Re: 8 kHz Tesla Coil

["Tesla list" <tesla@xxxxxxxxxx>]

Original poster: "Barton B. Anderson" <bartb@xxxxxxxxxxxxxxxx>

Hi Gerry,

I've searched high and low for the reference to the equation, but I 
can't find it. It is something I picked up from Googling and didn't 
save the document (dummy me). You are correct, Rdc added to Rac makes 
no sense. I'm sure I wrote it down as it was shown. I suspect this 
may have come out of a text book, but I think it's use was not 
interpreted as intended. I'm certain this was a skin effect approximation.

I've found another approximation calculator (excel throw-together) 
for Rac and skin depth in cylindrical conductors. It gave 37 ohms on 
your coil. If you remember, I showed 98 after adding the 61 Rdc (per 
the equation), which is a difference of 37 ohms! It appears the 
equation was using Rdc as part of it's approximation, but Rdc should 
have been removed to identify Rac, such as:

Rac = Rdc(1+(r/(2sD))-Rdc

where:

Rdc = DC resistance of winding
r = radius of wire diameter in inches
sD = skin depth in inches

For reference, another Rac approximation I found from Michael Mirmak 
of Intel Corp (was for pcb traces originally using trace heights and 
widths) I modified for a round conductor: It follows the excel 
calculator for high and low frequency's.

Rac = L(((3.318*10^-7)*F^0.5)/(4*d))
(should be ready for excel - just insert the values)

where:
L = Length of winding in inches
F = Frequency in Hz
d = wire diameter in inches

BTW, here's the excel file mentioned:
http://home.swipnet.se/2ingandlin/Skin_depth_calc.xls

Of course, we don't know how well any of these approximations work 
for our coils. It would be interesting to measure and find the 
proximity losses. Empirically, we could likely come up with something 
similar, which of course gets better with time. So far, no one is 
doing this. When I get my coils set back up, I'll have to perform 
some Q measurements.

Take care,
Bart

Tesla list wrote:

> Original poster: "Gerry  Reynolds" <gerryreynolds@xxxxxxxxxxxxx>
>
> Hi Bart,
>
> Maybe doing this would be better than nothing.  At least for close 
> wound coils, the assumption may be close enough and relative 
> comparisons would be valuable perhaps.
>
> In additions to Rdc and skin depth, having estimated Rac, and 
> estimated Q (based on Rac, Les, and Ces) would be good additions for JAVATC.
>
> BTW, I dont understand why you added Rdc to Rac to come up with Reff.
> This doesn't sound right.  Seems like Rac is Reff and is calculated 
> from the effective cross sectional area of the wire based on skin 
> depth (and perhaps an estimate of proximity effects thrown in).  As 
> freq -> 0,  Rac becomes Rdc and is the effective R.
>
> Gerry R.
>
>
> > Original poster: "Barton B. Anderson" <bartb@xxxxxxxxxxxxxxxx>
> >
> > That would certainly throw you in the ball park. We could use this 
> > or similar as a generic factor for now, but it's one of those areas 
> > that needs some time spent on it. Only then can we really identify 
> > a high and low Q design.