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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
>To: 	'tesla-at-pupman-dot-com'
>Subject: 	Skin Effect
>
>Hi,
>
>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 
>value/e.
>
>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)))
>	
>Where:
>	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)
>
>-------------------------------------------------

  Rob -

  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.

  John Couture

>
>