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Skin Effect - What it is, What it does

A magnetic field surrounds any conductor carrying electric current. Alternating current creates a field that varies, its movement generating an induced voltage within the conductor that acts to oppose current flow. That opposition is least towards the surface of the conductor, and greatest at its center, resulting in the current flow being greatest at the surface.

That is commonly called the "skin effect." The effective depth of current penetration into the conductor surface is calculable from conductor dimensions and resistivity as well as the frequency. At 50 Hz, most current flow in any round copper wire is within 9 millimeters of the surface. Hence, skin effect is insignificant at that frequency unless conductor diameter exceeds 18 mm.

As frequency increases, however, current penetration decreases rapidly. For example, at 1000 Hz, the depth becomes only 0.2 mm. To prevent overheating at the desired current flow, a conductor may have to be extremely large in diameter to provide enough area at so small a thickness. High frequency conductors are therefore often either thin sheets or hollow tubes.

Tubular conductors are also common at power line frequencies when currents are so large that the diameter of a solid conductor would far exceed twice the depth of penetration. For current density of 155 amperes per square centimeter, for example, conductor area at 5000 amperes would need to be 32 square cm. A solid round bar of that area would be 6.4 cm in diameter. But at 50 Hz, only the outermost layer 0.9 cm thick would carry most of the current. The diameter would have to be 11.3 cm to provide a later that thick of sufficient area. A tubular conductor 10 cm in diameter with a 1 cm wall thickness would suffice, with the added cooling provided by its inner surface.

Published formulae for penetration depth are usually in the simplified form of a numerical constant divided by the square root of frequency. These must be used carefully. The constant involves material resistivity, which varies with temperature, and the applicable value is seldom stated.

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