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The term “skin effect” refers to the non-uniform distribution of electric current over the surface or “skin” of the conductor carrying a.c. In other words, the surface of the conductor has a higher concentration of charge than the conductor’s core. Due to the concentration of current on the conductor’s surface, the conductor’s ohmic resistance rises.
With an increase in frequency, skin effect grows. The current density is slightly higher near the conductor’s surface at low frequencies like 50 Hz,. But practically all of the conductor’s current flows there at high frequencies like radio frequency. When d.c. current (frequency=0) is passed through a conductor, the current is evenly dispersed across the conductors’ cross-section.
Why skin effect occurs?
Let’s imagine that the conductor is composed of several concentric cylinders. Magnetic flux is induced in a conductor when a.c. is applied to it. The magnetic flux connecting one cylindrical element close to the conductor’s centre is greater than the flux connecting another cylindrical element close to the conductor’s surface. This is because the external cylindrical element is only surrounded by the external flux, whereas the centre cylindrical element is surrounded by both the internal and external flux.
The inner cylindrical element has a higher self-inductance and will therefore have a higher inductive reactance than the outer cylindrical element. The current has a propensity to concentrate towards the surface or skin of the conductor as a result of this difference in inductive reactance.
The conductor’s surface has the highest current density and its centre has the lowest. The conductor’s effective resistance rises as a result of the effect, which is equivalent to a reduction in the conductor’s cross-sectional area.
Factors affecting skin effect
- Frequency :– Skin effects increases with the increase in frequency.
- The shape of the conductor :– The skin effect is greater in solid conductors. And less in stranded conductors because solid conductors have a larger surface area.
- Diameter – It increases with the increase in diameter of the conductor.
- Type of material :– Skin effect increase with the increase in the permeability of the material .(Permeability is the ability of material to support the formation of the magnetic field).
- The Skin effect is negligible if the frequency is less than the 50Hz. And the diameter of the conductor is less than the 1cm.
- The majority of the current in stranded conductors like ACSR (Aluminium Conductor Steel Reinforced) flows through the aluminium layer on the outside,. While the steel layer close to the centre conducts no current and gives the conductor high tensile strength. The use of the ACSR conductor was made possible by the concentration of current near the surface.