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I measured the AC resistance of my customized inductor with a network analyzer of core E70. I got 23 mΩ AC resistance and 2.3 mΩ DCR for a 14 μH inductor with 8 turns using Litz wire (1400) with a 4.3 mm diameter.

I also got 30 mΩ AC resistance and 11 mΩ DCR for a 54 μH inductor with 16 turns using different Litz wire (1050) with a 2.8 mm diameter. Both were measured at the same frequency.

I am confused now; which factor affects the AC resistance? I thought I should have gotten more AC resistance using the 54 μH inductor. Does it depend on the type of Litz wire we use? Or the number of the small wire? How can I reduce the AC resistance?

ocrdu
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Manjesh Gowda
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  • The resistance will depend upon the length of the wire and its diameter (and also it's composition, but I assume both wires were electrical grade copper). If you want to decrease the resistance of an inductor without changing the inductance or changing the core (if it has one) then you need thicker wire or multiple wires connected in parallel. – Math Keeps Me Busy Feb 11 '21 at 13:28
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    *Both measured at same frequency.* - here's the problem - I'm struggling to see what you mean here because DC resistance has to be measured at DC or it will contain AC resistance artefacts. – Andy aka Feb 11 '21 at 13:29
  • Also, try measuring both with the core removed. – Andy aka Feb 11 '21 at 13:37
  • @Andyaka I measured both AC and DC in different equipments, here thats not an issue. What I mean is I was supposed to get more AC resistance for 54uH inductor right? because the wire dia is less compared to other. I still could not understand on what parameters does AC resistance depend on? – Manjesh Gowda Feb 11 '21 at 13:48
  • The AC resistance measurement has to include the DC resistance measurement because it cannot tell the difference. So now you are comparing 20.7 mohms (L1) with 19 mohms (L2). I.e. both are about the same for "true AC resistance". But, you are not measuring "true AC resistances" of the wire but also core losses because the core is in place. If you removed the cores and measured again you'd get virtually true value for AC copper resistance losses. – Andy aka Feb 11 '21 at 14:01
  • core is same for both but having different airgaps. The 14uH is having 3.9mm gap on each leg and 54uH is having 4.2mm gap. From this i understood that if we measure with core, air gap also comes into effect? it is 23mohms for 14uH and 30mohms for 54uH – Manjesh Gowda Feb 11 '21 at 14:09
  • @Andyaka I measured without the core and it gives 4mohm for 14uH inductor. It gives me a AC copper loss. What I dont understand is, measuring with core, I am not sure what exactly it gives! Can we calculate some losses using that? I am confused. Please let me know – Manjesh Gowda Feb 11 '21 at 19:58
  • You will see an increased resistance when on the core because you measure also the core losses. What do both coils produce without the core in place. – Andy aka Feb 11 '21 at 21:26
  • @Andyaka without the core i got 4mohm. So, with the core on the I got the resistance of 23mohm, What does it exactly indicate? Does it help us to get an rough idea about the whole losses or about the temperature? I do not understand that part. Please help me with this – Manjesh Gowda Feb 12 '21 at 07:50
  • Your question (above) is about comparing two inductors and you've only provided one value of AC resistance without the core so, that's why I asked you for the details of the other winding when not using a core. So, I'll say again - *What do both coils produce without the core in place* - I'm not going to be drawn into side questions until I can make a decent answer on your formal question and that involves testing both without the core. – Andy aka Feb 12 '21 at 08:00
  • one gives 4mOhm and another one gives 12mohm. – Manjesh Gowda Feb 12 '21 at 14:41
  • Perhaps a past [post](https://electronics.stackexchange.com/questions/586278/how-do-skin-effect-and-proximity-effect-behave-for-flat-conductor-power-inductor/586317#586317) will shed some light. – qrk Dec 04 '21 at 09:10

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The AC resistance of your inductor is determined by the litz wire used. See the New England Wire website for detailed information on litz wire selection. https://www.newenglandwire.com/traditional-litz-wire-theory/

Neils Kruse
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