To mitigate the effects of magnetic induction heating in a three phase AC system, it is common practice to group the conductors and to keep them in the same raceway. Also, three phase power is generated with each phase is 120 degrees out of phase from the others. Since Three phase power is generated by a rotating assembly (generator), and each conductor has a rotating magnetic field which surrounds it along the length of the conductor, are the magnetic induction effects reduced by the action of combining the three rotating magnetic fields into a 360 degree rotating flux envelope when grouping the conductors?
Mitigate the effects of magnetic induction heating in a three phase AC system
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You are correct that in each winding, the magnetic field varies in phase with the current in the windings. What you're having a problem with is the concept of flux being 'annihilated' at where the cores are joined.
It's helpful here to think about 'magnetic circuits'. Think about a single phase transformer for a moment; the core completes a loop that passes through the windings, so the field from the windings has a closed path. Now think about a three phase transformer. Look at the phase A winding. It has a certain amount of field that needs to be returned from one end of the winding to the other. You could just close it on itself, and do the same with phases B and C, and have three separate single-phase transformers, and it would get the job done, but it would be wasteful of material. Consider that the phase relationship of the currents means that, at any given moment, the fields from phases B and C added together are equal and opposite to that of phase A. It doesn't matter which phase you look at, the fields from the other two add to cancel. You see, where you were surmising that the fields annihilated eachother, what in fact happens is that they complement one another, and provide the right amount of magnetic return path. This lets you use less core material, and so economics dictates that's the way to go.
It's a bit like what happens to the currents in a Y-connected three phase load; the currents sum to zero, but it's not that they annihilate one another, it's that they form balanced return paths for one another.
Not having used said app I believe it is a master of assumptions and averages.
The far field of a transmission line with two opposite current conductors is essentially zero. However the near field on anything other than a solid screen co-ax will always have a unbalanced component around it.
The maximum unbalance will be when the two current conductors are 'eclipsed' and the near field will be mostly due to the nearer conductor.
What I suggest for better trials is that you take a mains cable and carefully remove the outer covering for 12 inches or so to separate the internal wires (do this with a spare kettle cord if you like). Then run your tests with the phone placed next to just one of the live wires and see if it has repeatable results.
It is also important to remember that the magnetic field sensor in the phone should be a 3 axis device so it can determine the magnetic field 'direction' and infer the conductor placement to a degree and correct a bit.
Accuracy of 30% compared to a somewhat more predictable device I think is pretty phenomenal like you have found. Getting better will obviously be much harder given the loose test conditions required.
It does remind me of a totally passive DC ammeter that my friend had that was over 50 years old. It was merely placed against a (single) conductor (such as the alternator charging lead in a car) and would deflect its needle to indicate the local induced magnetic field proportional to the current. Orientation was important and it would not have read accurately if a return conductor was nearby, however the principal is pretty much the same.
Best Answer
The magnetic field around each individual conductor does not "rotate".
And when the three conductors are bundled together, the net current through the bundle is zero, which means that there's negligible field outside the bundle. This is why power circuits are always routed together.