Figs. 2,683 to 2,687.—Skin effect and shield effect. Fig. 2,683, section of conductor illustrating skin effect or tendency of the alternating current to distribute itself unequally through the cross section of a conductor as shown by the varied shading, which represents the current flowing most strongly in the outer portions of the conductor. For this reason it has been proposed to use hollow or flat instead of solid round conductors; however, with frequency not exceeding 100, the skin effect is negligibly small in copper conductors of the sizes usually employed. In figs. 2,684 and 2,685, or 2,686 and 2,687, if two adjacent conductors be carrying current in the same direction, concentration will occur on those parts of the two conductors remote from one another, and the nearer parts will have less current, that is to say, they will be shielded. In this case, the induction due to one conductor will exert its opposing effect to the greatest extent on those parts of the other conductor nearest to it; this effect decreasing the deeper the latter is penetrated. After crossing the current axis, the induction will still decrease in magnitude, but will now aid the current in the conductor. Hence, the effect of these two conductors on one another will make the current density more uniform than is the case where the two conductors adjacent to one another are carrying current in opposite directions, as in figs. 2,685 and 2,686, therefore, the resistance and the heating for a given current will be smaller. If the two return conductors be situated on the line passing through the center of the conductors just considered, the effect will be to still further concentrate the current; the distribution symmetry will be further disturbed, and the resistance of the conductor system increased. It is therefore difficult to say which of the two cases considered holds the advantage so far as increasing the resistance is concerned. The case, however, in which the phases are mixed has much the smaller reactive drop.
If the conductor be large, or the frequency high, the central portion of the conductor carries little if any current, hence the resistance is therefore greater for alternating current than for direct current.
Ques. For what condition may "skin effect" be neglected?
Ans. For frequencies of 60 or less, with conductors having a diameter not greater than 0000 B. & S. gauge.
Ques. How is the "skin effect" calculated for a given wire?
Ans. Its area in circular mils multiplied by the frequency, gives the ratio of the wire's ohmic resistance to its combined resistance.
That is to say, the factor thus obtained multiplied by the resistance of the wire to direct current will give its combined resistance or resistance to alternating current.
The following table gives these ratio factors for large conductors.
| Cir. mils. × frequency | Ratio factor | Cir. mils. × frequency | Ratio factor |
|---|---|---|---|
| 10,000,000 | 1.00 | 70,000,000 | 1.13 |
| 20,000,000 | 1.01 | 80,000,000 | 1.17 |
| 30,000,000 | 1.03 | 90,000,000 | 1.20 |
| 40,000,000 | 1.05 | 100,000,000 | 1.25 |
| 50,000,000 | 1.08 | 125,000,000 | 1.34 |
| 60,000,000 | 1.10 | 150,000,000 | 1.43 |
Corona Effect.—When two wires, having a great difference of pressure are placed near each other, a certain phenomenon occurs, which is called corona effect. When the spacing or distance between the wires is small and the difference of pressure in the wires very great, a continuous passage of energy takes place through the dielectric or atmosphere, the amount of this energy may be an appreciable percentage of the power transmitted. Therefore in laying out high pressure transmission lines, this effect must be considered in the spacing of the wires.