# Magnetic flux conduit for flux concentration?

1. ## Magnetic flux conduit for flux concentration?

I came across a patent for a thick wall copper tube with an electrical gap in its side so there won't be current flow. Does this type of design actually work for concentrating flux? Like this: https://patents.google.com/patent/US6720855B2/en

If this flux conduit doesn't work, how do people usually create an area of more concentrated flux inside a coil?

If it does work, how does it work when no current flows in the thick copper tube?

Thanks, George

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2. ## Re: Magnetic flux conduit for flux concentration?

I suspect the author hopes that some method of wireless power transfer can derive from his idea of a 'flux conduit' made from a copper tube. Or possibly a directional energy beam?

Some aspects of the theory could work, however there is the opposing effect of eddy currents which are generated within the copper. These eddy currents result in a physical force which resists moving magnetic fields, including the very magnetic field which created them. Of course there are gaps in my knowledge.

I'm reminded what happens when I drop a neodymium magnet inside a thick-walled copper pipe. The magnet creates eddy currents which oppose the magnet's motion. Like magic the magnet defies gravity, taking a few seconds to drop through the pipe. As soon as it emerges, it falls freely as gravity dictates.

Videos on Youtube show how a moving magnet slows down just before it impacts a slab of copper. Or in another experiment, a magnet levitates on a string held above a spinning slab of copper.

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how do people usually create an area of more concentrated flux inside a coil?
A ferromagnetic substance normally is the core of inductors. Usually you want to minimize eddy currents because they waste power by generating heat. Thus transformers are built up from many thin iron plates, or else formed from slurry which contains ferromagnetic particles separated by non-conductive material.

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3. ## Re: Magnetic flux conduit for flux concentration?

The slot stops -eddy- currents from an AC magnetic field.
It's not confining magnetic flux.

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4. ## Re: Magnetic flux conduit for flux concentration?

This will not work with a DC magnetic field; a constant magnetic field will penetrate the copper tube.

If the coil (as seen in the picture) is fed with AC the magnetic flux will penetrate the copper sheet only to a small extent (the skin effect): the copper will effectively repel the magnetic field.

If the copper is replaced with a superconductor, the magnetic field will not penetrate the superconductor and will follow the conduit. This is the famous Meissner effect. See, e.g., https://en.wikipedia.org/wiki/Meissner_effect

A constant magnetic field can be shaped only with a superconductor.

The slot in the copper tube must be very thin: just sufficient to stop current but much smaller than the skin thickness (else flux will leak happily)

I do not know any practical application of this device.

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These eddy currents result in a physical force which resists moving magnetic fields, including the very magnetic field which created them. Of course there are gaps in my knowledge.
The magnetic field is parallel to the axis of the solenoid; the induced eddy currents will be antiparallel to the current flow in the solenoid. But due to the presence of the slot, the eddy currents will not complete the circuit.

Same case applies for a common power transformer: the eddy currents will be antiparallel to the current flow in the coils and the laminations will prevent that.

However, for some part of the current flow, the laminations will be parallel to the direction of the current and we shall need (i) thin laminations and (ii) high resistance steel sheets.

Eddy losses can be significant even with a laminated core; you may use an insulated iron wire shaped like a core (something like a toroidal core).

5. ## Re: Magnetic flux conduit for flux concentration?

Later yesterday, after posting my questions, I came across what people are calling a "Lenz lens" which is utilized to concentrate flux. It uses a similar gap but they are usually flat or wire frame and not conduit shaped.

Also later yesterday I heard back from the patent's listed inventor. She confirms that they have built these in their university lab and they really work.

But I'm still trying to understand them better when conduit shaped.

George

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6. ## Re: Magnetic flux conduit for flux concentration?

A point to consider is that the flux conduits other than a magnetic core are not reducing reluctance but increasing it. Means you need many ampere turns to achieve a certain flux at specific point of the conduit. I would say, the flux "conduit" is only able to displace flux but not concentrate it. It's no method to make the flux density in any spatial point considerably higher that without the conduit.

I do not know any practical application of this device.
I agree with c_mitra.

I know applications of similar slotted metal screens used in transformers to reduce leakage inductance.

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