[John L Stewart]

Some Applications of Skin Effect at Power Frequencies…..and other

For a large 3-phase induction motor there are certain rotor-slot arrangements which enable the designer to achieve high resistance at starting & a low resistance during running. That satisfies the requirements both a high starting torque & high efficiency.

Rotor slot arrangements make this possible. The operation of these motors is based on the skin effect phenomenon. While starting the rotating field results in relatively high frequencies being induced in the copper bars of the rotor. So the current induced in the rotor bars moves primarily near the perimeter of the rotor. Altho the volume resistivity of the copper rotor is everywhere uniform, the AC resistance is determined by the skin effect. As the rotor speeds up the induced frequency becomes less, so at rated speed the AC & DC resistances are almost the same. So high efficiency results.

High voltage transmission lines need to be mechanically strong while at the same time relatively light & efficient. One solution is the use of a steel core surrounded by aluminum conductors. Skin effect causes most of the current to migrate to the outer aluminum conductors. The inner steel provides the strength but carries little of the load current.

One of the transformers I worked on back while I still had hair was intended for inside an apartment building. The LT was 3-phase, 120/208. Each phase was something like 4000 Amps. That would light up a lot of 6L6GC’s!

The LT copper conductors coming out had a cross section of something like 6 inches by 3/8” thick. Where all three were positioned near one of the steel studs meant to be a fastener there was no problem. But where the single 4000 Amp conductor past near a similar attachment point on the other end the induced field caused a lot of heating. Back to the drawing board for that one! Can’t remember what the HT was but probably 2400v or 4800v, common in this part of the world.

Interleaving of windings in audio transformers is an attempt to mitigate the effects of skin effect. If you think of it, the current in any part of a section is the same & so is the DC resistance. But the inner conductors of the pack have higher AC resistance due to the total induced AC magnetic field. Not a problem at power frequencies but at 10 KHz it sure gets in the way!

Cheers. John