Re: {Collins} KWM-380 Power Transformer..two ways to eliminate mag field


Hi. Recently, as part of this thread, I had a fairly similar email discussion with Heinz (DH2FA) on the same.

Now, you are actually thinking of the second major way to "stop" if you will, a magnetic field from extending outward any further.

The truly magnetic technique of blocking the field can be done by surrounding it, or the object being interfered with, is with mu metal and related materials.

It has that property.

This is where the concept of permeability and the like comes in.

But let me ask you to completely put aside these concepts of permeability, or of somehow controlling the magnetic field.

That is not the technique we are discussing here.

What we are doing here is not "blocking" the field with high permeability material or other clever material or approach.

In this transformer application, we can effectively, to a very large extent, stop the field from meaningfully extending outward any further, by simply taking all its energy from it.

If we take all of its energy then there is nothing left (the magnetic field) to further leave the transformer, and be seen by another component.

We are using a very cheap method, of utilizing a simple shorting turn to absorb all the passing (cutting the strip with lines of force, and depleting these lines of force) energy of the field.

So this is how it works.

Picture a simple real world winding within the core, resident with all the other windings.

You look at the output voltage and it is 6.3 volts and can supply you current as well.

This energy comes from the magnetic field that the winding intercepts (that cuts through it).

The magnetic field cutting through the winding is diminished in strength as it gives up the energy demanded by the winding; and to the amount of energy that this winding furnishes as current (let the winding light a tube).

And that's the key.

Recall that the escaping remnant field lines, now on the outside of the transformer "cut" the copper shorted turn (induce a reversed polarity voltage which counters the original field) and they give up their energy, by first inducing a voltage, which then results in a current (connect the winding to a filament).

It's the current itself that burns up the energy that was obtained from the field. The current, flowing through the copper strap's own very low resistance burns up the energy as dissipated heat.

The better the shorted turn, the more energy the passing field must give up.

Hopefully all of its stored energy, so the field starts to effectively disappear as it interacts with this single turn winding, mounted ONLY, on the OUTSIDE of the transformer.

A last point.

The magnetic field can be thought of as simply another perspective of current. They are for the most part the same thing.

So very simply on this thought; burn up the current as dissipated heat, and you have eliminated the field that created this current.

To digress.

Mu metal and high permeability material techniques are used a great deal to shield (block the field from the tube, or vice versa) cathode ray tubes from a local interfering magnetic field.

Cathode ray tubes (the electron beam itself) are extremely sensitive to any nearby unintended magnetic field.

I have held a little toy bar magnet 6 feet from an open unshielded CRT on the bench, and watched the beam distort.

For very precise applications, like oscilloscopes, any distortion of the screen image must be avoided; so you mu metal magnetic shield it.

The down side is that mu metal is very expensive, so only certain applications seek to use it.


Louis N3OL

For convenience. I included my original text.

Hi JP;

What you left out was easily inferred and easily understood.
I wish I could catch every nuance in my writing, but fortunately the readers can see through it.

And you are quite right, the presence of the area of metal will act as an electrostatic shield, as surely as the IF cans surrounding the IF transformers.

Now, in the context of this, the "flux shield" term you quoted relates to this:
The magnetic field, for calculation and engineering sake is referred to as "magnetic flux" .
It's a math term that is a measure of the number of "lines of force" or density (strength) of the field as it moves through an area (a true area of length and width) of metal, or paper, or what have you.

So this "flux shield" term is what we have been talking about all this time.
The magnetic "Flux" is absorbed by inducing the highest possible current into the very low and wide copper strap turn; and is thus quite well "shielded' from escaping the transformer.

The copper strip' width serves to act as an extremely low resistance winding, draining the strength of residual field outside of the transformer, very well.
And in addition, the wide physical width also intercepts the field lines that may try to escape out of an otherwise unblocked window, and possibly miss the shorted strap.
Hence, it is often as wide as the true windings.

Remember also that the intended operation of the transformer, what the designer indeed wished to do or compromise on, and the cost; will drive all the variations of this.

In some present correspondence I had with Bob (N2OAM), I noted his pickup on the obvious question of where the flux shielding shorted turn is.
He pointed out that any shorted turn, as a winding, would certainly overheat.

But we note that this "turn" is, outside, of the windings core, and intercepts the very low remnant field, that tries to radiate away from the core.
And so while it absorbs the residual magnetic flux, it typically is not significant for heating  and interference to the normal transformer action.

Louis N3OL

From: J.M. Polinsky [mailto:k4jmp@xxxxxxxxx]
Sent: Tuesday, March 22, 2011 4:18 PM
To: Collins; collins@xxxxxxxxxxxxx; Poli, Louis (US SSA)
Subject: RE: {Collins} KWM-380 Power Transformer


Something is still missing here as copper has a permeability fairly close to 1... I guess what I am trying to politely state is that your assertion that this thin copper strap is canceling out the magnetic flux, seems irrational. I wish I could have said that a different way but my patience did not allow it. JP

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