Induced/eddy current cancelation in a loop

[artis]

Hi, I have a theoretical problem, I am wondering would it be possible to cancel the otherwise induced currents within a planar sheet/foil conductor.

you can see in the image I have a flattened conducting sheet which in the middle is split up in individual smaller conductors (for the sake of minimizing eddy currents) the middle is also the part which has a changing B field going through it.
But even with the individual conductors I suppose there would be a loop current in the outermost conductors, how could I cancel this unwanted effect?

 

[Rive]

By my humble opinion in this configuration the voltage/current on/through the endpoints is not effected.
Are you sure you have to care?

 

[artis]

I too think you are right that it doesn't have an effect on the up/down or down/up vertical flow of current but such eddy currents would heat up the affected middle part more and probably most importantly weaken the B field passing through the middle by creating an opposing B field due to Lenz's law.

 

[Rive]

If that matters then:
- if possible, you can wrap up the foil
- turn it aside, parallel with the field (if the direction of the field stays the same with time)

 

[Baluncore]

Each opening that has flux will have a circulating peripheral current. Those directional currents will cancel on the internal bars, but will not cancel for the two outermost bars.

1. By replacing the bars with a single thick bar there will be no eddy currents.

2. The holes should all have the same peripheral currents, so the currents will cancel exactly. The areas should carry the same flux. The outer bars should be thicker so they can carry the current without getting too hot.

There are other ways to lay out electrostatic screens.
I would like to know why bars with closed loops are used since they will have eddy currents.

 

[artis]

Well it is not meant so much as a screen rather just a way of passing wires through an area of changing flux without that flux having an impact or as little as possible.

@Baluncore one single thick conductor would do but here is the thing, imagine the drawing I posted, the green lines represent B field direction, which is perpendicular , just imagine there is a solenoid core there where the green lines are , the airgap in the core is where the vertical sheet passes through, so let's say the gap is 0.5mm, since copper has roughly the same permittivity as air we can consider it an airgap.
Surely enough as drawn in my situation there would be a current set up in the outermost bars , what would happen if I took each outermost bar and after the bar transverses the length of the core section it then instead of connecting to the sheet above it makes a one turn around the solenoid core and only then connects back to the vertical sheet, let's say that the bar at each side makes the turn around the core in the opposite direction, this should cancel the induced currents in the loop should it not?

 

[Baluncore]

... since copper has roughly the same permittivity as air we can consider it an airgap.

Too many similar names. I think you mean magnetic permeability rather than dielectric permittivity.

It takes time for flux to penetrate an area from the side of a conductive sheet, that is what the eddy currents are doing. Much will depend on the flux variation or movement of the solenoid relative to the grid. Is that an AC flux or DC, how did it get there , or did the grid slide into position pushing the field aside as it did ?

 

[artis]

the grid is already there as well as the flux cutting it is, their both stationary mechanically, yes I meant magnetic permittivity as that is what we are considering in this case.
the flux from the core intersecting the sheet in the middle region is AC.
Basically model the situation as simply an AC current passing through an AC magnetic flux which cuts the ac current perpendicularly, the needed end result being that both the flux in the core as well as the current passing in the airgap conductor are not disturbed or affected as minimally as possible.

Now I hope you can understand from my attached drawing my idea, in any case I simply connect the central wire and let it span the entire flux area but as for the side wires, I either loop them around the solenoid in a single loop or loop each sides side wire around in an opposing manner,

If I loop the wires all around the same loop then what I could expect is that if the waveforms are identical of both the current carrying sheet and the coil driving the flux through the core that cuts it that then I would get constructive interference where the side wires get added induced emf that adds to the one already present within them right ?

if I wire the side wires where each side loops in opposite direction around the core then I guess I cancel both any constructive interference as well as any possible eddy currents?

 

[Baluncore]

the grid is already there as well as the flux cutting it is, their both stationary mechanically, yes I meant magnetic permittivity as that is what we are considering in this case.

I have no idea what you are trying to do.

 

[artis]

Just think of a flat sheet like wire passing an area of AC flux perpendicular to the sheet, in that particular area in order to minimize the eddy currents one divides the sheet into individual bars/wires but still the outermost wires constitute a circulating current , so instead of the wires directly connecting back to the conducting sheet they make a single loop around the flux and only then connect to the sheet.
so in this way any opposing eddy currents get cancelled

 

[Baluncore]

As I see it, if you cut and reroute the outer bars to cancel their current, the current imbalance situation simply moves into the next pair of bars. The only solution is to have one bar without a hole. Cut and don't reroute the outer bars.

 

[artis]

one bar without a hole I suppose you mean to have simply one wire positioned in the middle and the outer bars basically should be taken away because if they don't form part of the circuit then they have no other use.

Now but I wonder say if I have only 3 strips of wire (bars) one in the middle and one at each outermost side , now the middle bar is safe from eddy currents so if we then reroute the outermost bars so that any unwanted induced current through them cancels , why would that make the situation worse? because the way I understand it is that the highest current always forms around the outermost parts of a conducting loop since those parts have the greatest number of flux lines through the loop, so if the outer parts are taken care of then the inner parts shouldn't be a worry?

 

[Svein]

Maybe you could get some ideas from this type of winding. It is called a bifilar winding.

 

[Baluncore]

because the way I understand it is that the highest current always forms around the outermost parts of a conducting loop since those parts have the greatest number of flux lines through the loop, so if the outer parts are taken care of then the inner parts shouldn't be a worry?

Why must there be any loop?

 

[artis]

because there is a loop , if you look at the drawing in post #1 the flux goes through the middle region where the bars are located, so the outermost bars as well as the upper and lower side of the flat vertical sheet form a loop don't they?

 

[Baluncore]

What would happen if there was no loop because you cut all but one of the bars?

 

[artis]

the same effect I agree the only problem is I would also "cut" the current carrying ability of my sheet conductor.

 

[Baluncore]

Unless you are measuring the strength of that magnetic field using the Hall Effect, you might like to move the single track sideways, out of the concentrated magnetic field.
https://en.wikipedia.org/wiki/Hall_effect

 

[artis]

Ok let me ask like this again with an added drawing, is it possible to cancel the eddy loop current that would otherwise form if the sidebars were connected directly from the lower to the upper sheet conductor by simply wrapping them around the very core which holds the flux that creates the eddy currents in the first place?

Now if I am not mistaken then eddy currents form in a similar way a current forms in a transformer's secondary winding, aka 180 degrees out of phase or in opposite direction than primary current in order to oppose the primary current?
So by wrapping the outer bar wires around the core in the opposite direction than the primary winding the currents from eddies and the loop should cancel, although given that the current in each sidewire is looped around the core in the same way and together that alone I think should mostly cancel any currents that would want to run in opposite directions in each of the wires should it not?

 

[Baluncore]

You appear to be fascinated by increasing the complexity to the point where you cannot understand the situation.
You need to study the theory of knots. You are designing a magicians slip-knot, that is not a knot. When you pull on the ends the knot disappears and you have no topological difference.

So why try to invent an involved cancellation, for the sake of cancellation, if you end up with the equivalent of zero? You may as well reduce the complexity to the zero it is, by deleting the whole thing.

 

[alan123hk]

Ok let me ask like this again with an added drawing...

I agree.
The induced current in the conductive path shown in the drawing should be substantially eliminated or at least to some extent cancelled.
But this is just fundamental behavior of electric circuit, nothing special.

Besides, I prefer to call it induced current rather than eddy current.

 

[Baluncore]

@artis
There is another more general problem with this cancellation technique that cannot be seen because of the complexity.
If the turns around the solenoid produce unit voltage per turn, then the smaller area loops on the conductor pattern will develop lower voltages, so you are always going to be significantly overcompensating.

 

[artis]

ok @Baluncore I am not dismissing your criticism , but besides giving it can you also maybe give a solution that you would see fit?
Now surely I could just eliminate the side wires of the vertical conductor, but that leaves me with a single mid conductor only and given the airgap is of limited size , say 0.5mm that also limits the wire thickness and the current capability of the whole vertical conductor.

Now for the sake of simplicity let's say that both the solenoid coil as well as the vertical conductor are connected to the same AC source and have the same phase waveform going through them, so far so good, the problem as we obviously see is that in the part where the solenoid flux crosses the vertical conductor a transformer effect is taking place and a secondary current is induced , so we also know this secondary current does two things 1)needlessly heats up my vertical conductor section 2) decreases the flux strength of my solenoid by opposing it.

So given this is what we know, how would you solve it?

My idea with the side conductor wrapping was simply to try to "cheat" physics by redirecting the path where the induced current would normally travel in a way such that the current cancels itself and is canceled by the very flux of the solenoid. I do see that in a way this also takes energy from the solenoid flux to do that as would also happen if I did nothing and left the side bars in place.
Ok I am hoping to hear maybe something interesting from you.

 

[Baluncore]

Closed loops of copper are a problem because they fill with field quickly and support large eddy currents. On the other hand the solenoid field will be slow to penetrate a single central conductor. Remember that the magnetic field will be deflected by a conductor. In the short term, the field will be deflected to flow around the copper rather than through it.

How many turns do you have on the solenoid? That will be vector summed to the current in the conductor. The angle of the resultant vector may be dominated by the solenoid.
It seems irrational and unproductive to cross two in-phase fields. You could tilt the solenoid by a degree or two and eliminate the conductor in the gap. That will achieve the same outcome without eddy currents.

You cannot expect me to design what seems like a perpetual magnetic mare's nest while I am wearing a blindfold. If it is just a thought experiment you should abandon it.
You need to be honest and explain clearly what is the aim of the experiment.

 

[artis]

@Baluncore I don't quite get the "being honest" part, as if I had broken a promise or owe you money, how can me asking a question can be labeled as dishonest?
The only reason I usually don't go into every detail is because it is already hard enough to make these threads simple to read and if I would write down every bit of my thought no one would read or answer these. One of the things I have learned over the years is to make short precise questions instead of rambling.

All in all I just simplified this scenario to a solenoid, actually this is just an attempt to measure Lorentz force generated voltage within a conductor moving perpendicularly to a B field if the B field is that of a time varying current, aka AC.
To understand how much comes from the lorentz force itself and how much is added by the changing field, whereas with a static field there would be no additional voltage from the field itself.
So you now see that I am not exactly crazy and I can't simply eliminate the conductor in the magnetic flux path.
Ideally I need the flux to cut the conductor as to have an effect on the electrons within it but at the same time arrange the conductor such that it doesn't get adverse effects from secondary currents or as you said flux deflection.

Now since the Lorentz force and induced currents are of different origin I am ok with canceling as much as I can of the induced currents while having as much flux as possible also penetrating the conductor it cuts.

Does this make sense now?

 

[Baluncore]

Does this make sense now?

No. You can't measure something if you cancel it.

I don't quite get the "being honest" part, as if I had broken a promise or owe you money, how can me asking a question can be labeled as dishonest?

I asked you to be honest in future, because things just did not make sense at that point.

Now for the sake of simplicity let's say that both the solenoid coil as well as the vertical conductor are connected to the same AC source and have the same phase waveform going through them,...

That was not true. There was no solenoid.
You owe me for my time that you have wasted on your mare's nest.

 

[artis]

I said in the beginning in the post that there is a core with a coil around it , normally having a core and a simple coil can be labeled a solenoid, but what has that got to do with the problem at hand ?

I am honest @Baluncore but it seems it doesn't help me much.

I am not sure why you give the impression that there is something new in this, I already stated the problem as clear as I can, my only goal is to use the flux in the core airgap as much as possible and reduce the secondary induced currents as much as possible, I guess one way of doing this would be to make the cross section of the pole-pair as thin as possible practically so that I can get away with a single conductor on each pole, Since poles normally go in pairs like N-S, I won't have problems with eddies across neighboring poles just have to optimize the situation within a single pole.

PS. why you always come across as bit harsh @Baluncore ?