Electron Flow

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Tourcosta

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I was talking to two guys in our company who said that they were told by Mike Holt in a class that electrons only flow on the outside of conductors and because of this twisted wire will move more electrons than solid. Perhaps I completely misunderstand electron flow but I understand that electrons move to adjacent atoms regardless of their relative physical position in a conductor. Can someone shed light on this?
 
I know that with high currents and high frequencies, there is a "skin effect" where the flow is higher at the periphery. I really don't know about the stranded wire bit.
 
This has been a point of contention with me for a while now. I had professors in school who would assure me that the current only flows on the surface of the wire. However, when I would point out that the formula for the resistance of a wire involes the cross-sectional area of the wire, not it's circumference, they would just get exasperated with me. Either way, I never got a good answer. All I know is that silver-clad copper is just about the best conductor there is, so it must have some basis in truth.
 
the travel of electrons in the rim of the conductors (skin effect) is noticable in higher voltages. which is why AC transmission won over DC in those old days of electricity. the skin effect was made more economical by making the conductors stranded - more skin to travel. unlike in DC, you have to increase the diameter of the conductor to have more current flow through.
 
Skin effect is all but zero at 60Hz until the conductor becomes larger than 266kCMIL. At 500kCMIL the 60Hz skin effect is only about 1.1 and is usually already included when ampacity tables are created.

The normal "7x" stranding of conductors is to make them easier to manufacture and install. As far as skin effect goes this normal stranding is viewed as a solid conductor in all calculations I have ever seen, including my 1969 engineering handbook.

At 50/60Hz the "proximity" effect should be more of a concern than skin effect but most people have never even heard of it.

At 50/60 the use of the term "skin effect" needs to go the same way as the use of "current takes the path of least resistance".
 
The stronger electric field in higher voltages forces the electron towards the outside of a conductor, so current will flow on the outside or skin of a conductor more as voltage increases. Utillities use al tubes in substations because the center of the conductor is not beaing used at transmission voltages. The amount of current has nothing to do with skin effect. Frequency also will cause a skin effect as described correctly by Jim.

AC won over DC because AC could be stepped up to a higher voltage with a transformer allowing for transmission over longer distances without experiencing voltage colapse (Westinghouses low bid for the 1893 Chicago wolrds fair display didnt hurt either)
 
zog said:
AC won over DC because AC could be stepped up to a higher voltage with a transformer allowing for transmission over longer distances without experiencing voltage colapse
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Actually, DC is preferred for really long transmission lines (350+ miles is a rough generalization). No reactance losses. I don't even want to hazard a guess how expensive the terminal equipment is for the 500kV DC line operating on the West Coast.
 
"Actually, DC is preferred for really long transmission lines (350+ miles is a rough generalization). No reactance losses. I don't even want to hazard a guess how expensive the terminal equipment is for the 500kV DC line operating on the West Coast."

This is true today, but in the late 1800's this technology did not exist, you were stuck with the DC voltage you generated.
 
jim dungar said:
...At 50/60Hz the "proximity" effect should be more of a concern than skin effect but most people have never even heard of it....
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Jim -
I'm one of the "most". Can you give some education, or perhaps some references. I don't mind doing my homework. But a jumpstart would help.

carl
 
bcorbin said:
This has been a point of contention with me for a while now. I had professors in school who would assure me that the current only flows on the surface of the wire. However, when I would point out that the formula for the resistance of a wire involes the cross-sectional area of the wire, not it's circumference, they would just get exasperated with me. Either way, I never got a good answer. All I know is that silver-clad copper is just about the best conductor there is, so it must have some basis in truth.
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I'm guessing these weren't EE professors. The resistance of a wire does involve the cross section. But it also varies with other factors, especially the frequency of the current. At high frequencies the resistance can be a multiple of the DC resistance, largely due to skin effect significantly reducing the effective cross section. As the frequency increases and the skin depth decreases, the current increasingly favors flowing near the surface. At 60hz, copper has a skin depth of a little over 1/3". At 10KHz, a higher audio frequency, the skin depth is only about 0.025". At RF frequencies we used copper clad steel on large (acres!) antenna arrays. The steel supplied the necessary strength and the copper cladding carried all the current. Silver is the best metal conductor, but what goes in the middle varies with frequency. At Gigahertz frequencies it can be glass.

Some have stated high voltage is a factor, but only in the sense that power stations operate at such high currents that the required conductors exceed the skin depth of copper, and thus can be hollow.

[edited to correct error in 10KHz skin depth and add italicized text.]
 
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Skin effect is indeed caused by current, and not voltage, because it is a magnetic-based phenomenon. Voltage in a conductor creates a voltage field around it, but current creates a magnetic field.

The AC magnetic field causes rings of flux around a conductor that expand and contract twice per cycle. These 120-Hz pulses of magnetism induce an inverse voltage in the surrounding conductor material.

Think of the conductor, whether solid or stranded, as an infinite number of strands, each exhibiting the magnetic pulses and inducing the inverse voltage in its neighboring strands. Note that this only happens with AC.

The strands at the perimeter of the conductor have the fewest neighboring strands, so they suffer the least effective increase in resistance. This is a little like a buck-boost transformer in buck mode.

Because the strength of an induced magnetic field depends on frequency as well as current, the higher the frequency, the greater the effect at a given current. TV transmitters use coaxial copper pipe.
 
I don't believe ordinary stranded wire makes much difference in regard to skin effect. The strands need to be insulated from each other to prevent current from flowing outward from the center. Litz wire, which is used to reduce losses due to skin effect and proximity effect, is braided from individually insulated strands.
 
steved said:
I don't believe ordinary stranded wire makes much difference in regard to skin effect. The strands need to be insulated from each other to prevent current from flowing outward from the center.
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Correct. Multi-stranded wire usually has so much intimate contact among the strands that the difference is negligible. Power stations often use tubular conductors for both skin effect and weight reduction (the former allowing the latter).

Litz wire, which is used to reduce losses due to skin effect and proximity effect, is braided from individually insulated strands.
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True, and as I remember, was often braided around a cotton core. Very flexible and fragile, and stank when soldering, which was very difficult.
 
Back to electrical theory 101. The answer is simple. There are more free electrons on the outside surface of the conductor and electricity DOES take the path of least resistance. All this talk about high voltage and frequency has nothing to do with it. Same theory holds true for AC or DC. This is just simple first year basics.
 
The POCO can use much smaller wire than us because their wires have no insulation and the higher the voltage the lower the current. They use step-up transformers every few miles because they do have voltage drop. But because higher voltage creates lower current, resistance becomes less of a factor. Conductor ampacity can be much lower. There is current inside the conductor. The electrons on the inside move slower than the ones on the outside. The outside electrons have more room to move.
 
Fink's Two Cents:

Fink's Two Cents:

According to Fink's Electronic Engineer's Handbook:

Skin effect occurs because the inductive reactance of a wire is less in the outer layers of the wire. (Think of a wire comprising many telescoping tubes. The inductance of the larger diameter tubes is less than that of the smaller tubes). Then, current tends to follow the path of least impedance, that is the outer layers. This effect becomes significant at higher frequencies. No mention is made of voltage or current levels being factors in this phenomenon.
 
"No mention is made of voltage or current levels being factors in this phenomenon."

Maybe because this is from an electronics book, where voltages and current are typically low.
 
You must always mention the frequency when discussing skin effect. Skin effect is definitely a problem in "communication" systems but not at the 50/60Hz of power systems. If you use an electronics book to investigate skin effect you will probably get a different viewpoint than you will from a power engineering book.

Current does not take the path of least resistance it takes ALL paths.
Skin effect does not cause current to flow on the outside of the conductor, it simply causes the current distribution in the conductor to be unequal (more towards the outside and less towards the inside).

The 60Hz Skin-effect Ratio for standard concentric stranded copper is:
300kCMIL = 1.006
500kCMIL = 1.018
600kCMIL = 1.026

These ratios can be used with the DC resistance values in NEC Table 8 to calculate the effective AC resistance (which look like the AC values in Table 9). But, we don't have to do this as the tables in 310 already consider any possible skin effect.

Proximity effect is a similar phenomona of unequal current distribution, but it is caused by conductors being close to each other and is similar to the concept that "like poles of a magnet repulse each other". So the current in two parallel phase conductors will tend to travel on the parts of each conductor which are furthest away from the other conductor. In skin effect the current distribution is in layers like an onion, but in proximity effect it is more like a crescent moons.
 
 
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