Re: Skin Effect
From: John H. Couture[SMTP:couturejh-at-worldnet.att-dot-net]
Sent: Thursday, July 31, 1997 3:25 AM
To: Tesla List
Subject: Re: Skin Effect
At 11:53 PM 7/29/97 +0000, you wrote:
>From: Robert Davis[SMTP:RE.Davis-at-btinternet-dot-com]
>Sent: Tuesday, July 29, 1997 4:35 PM
>Subject: Skin Effect
>I have the formula for skin depth as:
> Depth (mm Copper) = 66/sqrt(freq)
>Where the depth is the attenuation of current density from the surface
>Question 1: Is the surface value the same as the DC resistive value for
>copper wire of a given size?
>Question 2: From the above the dynamic resistance of a copper wire at a
>given frequency appears to be:
> Resistance = L/(Cr X ((PI X R^2) - (PI X (R-D)^2)))
> L = Length of wire
> D = Skin depth
> C = Conductivity of copper
> R = Radius of conductor
> Is this correct?
>Question 3: The skin effect calculation assumes that there is only one
>wire, skin effect being then caused by the magnetic field motion form the
>oscilating currents in that wire, BUT we are winding coils with wires in
>close proximity, does anyone have a correction factor to take into account
>fields from adjacent wires?
>If this is the case then the thickness of insulation of wire used (assuming
>the coil is close wound) will have some effect on the dynamic resistance of
>the coil and hence the Q of the coil, I think proximity of other turns will
>increase the dynamic resistance, is this correct?
>Thank you in advance.
>Rob Davis (UK)
You are talking about effective resistance which includes all of the
effects you mentioned. The effective resistance Reff is found by
Reff = Xl/Q
The Xl is easily found and you can find the Q using a scope as I show in
the Tesla Coil Notebook. This is the only way to find the Q of a coil
operating at high voltage. The Q is found by
Q = 1.364 x A where A = 10% amplitude from the log dec.