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"Core" outside of a coil

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Harvey:
The total flux (Φ) may be identical. However the flux density (B) may be considerably less in the outside loop because of the increased area of the exterior core. B = Φ/A

In addition to this, the entire magnetic circuit has some value of maximum reluctance which limits the flux similar to the way a resistor limits the current in an electrical circuit.

And even further, the coil inductance also limits the flux based on the time rate of change of current in your coil. Since the inductance is linked to the magnetic path, the two are interactive.

Just as the spokes of a wheel are closer at the hub than at the rim, flux density is greater in the center than around the outside.

To get the same results in both tests, I would suspect the best approach would be to choose the desired inductance and provide the geometry (and permeability) that satisfies the same flux density for both areas, internal and external to meet the inductance goal.

 8)

BEP:

--- Quote from: exnihiloest on 2012-03-07, 17:21:48 ---I re-read it, and it seems not more relevant than the first time I read it.



--- End quote ---

Ah! Why am I not surprised?

Never the less, the answer I supplied is the reason inductive devices are not designed as you have in your experiment. I seriously doubt those same designers worry about the closed loops of the magnetic circuit being non-conservative.

Forgive me, but I see your experiment as if a scientist is holding a bucket of water with many holes in the bottom while he puzzles over why the water level drops.

Yes, indeed, the high permeability of the ferrite will act as a lower reluctance path for the magnetic loops. This does not mean all of these loops will take that path. The very high reluctance portion of the circuit (between the ends of the coil and the ends of the outer core ferrite) provide an area where the loops take an equal preference of increasing the distance between them.

Magnets do not repel only when like poles face each other. They also repel when like 'equators' (bad terminology but should be clear) face each other. All that is required is the smallest of gaps. You have a huge distance allowing leakage. It is no different than a loosely wound coil over a sensible core causing magnetic leakage.

The above is only one half of the problem you see. The other half is exactly as .99 has described. In a central core the magnetic circuit direction is only one direction (simplified by not involving Lenz).

In any outer core there are a minimum of two magnetic circuit paths. The net magnetic is almost null. The only reason it isn't null is because the larger diameter of the outer perimeter presents a greater path than the inner diameter.

You should be able to link that minute 7% change to the difference between the inner and outer diameters of the outer core  8)

Here is an idea to ponder....

Make an outer core that is of Mobius form. This would provide the same path surface area for inner an outer diameters and should result in no effect on the inner coil.....

Wait a minute!  Isn't that why non-inductive resistors and capacitors use that form?  ;D 

 

exnihiloest:

--- Quote from: poynt99 on 2012-03-07, 23:01:09 ---Ex,

As I mentioned before, I believe the flux in both directions cancels inside the ferrite when it is situated outside the coil.

--- End quote ---

Hi Poynt99,
There is only one flux direction in the ferrite, the opposite direction being in the air. I agree that in both cases, ferrite inside or outside, the flux in both directions (air/ferrite) cancels. This is well known for a coil with an inside core, all its flux is looped in space around and my last experiment confirms it is also the case with an outside ferrite: flux passing in air through the inside coil = flux in opposite direction in the ferrite, nothing outside.
The last question that remained was: why the change of the inductance due to the ferrite is not the same in both cases? See Harvey's reply and mine to him (likely not a question of ferrite position but of the air path).


exnihiloest:

--- Quote from: Harvey on 2012-03-08, 00:03:30 ---The total flux (Φ) may be identical. However the flux density (B) may be considerably less in the outside loop because of the increased area of the exterior core. B = Φ/A

In addition to this, the entire magnetic circuit has some value of maximum reluctance which limits the flux similar to the way a resistor limits the current in an electrical circuit.

And even further, the coil inductance also limits the flux based on the time rate of change of current in your coil. Since the inductance is linked to the magnetic path, the two are interactive.

Just as the spokes of a wheel are closer at the hub than at the rim, flux density is greater in the center than around the outside.

To get the same results in both tests, I would suspect the best approach would be to choose the desired inductance and provide the geometry (and permeability) that satisfies the same flux density for both areas, internal and external to meet the inductance goal.

 8)

--- End quote ---

Hi Harvey
I synthesize what I understand now from your answer that enlightened me. We have two magnetic circuits. The first one with the ferrite inside the coil is constituted for one way by the ferrite cylinder and for the return by the entire air space outside and around the coil. The second one with the ferrite outside of the coil is constituted for one way by the air inside the coil and for the return by the same ferrite cylinder now around the coil.
Each magnetic circuit is far from saturation. Not the point. Signals are weak enough for not saturating the ferrite, and air is not saturable.
I agree with you that the flux is limited by the value of maximum reluctance. I think that we can accept without calculating that the reluctance of the second circuit is higher due to the lower crossed section of the air path inside the coil. From Φ=L*I we must conclude that a lower Φ is the consequence of a lower L and so the phenomenon is explained: it is not the ferrite outside or inside that explains the difference (I'm pleased  :)), but the consequent change of the section of the associated air path.

The same experiment by using a ferrite toroid having a big hole and wide diameter in order to severely increase the inside air path should clearly reduce the inductance change when coil inside and outside. I will try to confirm if time permits.

poynt99:
If you do this experiment up in FEMM, you should be able to determine why the difference.

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