Litz wire
in [Design]
|
From: Phil Hobbs on 22 Feb 2010 13:48 On 2/22/2010 9:45 AM, Phil Hobbs wrote: > On 2/20/2010 1:04 PM, Tim Wescott wrote: >> On Sat, 20 Feb 2010 05:38:23 -0800, dcaster(a)krl.org wrote: >> >>> On Feb 20, 5:47 am, life imitates life >>> <pastic...(a)thebarattheendoftheuniverse.org> wrote: >>>> On Fri, 19 Feb 2010 16:58:13 -0600, Tim Wescott<t...(a)seemywebsite.com> >>>> wrote: >>>> >>>> >>>> >>>>> On Fri, 19 Feb 2010 16:18:49 -0600, Tim Williams wrote: >>>> >>>>>> "George Herold"<ggher...(a)gmail.com> wrote in message >>>>>> news:550831a9-5935-4e3d-b37e- >> c664ebd9d752(a)o3g2000yqb.googlegroups.com... >>>>>>> We get Litz wire from MWS but nothing that big. If you don't need >>>>>>> that much have you thought of 'rolling your own'? >>>> >>>>>> I do sometimes, but only for small things. I'm contemplating 10A at >>>>>> 1MHz, so it needs to be pretty fine = way more strands than I'd want >>>>>> to deal with. >>>> >>>>>> I actually have some copper rope, which is about 1/4" diameter and >>>>>> looks to be made of 28AWG or so. I don't remember how many strands >>>>>> it is, but if I guess the rope is wound from 7 strands of 31 strand >>>>>> twist, that's 7*31 = 217. If 28AWG is good for ~200mA, 217 strands >>>>>> should be good for 40A, which sounds about right, I'd call it 8 or >>>>>> 10AWG equivalent. I salvaged this stuff from some old motor driver, >>>>>> which used a spool of this stuff for air-core inductors. >>>> >>>>>> Tim >>>> >>>>> Isn't there some magic braiding pattern for Litz wire? >>>> >>>> No. The wire strands have to be mag wire, which segregates them >>>> from >>>> each other, allowing the skin effect to be taken advantage of. Without >>>> strand segregation, it becomes a single strand, from the POV of the >>>> current flowing in it, with only one skin for the entire mass. >>> >>> No again. The wire strands have to be segregated and also braided so >>> that some of the time a strand is on the outside of the bundle and >>> sometimes on the inside of the bundle. See attached from Wiki. >>> >>> Litz wire uses some different tricks. Instead of using one big >>> conductor, it uses lots of little conductors (strands) in parallel >>> (forming a bundle). Each little conductor is less than a skin-depth, so >>> an individual strand does not suffer an appreciable skin effect loss. >>> However, that is not the complete story. The strands must be insulated >>> from each other -- otherwise all the wires in the bundle would short >>> together, look like a single large wire, and still have skin effect >>> problems. Furthermore, the strands cannot occupy the same radial >>> position in the bundle: the electromagnetic effects that cause the skin >>> effect would still disrupt conduction. The bundle is constructed so the >>> individual strands are on the outside of the bundle (and see low >>> resistance) for a time, but also reside in the interior of the bundle >>> (where the EM field changes are the strongest and the resistance is >>> higher). If each strand sees about the same average resistance, then >>> each strand will contribute equally to the conduction of the entire >>> cable. >> >> Why, then, does it work to make up multi-strand wire bundles for SMPS >> service where the wires are just lightly twisted into a bundle? Are >> these getting one part of the way there, but not all? >> >> I had heard about the Litz wire 'gotta be a magic braid pattern', but >> then I've seen all these SMPS transformers that just have almost-parallel >> strands. >> > > 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 > <correction--16500 ohms (33k per string)--was thinking 1k/100 ohms> > 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? <correction--60k // 6k, or 5455 ohms. Argument still holds.> > > > 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 > > 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
From: life imitates life on 22 Feb 2010 20:28 On Mon, 22 Feb 2010 09:45:48 -0500, Phil Hobbs <pcdhSpamMeSenseless(a)electrooptical.net> wrote: >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. 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.
From: life imitates life on 22 Feb 2010 20:56 On Mon, 22 Feb 2010 11:02:09 -0800, qrk <SpamTrap(a)spam.net> wrote: >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. Bulloney. Maybe it is "rarely taught" at that thing you call the school that you went to... maybe.
From: JosephKK on 23 Feb 2010 21:00 On Mon, 22 Feb 2010 09:45:48 -0500, Phil Hobbs <pcdhSpamMeSenseless(a)electrooptical.net> wrote: >On 2/20/2010 1:04 PM, Tim Wescott wrote: >> On Sat, 20 Feb 2010 05:38:23 -0800, dcaster(a)krl.org wrote: >> >>> On Feb 20, 5:47 am, life imitates life >>> <pastic...(a)thebarattheendoftheuniverse.org> wrote: >>>> On Fri, 19 Feb 2010 16:58:13 -0600, Tim Wescott<t...(a)seemywebsite.com> >>>> wrote: >>>> >>>> >>>> >>>>> On Fri, 19 Feb 2010 16:18:49 -0600, Tim Williams wrote: >>>> >>>>>> "George Herold"<ggher...(a)gmail.com> wrote in message >>>>>> news:550831a9-5935-4e3d-b37e- >> c664ebd9d752(a)o3g2000yqb.googlegroups.com... >>>>>>> We get Litz wire from MWS but nothing that big. If you don't need >>>>>>> that much have you thought of 'rolling your own'? >>>> >>>>>> I do sometimes, but only for small things. I'm contemplating 10A at >>>>>> 1MHz, so it needs to be pretty fine = way more strands than I'd want >>>>>> to deal with. >>>> >>>>>> I actually have some copper rope, which is about 1/4" diameter and >>>>>> looks to be made of 28AWG or so. I don't remember how many strands >>>>>> it is, but if I guess the rope is wound from 7 strands of 31 strand >>>>>> twist, that's 7*31 = 217. If 28AWG is good for ~200mA, 217 strands >>>>>> should be good for 40A, which sounds about right, I'd call it 8 or >>>>>> 10AWG equivalent. I salvaged this stuff from some old motor driver, >>>>>> which used a spool of this stuff for air-core inductors. >>>> >>>>>> Tim >>>> >>>>> Isn't there some magic braiding pattern for Litz wire? >>>> >>>> No. The wire strands have to be mag wire, which segregates them >>>> from >>>> each other, allowing the skin effect to be taken advantage of. Without >>>> strand segregation, it becomes a single strand, from the POV of the >>>> current flowing in it, with only one skin for the entire mass. >>> >>> No again. The wire strands have to be segregated and also braided so >>> that some of the time a strand is on the outside of the bundle and >>> sometimes on the inside of the bundle. See attached from Wiki. >>> >>> Litz wire uses some different tricks. Instead of using one big >>> conductor, it uses lots of little conductors (strands) in parallel >>> (forming a bundle). Each little conductor is less than a skin-depth, so >>> an individual strand does not suffer an appreciable skin effect loss. >>> However, that is not the complete story. The strands must be insulated >>> from each other -- otherwise all the wires in the bundle would short >>> together, look like a single large wire, and still have skin effect >>> problems. Furthermore, the strands cannot occupy the same radial >>> position in the bundle: the electromagnetic effects that cause the skin >>> effect would still disrupt conduction. The bundle is constructed so the >>> individual strands are on the outside of the bundle (and see low >>> resistance) for a time, but also reside in the interior of the bundle >>> (where the EM field changes are the strongest and the resistance is >>> higher). If each strand sees about the same average resistance, then >>> each strand will contribute equally to the conduction of the entire >>> cable. >> >> Why, then, does it work to make up multi-strand wire bundles for SMPS >> service where the wires are just lightly twisted into a bundle? Are >> these getting one part of the way there, but not all? >> >> I had heard about the Litz wire 'gotta be a magic braid pattern', but >> then I've seen all these SMPS transformers that just have almost-parallel >> strands. >> > >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 I like it when people explain things from basics. I (re)learn more that way.
From: JosephKK on 23 Feb 2010 21:44
On Mon, 22 Feb 2010 07:08:54 -0800, John Larkin <jjlarkin(a)highNOTlandTHIStechnologyPART.com> wrote: >On Sat, 20 Feb 2010 21:22:51 -0800 (PST), George Herold ><ggherold(a)gmail.com> wrote: > >>On Feb 20, 8:38 am, "dcas...(a)krl.org" <dcas...(a)krl.org> wrote: >>> On Feb 20, 5:47 am, life imitates life >>> >>> >>> >>> >>> >>> <pastic...(a)thebarattheendoftheuniverse.org> wrote: >>> > On Fri, 19 Feb 2010 16:58:13 -0600, Tim Wescott <t...(a)seemywebsite.com> >>> > wrote: >>> >>> > >On Fri, 19 Feb 2010 16:18:49 -0600, Tim Williams wrote: >>> >>> > >> "George Herold" <ggher...(a)gmail.com> wrote in message >>> > >>news:550831a9-5935-4e3d-b37e-c664ebd9d752(a)o3g2000yqb.googlegroups.com.... >>> > >>> We get Litz wire from MWS but nothing that big. If you don't need that >>> > >>> much have you thought of 'rolling your own'? >>> >>> > >> I do sometimes, but only for small things. I'm contemplating 10A at >>> > >> 1MHz, so it needs to be pretty fine = way more strands than I'd want to >>> > >> deal with. >>> >>> > >> I actually have some copper rope, which is about 1/4" diameter and looks >>> > >> to be made of 28AWG or so. I don't remember how many strands it is, but >>> > >> if I guess the rope is wound from 7 strands of 31 strand twist, that's >>> > >> 7*31 = 217. If 28AWG is good for ~200mA, 217 strands should be good for >>> > >> 40A, which sounds about right, I'd call it 8 or 10AWG equivalent. I >>> > >> salvaged this stuff from some old motor driver, which used a spool of >>> > >> this stuff for air-core inductors. >>> >>> > >> Tim >>> >>> > >Isn't there some magic braiding pattern for Litz wire? >>> >>> > No. The wire strands have to be mag wire, which segregates them from >>> > each other, allowing the skin effect to be taken advantage of. Without >>> > strand segregation, it becomes a single strand, from the POV of the >>> > current flowing in it, with only one skin for the entire mass. >>> >>> No again. The wire strands have to be segregated and also braided so >>> that some of the time a strand is on the outside of the bundle and >>> sometimes on the inside of the bundle. See attached from Wiki. >>> >>> Litz wire uses some different tricks. Instead of using one big >>> conductor, it uses lots of little conductors (strands) in parallel >>> (forming a bundle). Each little conductor is less than a skin-depth, >>> so an individual strand does not suffer an appreciable skin effect >>> loss. However, that is not the complete story. The strands must be >>> insulated from each other -- otherwise all the wires in the bundle >>> would short together, look like a single large wire, and still have >>> skin effect problems. Furthermore, the strands cannot occupy the same >>> radial position in the bundle: the electromagnetic effects that cause >>> the skin effect would still disrupt conduction. The bundle is >>> constructed so the individual strands are on the outside of the bundle >>> (and see low resistance) for a time, but also reside in the interior >>> of the bundle (where the EM field changes are the strongest and the >>> resistance is higher). If each strand sees about the same average >>> resistance, then each strand will contribute equally to the conduction >>> of the entire cable. >>> >>> Dan- Hide quoted text - >>> >>> - Show quoted text - >> >>Cool, Thanks Dan. I'd never thought of that. >> >>A bit OT, but I remember seeing a video of high current experiments >>done at the Magnet Lab, (then at MIT circa 1960's) Where they were >>using several ~2-3" wide strips of copper. > >This is pretty good: > >http://www.newenglandwire.com/nepdfs/litz_Brochure.pdf > > >John Many of the types shown are twisted bundles, which may be assembled into higher order twisted bundles. Noted Type 7 is explicitly braided. |