Skin effect is the tendency of an alternating electric current (AC) to
become distributed within a conductor such
that the current density is largest near the surface of the
conductor, and decreases with greater depths in the conductor.
The electric current flows mainly at the "skin" of the
conductor, between the outer surface and a level called the skin depth. The skin effect
causes the effective resistance of the conductor to increase at higher frequencies where the skin depth is smaller, thus
reducing the effective cross-section of the conductor.
The skin effect is due to opposing eddy currents induced by the changing magnetic field resulting from the alternating
current. At 60 Hz in copper, the skin depth is about
8.5 mm. At high frequencies the skin depth becomes much smaller. Increased
AC resistance due to the skin effect can be mitigated by using specially woven litz wire.Because the interior
of a large conductor carries so little of the current, tubular conductors such
as pipe can be used to save weight and cost.
Figure Right : Skin depth is due to the circulating eddy currents (arising from a changing H field) cancelling the current flow in the center of a conductor and reinforcing it in the skin.
Skin effect: skin depth decreases with
increasing frequency.
The electrical
resistance of the conductor with all its cross-sectional area in use is known
as the “DC resistance,” the “AC resistance” of the same conductor referring to
a higher figure resulting from the skin effect. As you can see, at high
frequencies the AC current avoids travel through most of the conductor’s
cross-sectional area. For the purpose of conducting current, the wire might as
well be hollow!
In some radio applications (antennas, most notably) this effect
is exploited. Since radio-frequency (“RF”) AC currents wouldn’t travel through
the middle of a conductor anyway, why not just use hollow metal rods instead of
solid metal wires and save both weight and cost? (Figure below) Most
antenna structures and RF power conductors are made of hollow metal tubes for
this reason.
In the following photograph you can see some large inductors used
in a 50 kW radio transmitting circuit. The inductors are hollow copper tubes
coated with silver, for excellent conductivity at the “skin” of the tube:
The degree to which frequency affects the effective
resistance of a solid wire conductor is impacted by the gauge of that wire. As
a rule, large-gauge wires exhibit a more pronounced skin effect (change in
resistance from DC) than small-gauge wires at any given frequency.
Fig :High power inductors formed from hollow tubes.
Note:
Please remember that this figure is not impedance,
and it does not consider any reactive effects, inductive or
capacitive. This is simply an estimated figure of pure resistance for the
conductor (that opposition to the AC flow of electrons which does dissipate
power in the form of heat), corrected for the skin effect. Reactance, and the
combined effects of reactance and resistance (impedance), are entirely
different matters.
