Shielding a wire from magnetic flux

[artis]

Assume such a situation, if I have a wire and some part of that wire passes through a hole in the core of an inductor or transformer or any other high magnetic field environment , are there ways to shield the wire, the part that intersects with the B field in order to prevent the field from inducing currents in the wire?
AN how good the shielding can be made, can it be made as good as to cancel any effect the field may have on the wire?

I know coax cables etc but are they equally good for a high strength B field environment?

Look at the image attached, the green lines are the B field flux through the core , the grey rod is the wire which is passing through the core.

 

[.Scott]

There are, twisted pairs, shielded twisted pairs, coaxial, and double shielded twisted pairs.
Where shielding is used, the shield is grounded only at one end. Its purpose is to prevent currents from getting any further - so it is not allowed to form a current-carrying circuit.

I've never known there to be a problem with interference with shielded twisted pairs.

I've just done a shopping search for double shielded cable. The only things that are coming up are cat-6, cat-7, cat-8 cables. The "double shielding" simply refers to shielding that is made up of foil, braided wire, and perhaps a ground wire.

Running communication wires past electromagnets - such as the ballasts from overhead lighting, is not unusual. And there can be problems if shielding it not provided. But regular cat-6 wiring should provide ample protection.

In extreme cases, signal cables can be fiber-optic.

 

[artis]

but what about having a metal redirect the magnetic flux around the sensitive part ?, for example if I would put a hollow metal (stainless ?) tube inside the opening where the wire needs to go the metal tube would redirect any flux otherwise going through the hole around it and so the inside of the hollow tube would be free of flux?

such "faraday cage" type shielding definitely works for E field and for high frequency EM fields although I don't know whether one can "reroute" low frequency B field or static B field with such approach?

I can't use twisted pairs or coax because my application involves just having a single wire with low voltage and medium current and for text purposes it is important that no additional opposite currents are induced otherwise the currents would cancel and I would get almost no voltage output where I need it.

 

[Rive]

I have a wire and some part of that wire passes through a hole in the core of an inductor ... if I would put a hollow metal (stainless ?) tube inside

The core material is already a 'magnetic conductor' itself. If you put more 'conductor' into the hole within the conductor, then you would just shrink the hole, no other effect...

The hole within the core already works as a kind of 'shield'. If you want more then diamagnetic materials would have some limited effect, but it not really worth the effort.

I can't use twisted pairs or coax because my application involves just having a single wire with low voltage and medium current and for text purposes it is important that no additional opposite currents are induced otherwise the currents would cancel and I would get almost no voltage output where I need it.

If you can share more details then we might be able to provide a solution.
The main point of the twisted pair wire is that the forward and return current flows in opposite direction so the induced voltage cancels each other at the end. Usually it is considered as a solution, are you sure it is actually a problem in your application?

 

[jim hardy]

are there ways to shield the wire, the part that intersects with the B field in order to prevent the field from inducing currents in the wire?

actually the flux in a transformer stays mostly in the core
and it induces voltage because the wires encircle that flux

adding to your diagram

the purple meter and its wires form a loop in the same plane as your flux so they encircle none of it and that meter will read zero
the orange meter and its wire form a loop in a plane perpendicular to your flux and encircle half of it, so that meter will read something.

As @.Scott pointed out, use a twisted pair so you get equal and opposite voltages that cancel
or as @artis pointed out, use a magnetic shunt to direct flux someplace else. Steel conduit works fairly well.

 

[yungman]

Think of a loop antenna, current around the wire in the loop is proportion to the magnetic field going through the loop. To reduce the magnetic field through the loop you can do it two ways.

1) reduce the loop area.
2) There is a surface area inside the loop. if you tilt the surface area so the flat surface is PARALLEL to the direction of the magnetic field, you minimize the magnetic field going through the loop.

In 1) you try to put the RETURN wire as close to your wire, a twisted pair is the best, coax shield is the return wire, so the loop area is very small too.

In 2), think of the loop antenna of tv, the loop is flat, the area is a flat plane. Think of a NORMAL line that is perpendicular to the flat plane of the loop. If you tilt the loop so the normal line is PERPENDICULAR to the direction of the magnetic field, you minimize the magnetic field line going through the loop and you minimize the current induced along the wire in the loop.

That's the reason in layout, if you have two inductors, you put that at right angle so their axis are at right angle, thereby their magnetic field is at right angle to minimize interference.EDIT: I wrote it wrong, the NORMAL has to be perpendicular to the direction of the magnetic field. It's old age!

 

[artis]

Well I visualized the setup and to my best knowledge it isn't formed in such a way as there would be a current loop around the core and the magnetic flux .
So I guess I could just leave the wire as is an no current would be induced in it ?

There is one more catch to my setup , it is not good enough that there are no induced current , I also have to make sure there is no generated current due to Lorentz force because my wire loop is rotating and the part that goes through the inductor core has (theoretically) B field cutting the wire at 90 degrees and with a rotating wire being 90 degrees to a B field can create force on the electrons so I would be really well off if I could just somehow make sure that no or very little flux crosses the section that is going through the inductor core, of the overall wire length in the loop the section going through the B field is very small , say proportionally no more than 10% of the loop length.

 

[jim hardy]

Well I visualized the setup and to my best knowledge it isn't formed in such a way as there would be a current loop around the core and the magnetic flux .
So I guess I could just leave the wire as is an no current would be induced in it ?

how about posting a picture of it so we can visualize it too?

presumably the wire is part of a circuit
and the rest of that circuit closes the loop, probably encircling some flux so induction is probable.
That's what i represented with my voltmeters.
I was trying to help you envision Faraday's law.

One experiment is worth a thousand expert opinions...
Since you say you have a physical setup, grab a voltmeter and give it a try.?

 

[yungman]

Yep, you have to be a lot more specific. Also, there's always a loop, to every circuit, there is always a signal going out and a signal returning back. You said you have a loop already.

People has to be more careful, the circuit drawing in books are very misleading, you always see circuit drawing and just have a ground symbol. The ground is actually a return and that's the one that more likely bite you in the real world. Think of current going out, you have to have a return path to come back to form a circuit. Finding the return path is the key on this kind of question. Draw out your question in detail.

 

[artis]

Jim I added a picture to my last post, see the thumbnail?

The black rectangular is the wire loop and the green arrows show the flux direction.

 

[berkeman]

Jim I added a picture to my last post, see the thumbnail?

I don't see it. What post # did you edit? Why not just add it as a new Reply, to help keep the thread organized. Editing prior posts can be very confusing...

 

[artis]

i did not edit anything i wrote my reply in post #7 and uploaded a drawing. Personally I see it

 

[jim hardy]

The black rectangular is the wire loop and the green arrows show the flux direction.

Magnetic flux flows in closed loops

but if you want to consider just the wire segment traversing your core
which i just figured out is the dotted line
it poses an interesting question
"What is the force on a stationary charge in a changing magnetic field?
F = qv cross B and the charge is stationary, so what on Earth do we use for v ?"

You said this is a transformer core

so i assume it looks more like this
(courtesy https://physics.stackexchange.com/q...and-magnetic-field-flux-through-the-iron-core, i added the hole and red wires)

the flux above and right of the hole where your wire traverses the core is encircled by your loop .so will induce voltage in it
Can you find an orientation for your loop where voltage induced in it is zero ?

 

[yungman]

You really have to draw a complete diagram like Jim showed. If it's like what Jim showed, you definitely have induction. Magnetic flux is always in a loop, so is current. You have to draw out you EXACT setup. This is very important.

 

[artis]

yes sure in Jim's picture i get induction no doubt about it as that sort of a winding looks like the ones used for a shaded pole in a shaded pole induction motor.
Well my case is a bit more complicated , yes I know flux is always in loops. I quite frankly would have to work rather hard to make the full drawing because it gets complicated and my ability to draw is bad.

As Jim already understood (sorry I forgot to mention) my problem is only with the dotted line , the small length of wire that goes through the core , the rest of the circuit is both at "wrong" angles and away from direct flux so I am fairly certain there will be no current induced there, the thing in question is the part that goes through the inductor, my main concern is how can I make the flux going through that part of wire as small as possible , ideally none.PS. I apologize for my bad drawings so Jim and others disregard the flux lines the only ones that mater here are the ones going through the part of the core where the wire also goes through.

 

[Tom.G]

A simple answer to your question as stated:
If there is no change in the magnetic flux and the wire does not move then there will not be an induced voltage.

However:
If both of the above conditions are NOT met, there will be at least some circulating current in the conductor, but depending on the specifics, that tiny current and any consequent voltage may be small enough for you to ignore.

 

[jim hardy]

the rest of the circuit is both at "wrong" angles and away from direct flux so I am fairly certain there will be no current induced there, the thing in question is the part that goes through the inductor, my main concern is how can I make the flux going through that part of wire as small as possible , ideally none.

Well, from what you say I'm not sure that you believe Faraday's law.

But i think you answered your own question when you mentioned "shaded pole"
if you keep flux low enough to not need that corner of your core
a shading turn or two will by Lenz's Law reduce the flux linking your loop

since you say you have the core, give it a try ?

 

[yungman]

yes sure in Jim's picture i get induction no doubt about it as that sort of a winding looks like the ones used for a shaded pole in a shaded pole induction motor.
Well my case is a bit more complicated , yes I know flux is always in loops. I quite frankly would have to work rather hard to make the full drawing because it gets complicated and my ability to draw is bad.

As Jim already understood (sorry I forgot to mention) my problem is only with the dotted line , the small length of wire that goes through the core , the rest of the circuit is both at "wrong" angles and away from direct flux so I am fairly certain there will be no current induced there, the thing in question is the part that goes through the inductor, my main concern is how can I make the flux going through that part of wire as small as possible , ideally none.PS. I apologize for my bad drawings so Jim and others disregard the flux lines the only ones that mater here are the ones going through the part of the core where the wire also goes through.

I am not sure about that. The flux might be perpendicular with the wire where it enter the core, BUT the flux bends and circles around and back to the other end. Remember, the flux travels in a closed loop, you feed a wire through the core, you wire forms a loop already, the flux will go through the wire loop.

People always draw circuit like in Fig.1a, looks like a wire from the output of an amp going through a hole in the core to drive the load. Looks like only one wire only. BUT in reality, it's like in Fig.1b. In fig.1a, looks like the load just go to ground. But in reality, the ground has to go back to the amp to complete the circuit as shown in Fig.1b. This forms the circuit loop like a loop antenna. There is no ONE wire in the circuit.

Look at Fig.2, this is like what you tried to draw. But I complete the wire loop as shown in RED. It is a complete circuit. Look at the flux line ( in BLUE), they go around in a closed loop. The lower loop cut through the wire loop (RED) and will induce a current into the loop.

The only way to avoid ( say minimize ) induction is shown in Fig.3 where both the forward signal and the return pass through the same hole.

Read post #6 and post #9 again.

 

[artis]

Ok, I just thought about it, because of the specifics of the design induction is not a problem because equal and opposite flux is running through the total of the loop in different places so basically I am only worried about any impact the field may have in the very place the wire crosses the flux of the core.

So I think my best bet is to either somehow shield that part of he wire from the flux or maybe create an opposing flux at that place to cancel the flux of the core and have net zero flux, although I would prefer passive methods like shields or field reroutes because using coils to create "shaded poles" complicates everything.

 

[jim hardy]

Well good luck .beating Faraday out of his induction.

A search on "mumetal tube" turns up several sources, perhaps one would sample you a short piece.

https://www.mushield.com/material-sales/seamless-mumetal-tubing/

 

[yungman]

You might was to go to physics forum to run it by them in electromagnetic theory. I am quite rusty on that right now. I don't think you can use metal tube. It is the flux that goes through the circuit wire loop that induce voltage, it doesn't matter you shield that portion of wire running through the core.

It is obvious you have some design that you don't want to share. I cannot help you with the info so far. It is far far from being a complete picture. It is not that simple. There is a big field specialized on this called Signal Integrity Engineer. I work for a while in this, you really need to see the big picture.