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Back in the 90's I heard this story of a guy who in the 70-80's lived remotely near the power lines and that he was very good in electrical engineering and that he had free electricity by using the power line as a primary of an air cored transformer.

Besides the utter danger of going anywhere near the power lines I find it unlikely that having the second coil at a safe distance (which might be at least 10 meter) still can make this arrangement act like an air-cored transformer.

Is this theoretically possible or is it just a myth?

enter image description here

Disclaimer !!!

This question is for pure scientific/theoretical/possibilities curiosity.

Messing with the high voltage power line and its proximity is very dangerous, potentially lethal and most certainly illegal.

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    \$\begingroup\$ This sounds like one for "Myth Busters". \$\endgroup\$ – Bad Bob Nov 14 '14 at 11:40
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    \$\begingroup\$ You can make a fluorescent tube light up (faintly) just by holding it underneath a power line, so the concept is certainly not completely implausible. The only question is how big a coil you'd need to extract a useful amount of power at a safe distance. \$\endgroup\$ – Ilmari Karonen Nov 14 '14 at 13:14
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    \$\begingroup\$ See here, youtube.com/watch?v=DNf8KtWeW1s \$\endgroup\$ – George Herold Nov 14 '14 at 14:03
  • \$\begingroup\$ This is a duplicate question. Almost the exact thing was discussed some months ago. | Yes, it is entitely possible wityh correctly aligned and constructed coil and people have tapped off power this was and also been prosecuted for doing so. | The fluorescent tubes mentioned light because of the electrostatic field. \$\endgroup\$ – Russell McMahon Nov 14 '14 at 14:15
  • \$\begingroup\$ @RussellMcMahon when I searched didn't show up anything, maybe I didn't chose the right words \$\endgroup\$ – Eduard Florinescu Nov 14 '14 at 14:18
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This sounds like induction coupling and there is no real theoretical limit to extracting power but the practicalites of "removing" anything more than a few watts are complex. The first thing to remember is that there needs to be current flowing through the overhead wire - it is the current that produces a magnetic field that induces a terminal voltage on a somewhat distant receive coil.

No current means no stolen power.

It helps if you can use a large inductance "receive coil" because then you stand a chance of tuning it with capacitance to match the AC frequency (50 or 60 Hz). You might get a couple of orders of magnitude increase in terminal voltage by tuning.

But tuning at 50Hz is problematic - the receive coil needs to be largely "air-cored" to be able to gather some of the magnetic field and even if you wound a 100mH coil, at an AC frequency of 50Hz about 100uF is needed to resonate it. Series resistance losses in inductor are going to seriously reduce the Q of the circuit so you might want to use Litz wire.

It can be done but don't expect to power your home lighting from it.

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  • \$\begingroup\$ Have a look here: users.physics.unc.edu/~deardorf/phys25/rwp/… \$\endgroup\$ – Mr X May 20 at 20:00
  • \$\begingroup\$ If you reduced the distance of the coil to the transmission line down to 1 meter and used a wire loop with a radius of 50cm you'd only need 31 turns to get 120VAC(rms). Use a low guage wire with a resistance of say, 0.1 Ohm and you'd have 1200 Amps of current. \$\endgroup\$ – Mr X May 20 at 20:01
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Yes, this is possible. I think the confusion comes from the fact that you are used to thinking about transformers being coils of wire around a magnetic core. However, that's just a good way to make a deliberate tranformer. The physical principle behind this is that every moving charge causes a circular magnetic field around it. Coils of wire concentrate the relatively weak field from the individual lengths of wire into a small area.

In your picture, the long and relatively straight transmission line wire will have a magnetic field around it proportional to the current thru the wire. If you were to run another wire close by the first wire, most of the magnetic field caused by the first wire would also go around the second wire. Since this current <--> magnetic field coupling works both ways, you would get a induced EMF in the second wire. Basically, you'd have a long stretched out air-core transformer. Deliberate transformers aren't made that way because they would be physically big.

Now imagine this transformer as the distance between the wires is increased. Less and less of the magnetic field caused by the first will go around the second wire. In transformer terms, the coupling decreases. However, it does not go to zero.

Let's say a farmer ran a wire fence to one side and underneath the power line. The coupling between the transmission line and a wire in the fence would be very poor, but with a long enough fence, could still pick up significant power. Considering the 100s of Amps flowing thru the transmission line, even very weak coupling stretched out over a long distance can be a meaningful amount. Over a mile or more, I wouldn't be surprised if you can pick up 10s of Watts with a carefully placed fence. I'd use something like a barb wire fence because you get individual conductors you can make sure are continuous, and it wouldn't look out of place if your property happened to border the power line right of way.

I admit to not having done the numbers, but it seems plausible to me.

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It's entirely possible, and as long as the axes of the conductors of the "primary" and "secondary" aren't perpendicular to each other it should work, with maximum output from the secondary occurring when the conductors are parallel to each other.

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I have often considered moving into a van under some high-power power lines and doing this; so far as I can tell the physics works out, but there is an issue. Many transformers have a ferrous core, but that is just to guide the B-field, usually in a circle around through the other coils. Electromagnetic fields of course don't require a medium through which to travel, though, so the iron core in this case would not be necessary. The issue I mentioned earlier is that PG&E, or whoever puts up power lines, often runs them with the current going in opposite directions to cancel out the induced B-fields from the power lines, and without a changing B-field no current will be induced in the secondary coil. Or anywhere else, for that matter. If you live near a single power line or unpaired power lines, though, you could probably pull it off, though you would likely be caught. Side note, I don't think a barbed wire fence (suggested earlier) would work because the B-field from the power lines would be pointing straight down at the ground; that is, at the ground, the B-field is pointing down-- in other places it would not be pointing down. Anyway, point being, you would need a coil on its side so that the magnetic field could go through the middle of it.

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Given the right materials (especially a perfectly isolated cable for the secondary coil), induction power draw is certainly possible.

However, there are challenges:

  1. Getting up that line carrier tower without getting struck by power
  2. Actually climbing on the lines (certainly possible with the right equipment) and looping the wire around one of the conductors without managing to violate its isolation and creating a nice ground fault thereby
  3. Even if you manage to install that wire upon the tower, it will be spotted by maintenance crews. Okay, I have heard horror stories about the US grid and it may well be possible it's only inspected in case of a problem, but here in Europe all lines are regularly overflown by helicopter to check for stuff like trees and bird nests which could compromise the integrity of the power line.

In case this was just a 20kV or such power line, then an inductor would not make any sense - a tap, fed into a HV transformer, would be easier and far cheaper to install and more difficult to spot as normally no living soul looks into the middle-voltage grid...

But given the above, I would sort this story out as "urban legend" - as soon as you're dealing with anything higher than 1kV, risk and cable cost is just too high!

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    \$\begingroup\$ The story is that he had the secondary coil in its yard that he didn't touch the line or the tower \$\endgroup\$ – Eduard Florinescu Nov 14 '14 at 10:28
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    \$\begingroup\$ 1. Climbing up the tower isn't part of the proposition. 2. Looping the "secondary" around the high voltage line isn't part of the proposition, plus it won't work since its axis will be normal - or nearly so - to the high voltage line's axis. 3. "Installing" the secondary so that any part of it touches the tower or the high voltage line isn't part of the proposition. \$\endgroup\$ – EM Fields Nov 14 '14 at 11:53
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DON'T VOTE FOR THIS ANSWER

I will probably delete this once the current interest subsides. The two questions may end up being merged.

See my answer and others at

"TAPPING ENERGY FROM HIGH VOLTAGE TRANSMISSION LINES"


Related

This question relates to electromagnetic coupling.

The image below shows an example of electrostatic coupling as mentioned by Ilmari Karonen. Note that the site where this photos first appeared and others relating to fluorescent tube lighting frequently refer to electromagnetic coupling. Ibn fact (as far as I know) what is incolved is electrostatic or capacitive coupling.

enter image description here

The original of that photo appears to be from here but it is used in a number of places.

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