Skin Effect in AC circuit is the tendency of the current to
crowd toward the outer surface. This results in a current density that is
greater near the outer surface of the conductor.
Skin effect is more relevant in AC system since it involves
frequency. All electric currents generate magnetic fields that in turn can
affect the current (this is the principle behind electric guitar pickups). In a
direct current case everything is constant and so nothing seems to happen. With
an alternating current, however, there is a delay in the magnetic field's
response to the change in current and the 'old' magnetic field tends to push
the current towards the outside of the conductor.
Skin Effect causes the longitudinal element of the conductor
near the center of the axis to be surrounded by more lines of magnetic force
than near the rim. This results in an increase in inductance toward the center.
Skin Effect increases the AC resistance of the conductor
relative to the DC resistance. As the voltage drop in all wires is the same, a
larger portion of the current flows through the outer conductor layers causing
an increase of the effective resistance.
The decreased area of conductance causes apparent increase
in the resistance. At 60Hz, the phenomenon is negligible in copper sizes #2 AWG
and aluminum sizes #1/0 AWG and smaller. As the conductor sizes increase, the
effect becomes more significant.
As the frequency increases, so does the effect until at very
high frequencies the entire current flows in a very narrow skin on the
conductor--hence the name.
Skin Effect contribution for round conductors at 60 Hz can
be approximated using the following formula:
Ycs = 11.18 / (Rdc^2 + 8.8)
Where Ycs
= Skin Effect expressed as a number to be added to the dc resistance
Rdc
= dc resistance of the conductor in micro-ohms per foot at operating
temperature
2 comments:
good explanation. But diagrams and animations should have been included.
is skin effect directly proportional to ac resistance as if in the article the skin effect increases with decrease in cross sectional area... but later i found in some of the text books saying this skin effect is directly proportional to A
,permeability and frequency ??????????can you explain
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