The effect of a permanent magnet on an air core coil.

[broli]

I was doing some experimental setup related to the electron spin stuff. And right off the bat got confused about something. This may sound stupid but how is the following possible.

Given an air core coil with a certain inductance. When a permanent magnet is inserted in its center its measured inductance is increased. Aren't we taught that a permanent magnet would have no effect on the inductance of an air core coil?

This happens with both ferrite and neodymium magnets.

[F6FLT]

A magnet in an inductor has the same role as any other material. So the inductance will be modified according to the permeability of the magnet material.
The relative permeability of neodymium is 1.05, to which will be added in part that of the material that covers it, often chrome. It is therefore normal that the inductance increases.

It is likely that the permeability of the material of the magnet is reduced in the direction of magnetization. It would be interesting to see, with a spherical magnet, if the inductance varies according to whether the magnetic axis is coaxial to that of the coil, or transverse.

[broli]

A magnet in an inductor has the same role as any other material. So the inductance will be modified according to the permeability of the magnet material.
The relative permeability of neodymium is 1.05, to which will be added in part that of the material that covers it, often chrome. It is therefore normal that the inductance increases.

It is likely that the permeability of the material of the magnet is reduced in the direction of magnetization. It would be interesting to see, with a spherical magnet, if the inductance varies according to whether the magnetic axis is coaxial to that of the coil, or transverse.

From here:
https://physics.stackexchange.com/questions/301080/what-is-the-permeability-of-a-permanent-magnet

That relative permeability of 1.05 is just the change in magnetization caused by an external field. Probably at remanence. Permeability as in the formula you wrote is not defined for a ferromagnet - there is no proportionality, hysteresis makes magnetization dependent on what happened before.

That relative permeability of 1.05 is just the change in magnetization caused by an external field. Probably at remanence. Permeability as in the formula you wrote is not defined for a ferromagnet - there is no proportionality, hysteresis makes magnetization dependent on what happened before.

So to me it makes no sense at all especially for such a big inductance change.

Btw the nickel-plating would tend to reduce the inductance not increase it. I will later on also check with PTFE coated neodymium magnets.

[Grumpy]

Apply some HV impulses t that coil while the magnet is next to it...

[Smudge]

broli said

So to me it makes no sense at all especially for such a big inductance change.

The inductance change was 2% whereas the quoted mu of 1.5 would produce a 5% change.  So it does make sense unless my math is wrong.
Smudge

[broli]

broli said   The inductance change was 2% whereas the quoted mu of 1.5 would produce a 5% change.  So it does make sense unless my math is wrong.
Smudge

Yeah you're right I got too surprised at such a difference when expecting no difference at all. It means that even a permanent magnet acts as a sort of paramagnetic material with a low linear permeability.

[gyula]

....

Btw the nickel-plating would tend to reduce the inductance not increase it. I will later on also check with PTFE coated neodymium magnets.

Sorry to chime in.  Nickel is listed among the ferromagnetic materials.  http://hyperphysics.phy-astr.gsu.edu/hbase/Tables/magprop.html#c1 (scroll down)

Its magnetic permeability is between 110 and 600 so the nickel plating (whatever thin layer it is) should increase coil inductance to a certain degree. 

Gyula

[F6FLT]

broli said   The inductance change was 2% whereas the quoted mu of 1.5 would produce a 5% change.  So it does make sense unless my math is wrong.
Smudge

The µr of neodymium is 1.05 (not 1.5), but you have the right %.

The order of magnitude of the experiment is perfectly correct, considering that neodymium is obtained from agglomerated powder, so it doesn't fill its volume 100%, and the core doesn't fill the coil completely, nor around, as you can see on the Broli's picture.

[F6FLT]

From here:
https://physics.stackexchange.com/questions/301080/what-is-the-permeability-of-a-permanent-magnet

So to me it makes no sense at all especially for such a big inductance change.

The inductance changes in very good agreement with the theory, see my post above.

It is also important to know that the permeability is only meaningful for a given direction of the magnetic field. In general a magnetic material is isotropic, so it has the same permeability whatever the direction of the magnetic field.
But when it is subjected to a strong field, as is the case in a magnet, the permeability may be different along the magnetic axis than transversely.
For neodymium, this would not change much because its permeability is low.
For ferrite magnets, probably the permeability is much higher, it may no longer be isotropic but weaker along the magnetic axis.

I had noticed this with ferrite toroids used in a resonant LC circuit.
If we saturate the toroid with the field of a diametrically oriented neodymium magnet, which reduces the permeability, the resonant frequency changes considerably. But if we saturate the toroid transversely with the same magnet, i.e. the magnet and the toroid are in parallel planes, then its resonant frequency does not change. This is because the constant field of the magnet is perpendicular to the variable field along the torus, linked to the AC current, so it can not circularly saturate the torus. The transverse permeability and that along the torus are decoupled.

As a ferrite can be both saturated and unsaturated, and thus have different permeabilities, a question of axis, I had thought of exploiting the effect for experiments but without coming up with a good idea.

[Hakasays]

Thanks for posting your experiment Broli.  It's great to pull at the threads at anything that seems weird or counter-intuitive to see where they lead.

Apply some HV impulses t that coil while the magnet is next to it...

When a permanent magnet is saturated, what happens to the magnetic flux contained inside the core?
It would have to be ejected, right?

[F6FLT]

...
When a permanent magnet is saturated, what happens to the magnetic flux contained inside the core?
It would have to be ejected, right?

What do you mean by "saturated"? A magnet is already a saturated material, or nearly so, by its own field.
If you increase the flux by an external field, the two are simply surimposed, like stacking two magnets.

[Hakasays]

What do you mean by "saturated"? A magnet is already a saturated material, or nearly so, by its own field.
If you increase the flux by an external field, the two are simply surimposed, like stacking two magnets.

The magnet is only saturated to the flux density of the magnet itself.  As Broli demonstrated the magnet still has positive permeability.

This question is more about variation of permeability of the material itself, and how the magnetic flux and atomic structure might react to this change.
IE: If we were to vary the permeability of a ferrite or neo magnet up or down, how it affects flux contained or interacting with it?

(In practice this would likely be accomplished via a magnetic amplifier/saturable reactor/amplidyne setup where permeability can be modulated with respect to time.)

[Smudge]

The magnet is only saturated to the flux density of the magnet itself.

That is only true for magnetized ring cores where there is no demagnetization factor and no poles.  For any practical magnets there is demagnetization which reduces the flux (but not necessarily the magnetization so the term demagnetization is wrong).  Magnetic engineers draw a load line on the BH curve to obtain the working point.  Only when the magnet is driven into the first quadrant of the BH curve is it fully saturated where the mu becomes 1 because  there are no misaligned ripples to rotate or flip.

As Broli demonstrated the magnet still has positive permeability.

Because it is in the second quadrant and not fully magnetized.

Smudge

[F6FLT]

Sorry to chime in.  Nickel is listed among the ferromagnetic materials.  http://hyperphysics.phy-astr.gsu.edu/hbase/Tables/magprop.html#c1 (scroll down)

Its magnetic permeability is between 110 and 600 so the nickel plating (whatever thin layer it is) should increase coil inductance to a certain degree. 

Gyula

You're right, but I just did the experiment, and it's the opposite that happens!

My neodymium magnet (µr=1.05) is covered with nickel. So we would expect a drop in resonant frequency when we put it in a coil.

I took a speaker coil in series with a capacity of 2.2 nF. The resonance frequency is 290 KHz.
A ferromagnetic core transformer or a ferrite makes, as expected, lower the frequency.
But when I place my ball magnet inside, the resonant frequency rises to 330 KHz.
Changing the orientation of the magnetic axis of the ball in the coil does not change the resonance frequency.

Experiment made with a SIGLENT SDG2042X generator in sweep mode on channel 1: the swept frequency goes from 200 KHz to 480 KHz (20 KHz/div) in 280 ms, scope synchronized by a square wave signal of T=560 ms generated by channel 2 (synchronous of channel 1).

I'm sure that an expert of free energy would explain us the secret: it's that the permeability is less than 1, that scientists are once again wrong, and that the OU is not far... 
But as usual it is our experts who are wrong. I let you find out why, but not more than 5 mn...

[Allcanadian]

Broli

I was doing some experimental setup related to the electron spin stuff. And right off the bat got confused about something. This may sound stupid but how is the following possible.

Given an air core coil with a certain inductance. When a permanent magnet is inserted in its center its measured inductance is increased. Aren't we taught that a permanent magnet would have no effect on the inductance of an air core coil?

This happens with both ferrite and neodymium magnets.

The simple explanation...

Inductance is the tendency of an electrical conductor to oppose a change in the electric current flowing through it. The flow of electric current creates a magnetic field around the conductor. The field strength depends on the magnitude of the current, and follows any changes in current.

Inductance is not something but a measure of something which is actually self-inductance. The changing magnetic field due to the changing electron current cuts the same conductor(s) inducing an emf opposing the source emf, think Lenz Law. So when we measure "inductance" were actually measuring self-inductance or the induced emf in relation to the source emf as a changing voltage. I use a trick and when someone say's "inductance" my mind thinks "okay you mean self-inductance" which logically leads to induced EMF's.

If there is already an existing ie. permanent magnetic field present then this field influences the coils changing magnetic field. Here we need to consider the magnetic field density of "both magnetic fields" in time. If the PM field density is already at X then this tends to influence the coils changing magnetic field until it is greater than X. So the energy in the coil tends to dissipate like a resistance up to X then once the magnetic field density is greater than X it starts changing more and can induce an opposing emf to counter the source emf.

There's another trick to help understand inductance. If two coils in series face each other with opposing magnetic fields they do not cancel each others field. Each field influences the other raising the total magnetic field density, inhibiting the field change to induce an opposing EMF causing both coils to act more like a resistance than an inductance. So we need to consider the polarity of the field(s), the total field density, the field rate of change and/or magnitude of induced EMF. As we can see, we could probably fill a library with all the possible phenomena relating to the single term we call inductance...

Regards
AC

[partzman]

You're right, but I just did the experiment, and it's the opposite that happens!

My neodymium magnet (µr=1.05) is covered with nickel. So we would expect a drop in resonant frequency when we put it in a coil.

I took a speaker coil in series with a capacity of 2.2 nF. The resonance frequency is 290 KHz.
A ferromagnetic core transformer or a ferrite makes, as expected, lower the frequency.
But when I place my ball magnet inside, the resonant frequency rises to 330 KHz.
Changing the orientation of the magnetic axis of the ball in the coil does not change the resonance frequency.

Experiment made with a SIGLENT SDG2042X generator in sweep mode on channel 1: the swept frequency goes from 200 KHz to 480 KHz (20 KHz/div) in 280 ms, scope synchronized by a square wave signal of T=560 ms generated by channel 2 (synchronous of channel 1).

I'm sure that an expert of free energy would explain us the secret: it's that the permeability is less than 1, that scientists are once again wrong, and that the OU is not far... 
But as usual it is our experts who are wrong. I let you find out why, but not more than 5 mn...

IMO, the result you see is due to the fact that the nickel coating is acting as a shorted turn resulting in the lowered inductance.

Pm

[gyula]

You're right, but I just did the experiment, and it's the opposite that happens!

...

Hi F6FLT,

Interesting.  Let me show you a test I just made, out of curiosity (I accept your test result of course).

1st picture:  multilayer air core coil has 3.00 mH inductance.
2nd picture: I inserted two stacked cylinder Neo magnets into the coil, coil inductance went up by at least 20 uH.
3rd picture: I pulled out quasi half way the magnet stack from the coil as shown, inductance went up by at least 50 uH.

The measuring frequency of the L meter was about 206 Hz (checked with an oscilloscope) when it measures around 3 mH inductance. I include this because your measuring frequency was much higher with respect to mine. 

So it seems my Nickel coated Neo magnet increases coil inductance.   

Gyula

[Hakasays]

Gyula, interesting results.   Did you try with the neo magnets facing both directions?
(I wonder if the meter may have a DC bias when doing inductance measurements, so N-S vs S-N might give you a different result)

[F6FLT]

IMO, the result you see is due to the fact that the nickel coating is acting as a shorted turn resulting in the lowered inductance.

Pm

Good point, Partzman!

Not only the nickel coating is conductive, but also the NdFeB, and therefore as you say such a core constitutes a conductive loop like a shorted transformer secondary.

If i1 is the current in the coil and i2 is the current induced in the magnet causing a field that opposes that of i1, a voltage U applied to the coil is such that :
U = L*di1/dt - M*di2/dt where M is the mutual induction coefficient. The equivalent inductance L' is therefore such that L'*di1/dt = L*di1/dt - M*di2/dt so L'= L - M * di2/di1.
As di1 and di2 are proportional, di2/di1 is a positive constant depending on the transformation ratio and the resistances of the inductor and the loop constituted by the magnet,
So we have L' < L.

...
So it seems my Nickel coated Neo magnet increases coil inductance.   

Gyula

This is also true! As neodymium and nickel have a permeability > 1, they increase the inductance, while the shorted secondary loop effect decreases it. So we have two antagonistic phenomena that will give opposite results depending on their parameters. I think that in your case, the effect of the permeability dominates because the core fills the coil better than the ball magnet in my case, so the effect of the increase of the permeability which increases the inductance takes over the effect of the coupling which reduces it.
So we have L' > L.


The devil is always in the details

The lesson I learned is that the magnetization of the magnet plays no role. Only the permeability of the medium and the possible couplings leading to currents in the core, play on the inductance.
By using a copper tube as a core, I predict that the inductance will decrease significantly.

[gyula]

Gyula, interesting results.   Did you try with the neo magnets facing both directions?
(I wonder if the meter may have a DC bias when doing inductance measurements, so N-S vs S-N might give you a different result)

Hi Hakasays,

Yes I did. The meter puts a (very) little DC bias onto the coil indeed but no any change manifested in the inductance value when I flipped the orientation of the magnet stack in the coil.

Gyula

[gyula]

Hi F6FLT,

I agree with both of your comments on the increase and decrease of the inductance.  Additionally, I think the measuring frequency also plays a certain role in the resulting behaviour.

...
By using a copper tube as a core, I predict that the inductance will decrease significantly.

Yes, I recall the existence of coil "tuning" rods used by radio repair men and radio technicians of the 60's and 70's.  A plastic or wooden rod (OD 8-10mm, length 10-15 cm) had a soft ferrite piece embedded into one of the rod ends and a copper sleeve (a short pipe) was fixed on the other end. Depending on which end of the rod was put near to or into a coil the result was an increase (ferrite end) or a decrease (copper end) in coils inductance, so it was a good help in the tuning of the radio equipment having mainly slug tuned coils back then.

Gyula

[F6FLT]

@Gyula

I wasn't aware of that, thank you.
At first sight the idea of putting a short-circuited loop is a bit confusing, because we think that we add losses by induced currents. But if the resistance of the loop is really low, it's not the case and we have an efficient control system.
It is indeed clever.

[Allcanadian]

Gyula

Yes I did. The meter puts a (very) little DC bias onto the coil indeed but no any change manifested in the inductance value when I flipped the orientation of the magnet stack in the coil.

I think this is a good example of how we can be fooled by small details. In fact there are a few phenomena which are effecting your measurement...

1)Inductance is actually self-inductance, so when you added a permanent magnet as a core which is mostly Fe or iron it concentrated the field increasing the self-inductance. In effect you added an iron core.

2)The permanent magnets field decreased the self-inductance because the PM has it's own field partially saturating the air cores self-inducing field.

So the PM's iron core increased the inductance and the PM's magnetic field decreased the inductance. Here we should understand the PM field is separate from the PM material.

Regards
AC

[Magluvin]

IMO, the result you see is due to the fact that the nickel coating is acting as a shorted turn resulting in the lowered inductance.

Pm

Hey PM

Had gone through these things pretty far back and my thoughts on this is the existing dense field of the magnet tightens things up, like the string on a guitar.   Like a magnetically biased choke/transformer core(hitachi shows some examples)..  And the freq will increase even if it seems like the magnet as an extra core material should lower the inductance.  Like a simple boost converter, using just a choke coil that is biassed with a magnet, the coil can be driven to further peaks of stored energy because the drive for the coil will have to reverse the magnets field before saturation of the core.  So a small core can be used to do a bigger job like a larger core.

Mags

Mags

[gyula]

Hi Allcanadian,

...

1)Inductance is actually self-inductance, so when you added a permanent magnet as a core which is mostly Fe or iron it concentrated the field increasing the self-inductance. In effect you added an iron core.

Yes I added a "core" to the air cored coil and this core increased the self inductance of the coil by 0.66 % (up to 1.66 %).   This is ok, I agree, for the effective permeabilty of Neo magnets is around 1.05 so indeed the very small increase in self inductance is justified. The very small permeability of the Neo magnets (wrt air) come from their almost 100% magnetic saturation state. 

2)The permanent magnets field decreased the self-inductance because the PM has it's own field partially saturating the air cores self-inducing field.

So the PM's iron core increased the inductance and the PM's magnetic field decreased the inductance. Here we should understand the PM field is separate from the PM material.

Well, in my test setup I did not find any difference in the increased self inductance when I aligned the stacked magnets in the same position inside the coil but with flipped poles. I mention this again because although I accept that the PM field is separate from the PM material, the static field of the PM that penetrates the coil, always encounters an oscillating field (around 200 Hz in my test setup) which has flipping N and S poles being attracted or repelled by the PM field. And whenever attraction happens, the oscillating field can reach the surface of the Neo magnet, creating induction in the metal body (both in the Nickel coating and inside). So the shorted turn member Partzman mentioned and F6 continued with further comments still make sense. 

In my working practice I came across with the use of silver coated copper rings inserted into slug tuned single layer coils mounted on ceramic bobbins to fine tune them in the lower VHF bands. Such rings acted as a single turn shorted coil coupled to the main coil and was used for fine tuning it by adjusting the distance of the ring from the coil. The silver coating on the shorted turn served to have the smallest loss transformed back into the main coil. The closer the ring was placed to the main coil, the bigger inductance decrease happened in the main coil. At the desired distance the ring coil was glued to fix its position.

Regards
Gyula