When is a transformer?

[@rt]

Hi Guys
I was wondering about passing a conductor through a hole in a sheet of metal like every old mains lead passing through a gland of a steel enclosure.
If I turn a wire around a straight current carrying wire, that’s a transformer, so why isn’t the steel enclosure a transformer shorted turn?

[amspire]

If you pass a single wire through a metal sheet, it is a transformer with a shorted turn.

If you pass a mains lead through a hole, then the net current is zero, so there is negligible effect.

What you will find with the single wire is that there is not enough magnetic coupling at mains frequencies to cause a significant power loss in the wire. The effect of the shorted turn would add a very slight extra voltage drop in the cable, but it will probably be much less then the resistive drop in the cable.

When a single wire is used to carry a high RF frequency goes through a metal wall, feedthroughs with a fixed impedance (like 50 ohms or 75 ohms) are used. You can look up transmission line theory if you wan to find out how that works, but to put it simply, in a transmission line, there is both inductive and capacitive coupling and the two cancel each other out.

[@rt]

Ok thanks I suppose there's no difference if both the wire and metal are non ferrous? Since winding copper wire around another copper wire still makes a transformer.

[T3sl4co1l]

It's in parallel with the magnetic field lines, so no current is induced.

A transformer only transforms where there are two wires that run parallel.  If there's a core (that runs perpendicular, and encloses both), you get bonus points.

Tim

[@rt]

If you wrap malleable enamelled magnet wire around a rigid copper conductor what’s the difference?
In that case there’s potential across each end of the coil you made.

ps. If you wrap a few turns of some wire around your coax you can connect each end of the wire to a frequency counter
and read frequency when you transmit, presumably from reflected current on the sheath, but still the wires weren’t parallel.

[T3sl4co1l]

If you wrap malleable enamelled magnet wire around a rigid copper conductor what’s the difference?
In that case there’s potential across each end of the coil you made.

No different from if you laid the wire parallel, at the same average distance.

ps. If you wrap a few turns of some wire around your coax you can connect each end of the wire to a frequency counter
and read frequency when you transmit, presumably from reflected current on the sheath, but still the wires weren’t parallel.

If you have leaky coax, yes. But that's as much leaking E field as H.

Neither of these are a very good example of a pure magnetic situation, so you will get large errors from electric field if it's not properly shielded.

Tim

[@rt]

To get to the practical point, I want to pass approx 40kV at a frequency into the kHz on a 5mm rigid conductor
through the 34mm hole in this 4mm aluminium.
All that was really needed to eliminate all doubt was to make a cut across the orange line,
which is not part of the diagrams sent to the laser cutting company :O

[T3sl4co1l]

Definitely no worry, and anyway at that voltage, the electric field is WAY more important to any transmission line or loading effect.

The characteristic impedance Zo of a wire, kind of anywhere near that metal bulkhead really, is in the 100s of ohms.  If your current level is ~mA, then your circuit impedance is in the ~Mohms.  Zcircuit >> Zo, so it looks largely capacitive.

Tim

[dan.soethe]

The wire passing through is at a right angle. The wire has to be in parallel with the metal to generate current.
Only magnetic flux current is flowing in the metal...

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[dan.soethe]

Wrapping any wire around another wire or cable will only have capacitve coupling but the coil is perpendicular (90 deg) to the wire form so no inductive coupling, so no current will be induced.

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