SKIN EFFECT – BASIC DEFINITION AND TUTORIALS

What is skin effect?

Real, or ohmic, resistance is the resistance offered by the conductor to the passage of electricity. Although the specific resistance is the same for either alternating or continuous current, the total resistance of a wire is greater for alternating than for continuous current.

This is due to the fact that there are induced emfs in a conductor in which there is alternating flux. These emfs are greater at the center than at the circumference, so the potential difference tends to establish currents that oppose the current at the center and assist it at the circumference.

The current is thus forced to the outside of the conductor, reducing the effective area of the conductor. This phenomenon is called skin effect.

Skin-Effect Resistance Ratio. The ratio of the A.C. resistance to the D.C. resistance is a function of the cross-sectional shape of the conductor and its magnetic and electrical properties as well as of the frequency.

For cylindrical cross sections with presumed constant values of relative permeability and resistivity, the function that determines the skin-effect ratio is

where r is the radius of the conductor and f is the frequency of the alternating current. The ratio of R, the A.C. resistance, to R0, the D.C. Resistance.

Skin Effect On Steel Wires and Cables.

The skin effect of steel wires and cables cannot be calculated accurately by assuming a constant value of the permeability, which varies throughout a large range during every cycle. Therefore, curves of measured characteristics should be used. See Electrical Transmission and Distribution Reference Book, 4th ed., 1950.

Skin Effect of Tubular Conductors.

Cables of large size are often made so as to be, in effect, round, tubular conductors. Their effective resistance due to skin effect may be taken from the curves of Sec. 4. The skin-effect ratio of square, tubular bus bars may be obtained from semiempirical formulas in the paper “A-C Resistance of Hollow, Square Conductors,” by A. H. M. Arnold, J. IEE (London), 1938, vol. 82, p. 537.

These formulas have been compared with tests. The resistance ratio of square tubes is somewhat larger than that of round tubes. Values may be read from the curves of Fig. 4, Chap. 25, of Electrical Coils and Conductors.