Power transformer monitoring device

[KlausST]

Hi,

Use a small coil that can inductively couple to the transformer magnetic field and produce a few volts

You could try this, but I wouldn't be surprised if this trial fails.
Since it's an oil cooled transformer I expect it to be in a metal case...shielding the magnetic field.

Usually or better say "ideally" the magnetic overall field in a three phase transformer cancels out. For sure there is a magnetic field in each leg. I don't expect much of the fundamental frequency. Because of unlinearity of the magnetic core there will be overtones.
The third overtones will sum, because they are identical in phase in all three legs.
I wonder how much of this is "usable" outside of the transformer.

If it really works it's indeed the preferable power supply...but I assume each transformer type will have it's own hot spots of magnetic field. They will vary in size and location.

Klaus

[c_mitra]

Usually or better say "ideally" the magnetic overall field in a three phase transformer cancels out.

You are right, but this cancels only in the average sense. Please consider a single phase transformer, then the average cancels there too. But,,,

For a three phase case, you will get a 180Hz AC ripple voltage that can easily come out - the steel cage is a good protection but not 100% effective.

I shall try to make a plot ans see if I can convince.