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Re: Primary and copper (fwd)
---------- Forwarded message ----------
Date: Mon, 30 Mar 1998 08:40:23 +1200
From: Malcolm Watts <MALCOLM-at-directorate.wnp.ac.nz>
To: Tesla List <tesla-at-pupman-dot-com>
Subject: Re: Primary and copper (fwd)
Hi Antonio, all,
> Date: Thu, 26 Mar 1998 21:32:30 -0800
> From: "Antonio C. M. de Queiroz" <acmq-at-compuland-dot-com.br>
> To: Tesla List <tesla-at-pupman-dot-com>
> Subject: Re: Primary and copper
>
> Erik Schulz wrote:
>
> > The following is a list of the 75% current depth in copper at different
> > frequency.
> >
> > kHz, mils mil is 0.001 inches
> > 1000, 2.6
> > 900, 2.7...
>
> Or:
> skin depth=sqrt(resistivity/(pi*frequency)/u0) meters
> where u0=pi*4e-7.
> For copper (resistivity=1.724e-8 Ohm-m):
> skin depth=0.06608/sqrt(frequency) meters, or
> skin depth=2602/sqrt(frequency) mils.
>
> This value is valid for flat sheets. For round wires the usual approach
> is to consider a sheet wrapped around the wire, what results in:
> resistance=(length/diameter)*8.31e-8*sqrt(frequency) (any length unit).
> This works if the skin depth is much smaller than diameter/2, but fails
> totally for thinner wire or lower frequency.
> Do someone know the expression for wire resistance as function of frequency
> when the wire radius is not much greater than the skin depth?
At that thickness, skin effect is pretty well non-existent and Rac =
Rdc to all intents and purposes. Lest anything think this is good
value, I should point out that while copper utilization approaches
100%, the resistance is a lot higher than using thicker wire.
Malcolm