Skin Effect
The distribution of current over the cross section of the conductor is uniform for dc only.An alternating current flowing through a conductor does not distribute uniformly but tends to concentrate near the surface of the conductor.In case of an ac system no current flows through the core and the entire current is concentrated at the surface regions.As a result the effective area of the conductor is reduced causing an increase in ac resistance.The effective ac resistance is usually referred as the effective resistance of the conductor.This phenomenon is called the skin effect.Skin effect causes concentration of current at the skin of the conductor.
Fig:A Solid conductor Fig:B Non uniform distribution of current
Why skin effect occurs?
For the determination of skin effect,we consider a solid conductor composed of large number of annular filaments,each carrying a fraction of total current.The flux linkages due to the filaments lying at the surface link the whole of the conductor while the flux set up due to the inner filaments does not link with the surface or outer filaments.Thus the filaments near the center are of larger inductance than that near the outer surface.The high reactance of the inner filaments causes the current to distribute in such a way that the current density is less in the interior of the conductor than at the surface.Hence the non uniform distribution of current is shown in the fig B.
Skin effect increases with the increase in the frequencies.At the low frequencies the effect is very small.For commercial frequency of 50 Hz or less the increase in effective resistance is inappreciable for solid copper conductors up to 1 cm in diameter;about 2.5% for 2 cm diameter and 8 per cent for 2.5 cm diameter.For the case of aluminium wire the effect is the same as in a copper wire of equal conductivity.As the resistivity of copper is 0.6 times that of aluminium.The increased resistance due to skin effect on an aluminium wire of a square mm in cross section will be of the same percentage as on a 0.6 a square mm in copper wire.
Factors Affecting Skin Effect
The skin effect depends upon:
- Type of material :Skin effect increases with the increase in the permeability of the material.
- Frequency:Skin effect increases with the increase in the frequencies.
- Diameter of conductor :Skin effect increases with the increase in the diameter of the conductor.
- shape of conductor:Skin effect seen on the solid conductor is more due to more surface area of solid conductor.while the skin effect is less in the stranded conductor.
Note:
- The skin effect is much smaller with stranded conductors than with solid conductors.It increases with the increase of cross-section,permeability and supply frequency.
- Stranded conductors are invariably used for transmission and distribution lines and hollow conductors for solid bus-bar.
- Skin effect is negligible when the frequency is less than 50Hz and the diameter of the conductor is less than 1 cm.
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