OT Electrical question

If I may answer the question posed to Don Foreman, (Not that I could ever fill the master's boots, no way, no how) I would say with a true conjugate load apparent resistance would be the same as "RMS" resistance. Don't ask me about current, though.

Hi Bob

It might be more important than I thought to account for the transmission line characteristics of "household wiring" than I realized. I had considered it a defination that RMS loss is equal to DC loss. I have no trouble accepting jk's statements about the inductive quality of the parallel line that the AC generator sees. I sure have neglected the Transmission Line thinking as applied to household wiring. I'm surprised to learn that the resistance is so much different, DC to AC. Now I wonder if the difference is related to the inductive properties of the load. Either jk, or Don will probably answer the question.

Jerry

Michael A Terrill sez:

"Bullshit. There are a number of long distance HVDC transmission lines

Not so fast on the "Bullshit" . For the wire to become any sort of antenna it would have to approach 1/4 wavelength as well as being tuned with a lot of capacitance. A 1/4 wavelength at 60Hz is some 776 miles. Doubtful that those long DC lines are 776 miles or that there would be enough distributed C to approach resonance. Correct me if I'm wrong.

Bob Sw>>

Hi Bob

I dont have experience with this problem as applied to Power Transfer at

60Hz. But, remember the way two parallel lines, like Twin Lead, (with the antennas you used to develop), are actually a transmission line, with a characteristic impedance, and some radiation losses, There are probably voltage increases associated with those LONG power lines, from one end to the other unless the smart guys match them somehow. So, a power transmission line of even a small fraction of a wavelength could contribute to some appreciable radiation loss.

I'd bet these guys know something about 60 Hz power transfer that old guys like me never thought of.

Jerry

A purely resistive load is identical for AC and DC.

If you add the right amount of shunt inductance to cancel out the shunt capacitance at 60 Hz there would be no difference between AC at 60 Hz and DC. The capacitance and inductance would resonate at 60 Hz and cancel each other out. Likewise, if you could add series capacitance to cancel out the series inductance you could cancel out the series inductance effects at 60 Hz.

I don't know of any practical way to add series capacitance in parallel with the wire to cancel out the inductive effects of the wire however. So other than in theory, I don't know how it could be done.

Telephone lines have load coils added across them for this very reason. It helps cancel out the loss due to cross wire capacitance and extends the workable length of the wire. I don't know if this practice is as common as it once was. They might simply use better wire insulation these days which reduces the capacitance to a point where the load coils are normally not needed.

In yard lighting the load is already purely resistive -- and further, it is typically a distributed load.

Recall that voltage drop in the line due to reactance is not a "loss" because reactance is non-dissipative. Reactive voltage drop could theoretically be cancelled by resonance, but the (real) losses in the components used to accomplish this would probably exceed the gain except perhaps in very long (many miles) point-to-point lines.

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I think I detect a resonance in my string from being pulled!

Neglecting how conduction works in metals for the moment .... if your round-trip distance is 10 miles, the transit time for an electron making the trip would on the order of 60 microseconds -- which is 0.6% of a half-cycle at 60 Hz. So a "sway" would be about 155 round trips. Even inn the conga line analogy, if no electron moves more than 10% of the round trip then there must be 10 electrons in motion (and dissipating power in 1/10th of the line resistance) to shove one thru the load.

Wanna get unpopular real fast, build a 400 Hz transformer that isn't quite solid and tight. They don't just hum, they squall and it is very irritating. My name was "Dammitforeman" for a little while.

Hi Don

Your posts always make sense to me. But, I thought jk wrote that you were wrong when you wrote that the loss is the same for either AC or DC. Maybe he'll explain.

Jerry

Electrically short antennae, particularly non-resonated short antennae, are very inefficient radiators -- but power loss of even a few percent is not trivial in a megawatt feed.

I doubt that much 60Hz actually gets radiated because the vector sum of currents in a region enclosing a 3-phase t-line is zero. But there are certainly near-field losses: line-to-ground capacitances result in transverse ground currents which are lossy, and regions where alternating H fields don't quite cancel induce loop currents in the underlying ground which are also lossy. These don't amount to much in a lower-power line of a few miles, but their aggregate effects could be significant in a high-power long-distance line.

ooopps, sorry.

Upon recalculation it does appear that 60 Hz can radiate a little bit. The standard formula for free space loss for 60 Hz at a distance of 1 meter has it that power would be less by a factor of 6.3 X 10 to the -12 . That would be about -112 dB down.

Bob Swinney

I think you might have that wrong Don. The electrical wave might move that fast, but electrons move much slower than the wave front moves and your 60 ms number sounds way too fast to me. I'll have to dig and see if I can find some facts on how fast the electrons will actually move in a wire - or even an example.

None the less, this "cogna line" gibberish sounds like gibberish to me.

Yes, closer, but not the same. jk

But the losses in the resistance due to the reactive current, is real, and a real loss.

At those voltage and current levels, yeah.

jk

If conduction band electrons didn't move the metal wire would be electrically no different than a length of string. It is well known and shown that energy propagates down a wire and t-line at a significant fraction of c. Current at the yonder end is electrons in motion by definition.

One amp is a flow of 6.241 * 10^18 electrons per second -- but there is a GHS (great huge shitload) of electrons in each gram of conductor so current flow can commence at the yonder end as everybody moves over, per Bruce's conga line, before any particular electron makes the trip. That said, they must move along eventually or there'd be one hell of a pile of electrons (charge, hence potential) at the influx end.

10 miles in 60 microseconds is 166667 miles per second, or about 90% of light-speed... ok for Velocity of Propagation of signal, but nowhere near the actual Drift Velocity for electrons, which is snail speed. From 3rd URL below: "For example, in a copper wire of cross-section 0.5 mm², carrying a current of 5 A, the drift velocity of the electrons is of the order of a millimetre per second. "

Of course they move. The issue wasn't whether they move, but how fast they move. I was only questioning the speed you suggested. You said the electrons themselves can transit 10 miles in 60 ms. I didn't think they moved anywhere near that fast.

Here's a page I found on the subject:

He calculates the speed of electron flow in a lamp cord driving a 100W bulb is 8.4 cm per HOUR.

Or, as they move back and forth with the A/C current, they only wiggle back and forth .00002".

This is the type of speed I was thinking of.

Electrons move very slow in wire. The wave front they create however, as you said, moves at a significant fraction of the speed of sound. Just like a wave in water, the speed of the wave is not the speed of the water molecules.

Don -

Can you explain the inductive pickup lines used to tap power ?

How about the very high voltage lines - 750KV and 1MV lines that burn the everliving out of the grass and greens that try to grow underneath them. They glow like neon (Nitrogen) tubes and can be easily seen at night.

The problem with DC is the copper losses. The basic IR drop smacks them. But when you start high - what is a 10 or so percent drop over a distance.

The use is into Al plants or other smelting places I suspect. Can't think of much others.

Martin

Martin H. Eastburn @ home at Lions' Lair with our computer lionslair at consolidated dot net TSRA, Life; NRA LOH & Endowment Member, Golden Eagle, Patriot"s Medal. NRA Second Amendment Task Force Charter Founder IHMSA and NRA Metallic Silhouette maker & member.

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