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Re: Primary and copper (fwd)
---------- Forwarded message ----------
Date: Sun, 29 Mar 1998 00:11:00 -0600
From: Bert Hickman <bert.hickman-at-aquila-dot-com>
To: Tesla List <tesla-at-pupman-dot-com>
Subject: Re: Primary and copper (fwd)
Tesla List wrote:
>
> ---------- Forwarded message ----------
> 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?
>
> Antonio Carlos M. de Queiroz
> http://www.coe.ufrj.br/~acmq
Antonio and all,
If the radius of the wire is roughly the same as the skin depth, then
the HF resistance is approximately the same as the DC resistance. This
effect is sometines used for very high speed current shunts to remove
much of the frequency dependency. A thin-walled conductive tube can be
used to accurately measure high-amplitude current transients if the
tube's wall thickness is less than the skin depth of the highest
frequency component of the transient to be measured.
-- Bert --