Re: Skin Effect
Sent: Saturday, August 02, 1997 9:52 AM
To: Tesla List
Subject: Re: Skin Effect
To: Robert Davis
Could you expound on the equation for dynamic resistance of copper wire?
Is it C (conductance) multiplied by r (radius) multiplied by X (reactance
??????) or is the X a multiplication symbol? If X is a multiplication
symbol where is the freq (F) brought into the equation? Without some F
this equation won't work because the dynamic resistance certainly does
change with frequency buy I was unable to determine where you entered this
factor into your equation.
> From: Tesla List <tesla-at-pupman-dot-com>
> To: 'Tesla List' <tesla-at-poodle.pupman-dot-com>
> Subject: Re: Skin Effect
> Date: Thursday,July 31,1997 8:24 AM
> 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
> >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
> >oscilating currents in that wire, BUT we are winding coils with wires in
> >close proximity, does anyone have a correction factor to take into
> >fields from adjacent wires?
> >If this is the case then the thickness of insulation of wire used
> >the coil is close wound) will have some effect on the dynamic resistance
> >the coil and hence the Q of the coil, I think proximity of other turns
> >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
> 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