations given in E.C. Snellings "Soft Ferrites" book are pretty good. E.C. Snelling and Giles have another book on transformers. I've forgotten the title, but it also covers proximity effect.

--
Mark

Too early in the AM!

--
Dr Philip C D Hobbs
Principal
ElectroOptical Innovations
55 Orchard Rd
Briarcliff Manor NY 10510
845-480-2058
hobbs at electrooptical dot net
http://electrooptical.net

Proximity effect is little known unless you design transformers. It is rarely taught at the uni. An excellent reference is "Soft Ferrites" by E.C. Snelling. You can usually find this book at university libraries. There's another book, "Ferrites for Inductors and Transformers" by Snelling and Giles on transformer design. Soft Ferrites isn't the easiest read, but the information towards the rear of the book has all sorts of useful information about winding configurations. The program "Magnetics Designer" by Intusoft also deals with proximity effect.

Frequencies under 1MHz, proximity effect is dominant. I have run the calculations and tested transformer designs with impedance analyzers (HP 4195 & 4194). The calculated and measured come out fairly close.

Bunched conductors are parallel insulated conductors (magnet wire) which are twisted together, just like stranded hookup wire from Belden, but fewer twists per inch. In a pinch, I'll make my own bunched conductors and twist the bunch using an electric drill. For production, I use Litz wire from MWS. I use Litz since they won't make bunched conductors for me. Don't know why since this is considered one bunch in a Litz construction. Since we don't make that many transformers, the 5 pounds of wire we get will last a decade or two.

On the practical side, the bunched conductors I have made/used aren't much different from commercial Litz wire in my tests making 5kW pulse transformers for 400kHz service. I'm sure my office made bunched conductors weren't perfectly in the same position at an equal time. Holy crap, that's a lot to say! Just twist the stuff and everything will be happy.

--
Mark

E X A C T L Y !

A bundle of individual EM fields from individual strands does NOT generate a field that is the same as that of an equally sized solid conductor.

In fact, in a solid conductor, conduction CAN be pushed to the outside of THAT conductor. In the case of a bundle of individual conductors, the individual EM fields generated are NOT capable of forcing ANY of the center conductors' electrons to the outer shell of the bundle, and from within an individual conductor, IF the push was occurring, it would only 'push' those electrons out to one side of it, but it would still be 'out' at the skin, where it belongs.

More likely a mag wire manufacturers ploy to sell "specially configured" spools at exorbitant prices over cost.

We could have been making our own all along.

Bulloney. Maybe it is "rarely taught" at that thing you call the school that you went to... maybe.

Wescott

need

at

want

strands

strand

strands

driver,

Without

so

insulated

skin

the

almost-parallel

circuits.

wire.

=20

I like it when people explain things from basics. I (re)learn more that = way.