What will leakage inductance of transformer primary be? - Page 1
Forum section: electronics / general

author: Faringdon
author_rank: Super Contributor
timestamp: April 01, 2022, 10:10:10 pm
content: Supposing you have an ungapped PM62/49 core. Its “sandwich” wound Np1:Ns1:Ns2:Np2 ( 4 layers).Where Np1 = 11 turns and Np2 = 10 turns are in series. Ns1 and Ns2 are separate secondary coils of 9 turns each.What is the leakage inductance measured at the 21 turn primary with both secondaries shorted?Each layer of the 4 layers of turns spans the entire bobbin length.….The leakage inductance will bear some relation to the inductance of theair cored coil…..at least, the leakage inductance will definitely not be lower in inductance than this.The inductance (uH) of an air cored coil is (N^2 * D^2) / (18.D + 40.L)Where:N = number of turnsD = coil diameterL = coil lengthBut what is the leakage inductance as described? And what value do you take for “length” above?....because the primary is made of 2 layers in series…..the equation doesn’t deal with that.If we take “length = bobbin length” = 33mm, then the above equation gives the air cored coil inductance as 163uH. I just cant believe this….no way can an air cored coil of 21 turns (on a PM62/49 bobbin) have an inductance of 163uH.PM62/49 core:https://www.tdk-electronics.tdk.com/inf/80/db/fer/pm_62_49.pdfThis page gives the inductance of the air cored coil as 6.45uH (D=25.7mm, Length=33mm, N=21)https://m0ukd.com/calculators/air-cored-inductor-calculator/...however, leakage inductance , in some past experience, is about three times this value, so thats 19.5uH.But then suppose the core gets gapped to AL=1000......then that woud make the Primary coil open circuit inductance = 441uH.......and since gapping doesnt affect leakage inductance, that would mean the leakage inducatance being 4.3% of primary inductance....which is way too large.....leakage inductance of an interleave wound coil is never usually more than 1% of primary?So just what is the leakage inductance described above?

author: T3sl4co1l
author_rank: Super Contributor
timestamp: April 01, 2022, 10:15:59 pm
content: Are those single layer windings then? Roughly speaking, it's the equivalent of a parallel wire transmission line, edge-wound, with one side shorted out. So, the inductance comes from the length, of such a geometry.So, depending on wire/cable type, arrangement and insulation thickness, maybe in the 100 ohm range, so ~300nH/m times wire length.The air cored formula doesn't solve for mutual inductance between two coils and therefore cannot be used to subtract out the secondary's mutual inductance. That does however give close to the value expected for deep core saturation, not that that's important with respect to operating conditions.Tim

author: Faringdon
author_rank: Super Contributor
timestamp: April 01, 2022, 10:19:57 pm
content: QuoteThanks, yes they are.Incidentally, primary is made of 3 strands of 7/0.23mm TEX-ELZSecondaries are each made of 3 strands of 7/0.3mm TEX-ELZTEX-ELZ wirehttps://www.furukawa.co.jp/tex-e/en/product/texelz.htmlQuoteThanks, the primary coil length is 2.52metres, so by your kind formula, the air cored coil inductance is 756nH....So then you would say the leakaage inductance as described is maybe three times this? (my guess)Its an interelave wound transformer, so i just cant understand it if the leakage inductance of the primary with both secondarys shorted is more than about 4uH?I am thinking back to interleave wound flyback transformers...and the leakage inductance always ends up being really low.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: April 01, 2022, 10:53:30 pm
content: Three times what?The stranded cables will have somewhat higher inductance by themselves; using three in parallel probably about balances that out, I don't know.Or are you saying the three strands are in series?Tim

author: Faringdon
author_rank: Super Contributor
timestamp: April 02, 2022, 01:02:12 pm
content: QuoteThanks....its using 3 strands in parallel, to reduce overall winding resistance.