Questions about magnet material's effect on inductance

[eeron]

Hi,

I have some questions regarding this video, where the effects of magnet material on pickup inductance are demonstrated:





The part I'm a bit confused about starts at about 4:40.

The luthier in the video has a humbucker without a magnet, and it's inductance measures 4.13H. Then he proceeds to put different alnico magnets in (A2, A5, A8), which measure 4.53H, 4.37H and 4.23H respectively. This is all fine and good, but what I'm confused by is that when he puts in a ceramic magnet, the inductance drops to 4.09H which is even lower than without a magnet at all.

Now, I understand that a ceramic magnet is non-conductive, so there will be no eddy currents in the magnet increasing the inductance, but shouldn't the presence of a higher permeability material still increase the inductance of the coil? Isn't the whole point of eg. ferrite cores in inductors to increase the inductance?

At the end, he puts in a steel ruler, and then the inductance is 4.48H, which again I'm not surprised by because steel has high permeability and is conductive.

So my question is, how is it possible, that the pickup with a ceramic magnet has lower inductance than with no magnet at all?

As a follow up question, to what extent is the brighter sound associated with a ceramic magnet explained by the inductance not being increased, or does the higher strength of the magnet also contribute?

[ms]

I am not sure how deeply you want to go into this, so I will give a short, general answer.  The magnet does not alter the inductance because of its permeability.  The permeabilities of the magnets are low compared to that of the steel in the cores; how could they have any such direct effect?  Instead, the inductance is modified because magnetizing the steel alters its permeability, and this can be up or down depending upon the degree of magnetization.

[eeron]

Thanks,

That does clear it up quite a bit. So apparently the source of my confusion was that I ignored the pole pieces, and it's the pole pieces that are in an external magnetic field of the pickup magnet.

It's been more than a decade since I studied this stuff at uni, and of course we didn't specifically look at guitar pickups. So I was thinking more about the more usual case of current in the coil inducing the magnetic field and how materials around the coil respond to it, obviously the field of the pickup magnet is much stronger.

But am I right in thinking that it's the currents induced in the non-magnetized steel ruler which increase the inductance in that case?

Also, if you or anyone else can provide more insight as to the results in the video, I'd be interested to learn more.

[ms]

Apr 29, 2021 14:15:23 GMT -5 eeron said:

Thanks,

That does clear it up quite a bit. So apparently the source of my confusion was that I ignored the pole pieces, and it's the pole pieces that are in an external magnetic field of the pickup magnet.

It's been more than a decade since I studied this stuff at uni, and of course we didn't specifically look at guitar pickups. So I was thinking more about the more usual case of current in the coil inducing the magnetic field and how materials around the coil respond to it, obviously the field of the pickup magnet is much stronger.

But am I right in thinking that it's the currents induced in the non-magnetized steel ruler which increase the inductance in that case?

Also, if you or anyone else can provide more insight as to the results in the video, I'd be interested to learn more.

Eddy currents  reduce the inductance, not increase it.  But if you make the measurement at 120 Hz, the frequency is too low for there to be significant eddy current effects in a pickup.  The ruler just adds more steel to the system; exactly how this increases the inductance is a complicated question.  I would model a 2D approximation in FEMM to find out.

Maybe that guy really does believe what he is saying, but notice that when he told you that the strength of a magnet does not influence the inductance, and to prove that you demagnetize a magnet and then put it back in the pickup, he did not actually do that test.  For example, a very strong magnet saturates the cores, or at least gets to a flatter part of the curve, reducing their permeability maybe to just about one, and thus decreases the inductance relative to a weaker magnet.  It is all in the set of hysteresis curves. Certainly the strength of the field does influence the inductance, and by not understanding what he is doing and selecting results carefully, he can get results that reinforce his lack of understanding.  This is, after all, a complicated subject.

[eeron]

Apr 29, 2021 14:53:36 GMT -5 ms said:

Apr 29, 2021 14:15:23 GMT -5 eeron said:

Eddy currents  reduce the inductance, not increase it.  

Great, so I had that backwards too but that does make more sense now that I think about it more: The eddy currents oppose the change in the magnetic field arond the coils, and weakened field -> lower inductance. I guess I was just trying to make sense of the steel ruler increasing the inductance, but your comment of there just being more steel around the coils (in addition to the pole pieces) would explain it.

I also got the sense that the guy in the video doesn't really have a solid grasp of the subject, but couldn't put my finger on it. But it did seem very strange to me that he was talking about the materials of the magnets and their "iron content", which would lead me to believe the effects of the steel ruler and ceramic wouldn't be so different. The result seemed contradictory to me, given what he was talking about. Although I believe AlNiCo contains at least some iron as well?

Also, thanks for mentioning saturation and hysteresis curves, i think I'm understanding it better now.

[aquin43]

My explanation would be that, as ms said, the magnetisation due to the presence of the magnet will reduce the permeability of the pole pieces slightly.

In the case of the Alnico magnets, which have permeabilities greater than one, there will also be an increase in the mutual coupling of the two coils via the magnet. This explains the overall increase in inductance which is greatest for Alnico 2 with its higher permeability (see antigua's comparison).

In the case of the ferrite, which has a square BH characteristic and so very low permeability, we are left with only the effect of the magnetisation.

[ms]

Apr 29, 2021 15:18:47 GMT -5 eeron said:

Apr 29, 2021 14:53:36 GMT -5 ms said:

Great, so I had that backwards too but that does make more sense now that I think about it more: The eddy currents oppose the change in the magnetic field arond the coils, and weakened field -> lower inductance. I guess I was just trying to make sense of the steel ruler increasing the inductance, but your comment of there just being more steel around the coils (in addition to the pole pieces) would explain it.

I also got the sense that the guy in the video doesn't really have a solid grasp of the subject, but couldn't put my finger on it. But it did seem very strange to me that he was talking about the materials of the magnets and their "iron content", which would lead me to believe the effects of the steel ruler and ceramic wouldn't be so different. The result seemed contradictory to me, given what he was talking about. Although I believe AlNiCo contains at least some iron as well?

Also, thanks for mentioning saturation and hysteresis curves, i think I'm understanding it better now.

A key thing to remember is permanent magnets tend to have low permeability.  It sort of goes along with many magnetic domains already lined up and  not so easy to change. For example, vacuum has a (relative) permeability of 1. Neodymium magnets, very strong, are 1.05. AlNiCo is typically a few times a vacuum.  AlNiCo is high enough so that the effects of  putting steel (100+) in contact is not such a simple problem to solve sometimes.  This why modeling programs have become so popular.  My intuition on these sorts of problems is very approximate, and so I think a bit of computing is a really good thing.


AlNiCo materials here: en.wikipedia.org/wiki/Alnico


Simple test to show an effect of magnetic field strength on inductance: Wind a Fender type signal coil pickup using non magnetized pole pieces. Measure the inductance. Magnetize the poles. Measure again.
 

[ms]

(I can not get the images to post in the right order, but they are labeled air for air, stl for steel and cer for ceramic.)

The question remaining from that video is this: why does the ceramic magnet cause the inductance of the pickup to drop relative to no magnet?  We can find a plausible answer,  but not prove it behind any doubt.

First it is important to understand the characteristics of ceramic magnets.  We look here: en.wikipedia.org/wiki/Ferrite_(magnet).  Under "Hard Ferrites" (meaning permanent magnets) we see these two characteristics:
1.  Ceramic magnets are "very permanent", that is, it takes a very high field to affect their magnetization or cause demagnetization.
2.  They have high permeability.
Well, surely both of these things cannot be true, and so we need to run a test.

The only ceramic magnet I have is from a mini humbucker, and since I can measure the impedance of a pickup very accurately, I take one of the mini hb coils and measure the impedance with three "cores": air, steel, and the ceramic magnet.  The ceramic magnet is not intended to be used as a core for the coil, but it fits, so why not.  Air and steel are for mental calibration, and then we want to see where the ceramic magnet fits in that scheme.  So the first two attachments show information derived from the impedances of air and steel.  To get this information, the high frequency part of the impedance is used to derive the pickup capacitance by a fitting technique, and then the impedance of this capacitance is "unparlelled" from the while impedance.  This is better thought of as subtracting the admittance of the C from the whole admittance, and then converting back to impedance.  So the imaginary part of this impedance, call it Zu, is the inductive reactance.  It might not increase linearly with frequency because of the effect of eddy currents.  Air does not cause eddy currents, and the magenta line, Zu.imag, on the plot for air follows the dashed line (the inductive reactance of Lcoil, the inductance at very low frequencies extended to high frequencies as if it was the true inductance). 





On the other hand, steel is both permeable and conductive.  Lcoil has more than doubled from that of the air core, and the magenta line deviates downward from the dashed line.


So what is ceramic like?  The plot shows that it is within about 1% of air.  So it provides the high microscopic currents to make a strong magnetic field, but otherwise it almost might as well be air.





So how can it reduce the inductance of the pickup?  It is really very simple, I think.  With my Tesla meter I measure the field made by the ceramic magnet and some AlNiCo magnets. (They were actually for a regular humbucker, and so a bit larger, but never mind.)  The ceramic is about twice as strong as the strongest AlNiCo.  So I think what is happening is that the ceramic magnet moves the steel in the humbucker poles a bit further along the hysteresis curve to where the permeability starts to drop, that is, in the direction of saturation, but not nearly there.

[eeron]

Thank you for the thorough answers! It all makes much more sense now.