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Re: AC Resistance of wires - was 8 kHz Tesla Coil



Original poster: "Malcolm Watts" <m.j.watts@xxxxxxxxxxxx>

On 4 Oct 2005, at 5:27, Tesla list wrote:

> Original poster: "Gerry  Reynolds" <gerryreynolds@xxxxxxxxxxxxx>
>
> Hi Kurt,
>
> The Medhurst resistance tables assume "high frequency" and that the
> proximitry effect is "fully saturated".  The table was also developed
> for coils with a small number of turns (30-50).  The proximitry effect
> may be overestimated.

It should be "proximity" as in "close to".

Malcolm

> Gerry R
>
> >Original poster: "Kurt Schraner" <k.schraner@xxxxxxxxxxx>
> >
> >Hi Bart,Gerry,
> >
> >Medhurst's empirical Method for HF-Resistance of solenoids is
> >implemented in my TC-Plan Excel:
> >http://home.datacomm.ch/k.schraner/TCplan.zip
> >The relevant sheets are "Skineff." and "Proxi-eff.", whose results
> >are transferred to "TC calc."' cells E34:E38. In the light of more up
> >to date, more general methods, Medhurst may soon become of lesser
> >relevance, but I've tried to compare results from it, with Q of some
> >of my coils, estimated from bandwidth measurements at Fres. A summary
> > Excel of the results can be had at:
> >http://home.datacomm.ch/k.schraner/Qsec2.xls or here (view in fixed
> >width font, sorry for the possible mess!):
> >
> > TC-Secondary Q-Estimation by Medhurst's Empirical Method
> > for some of my Coils
> >
> > Geometric Data
> > Secondary Diameter Length Aspect L/D Turns
> > Name [cm] [cm] [ - ] [ - ]
> > Sk-Seibt 4 150.3 37.58 4187
> > Sk-UBTT 12.54 60.3 4.81 1680
> > Sk-5cm 5.11 41.1 8.04 934
> > Sk-12cm 12.12 58.5 4.83 894
> > Sk-Long 16.1 140.2 8.71 1976
> > Sk-20cm 20.52 66.8 3.26 943
> > Sk-38cm 38.4 44 1.15 346
> > Sk-B&W *)40.13 177 4.41 813
> > Sk-B&W *)40.13 177 4.41 813
> >
> > Geometric Data (cont.)
> > Secondary Wire-Copper Pitch Comment
> > Name [mm] [mm]
> > Sk-Seibt 0.319 0.359 Big error unexplained!
> > Sk-UBTT 0.319 0.359 Test-signal via low-Z amp.
> > Sk-5cm 0.4 0.434 no experiment
> > Sk-12cm 0.6 0.654 Test-signal via low-Z amp.
> > Sk-Long 0.64 0.71 Test-signal via low-Z amp.
> > Sk-20cm 0.63 0.708 no experiment
> > Sk-38cm 1 1.2 cardoard/cotton insul. wire
> > Sk-B&W *) 1.268 2.177 Test-signal via low-Z amp.
> > Sk-B&W *) 1.268 2.177 if R Sig-Gen=136 Ohm
> >
> >
> > Electrical Data
> > Secondary at1kHz Fres,bare DC Resist. Wire straigt
> > Name Lexp[mH] exp.[kHz] exp.[Ohm] with Skineff.
> > Sk-Seibt 18.13 366.11 114 240.82
> > Sk-UBTT 66.59 205.4 147.8 147.86
> > Sk-5cm 5.203 923.8 21.7 n.a.
> > Sk-12cm 18.2 402.8 21 27.61
> > Sk-Long 67.7 147.4 55.2 61.05
> > Sk-20cm 49 202.7 34.8 40.5
> > Sk-38cm 28.12 217.95 9.2 16.32
> > Sk-B&W *)54.67 119 14.5 22.3
> > Sk-B&W *)54.67 119 14.5+136 22.3+136
> >
> > Electrical Data (cont.)
> > Secondary Rac-calc. Qexp Qmedh. Qerror
> > Name with Proxy.E [ - ] [ - ] [%]
> > Sk-Seibt 698.9 156.5 70.8 54.8%
> > Sk-UBTT 440.65 211.8 186 12.2%
> > Sk-5cm n.a. n.a. n.a. n.a.
> > Sk-12cm 91.01 269 270 -0.4%
> > Sk-Long 190.6 164.4 168.4 -2.4%
> > Sk-20cm 128.0 n.a. 478.3 n.a.
> > Sk-38cm 49.79 104.3 803.7 -670.6%
> > Sk-B&W *) 44.02 141.7 576 -306.5%
> > Sk-B&W *) 44.02+136 141.7 141.7 0.0%
> >
> >
> >The method seems to underestimate Q of my small diameter coils, and
> >to (heavily) overestimate the Q of the big diameter coils. What seems
> >obvious is the dominant proximity effect over skineffect for usual TC
> >frequencies, as mentioned before on the relevant treads. Maybe
> >dielectric loss, not considered in the calculations, come to play
> >here...
> >
> >*) For the bandwidth measurements it seems important using a low-Z
> >signal source (--> Terry's low-Z amp), which was not the case for
> >"Sk-B&W". Otherwise the generators internal resistance will reduce
> >measured Q value. Anyway, bandwidth measurements seem prone to higher
> >error, 'cause of amplitude reading on the scope and frequency
> >precision reading from the sig-gen.
> >
> >Well ... my 2cents for the interesting threads about coil Q. If I see
> >coming out a method estimating Q with perhaps 10% error, I'd readily
> >adapt it!
> >
> >Best regards
> >Kurt
> >
> >
>
>
>
>
>