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Post by antigua on Feb 17, 2023 17:12:22 GMT -5
The field of a permanent magnet is not a function of the material's permeability. Permeability is a term relating to applied magnetic fields. With steel, the residual flux is low and the permeability is high, with ceramic or neodymium, the residual flux is high and the permeability is very low, and when you have a pickup with a ceramic magnet and steel pole pieces, then you take advantage of both properties, but with both being in different physical space, the magnet on the botton and the streel as the pole pieces. With AlNiCo pole pieces, a modest permeability and modest residual flux both coexist in the same material, in the same location, almost as if the steel and ceramic were mixed together in one place. Some of the domains in the AlNiCo are rigidly aligned and some are free moving, the rigid domains are what give it a residual flux and determine its coercivity value, the free moving ones give it permeability. The different grades of AlNiCo will result in more of one characteristic than the other, for example AlNiCo 2 and 3 have a higher permeability but a lower residual flux, where as AlNiCo 5 or 7 or 8 are the other way around.
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Post by ms on Feb 17, 2023 18:53:59 GMT -5
According to your reluctance theory, what would you understand to change if we replaced the permanent magnet with a HyMu-core solenoid at equivalent field strength (relative permeability at this H is about 250,000)? I do not have a reluctance theory. I would prefer that reluctance and pickups were not ever mentioned in the same sentence or paragraph. I am not sure you could get the equivalent permanent magnetic field of an alnico magnet without saturating, but if you assume you can, then you would get about the same inductance and output as steel. The reason is that a short open core does not benefit from very high permeability. The huge "air space" limits the effect. No matter how high you make the permeability, you cannot get more than something like three times the inductance as with an air core.
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Post by ms on Feb 17, 2023 18:55:07 GMT -5
At the very least, I have confirmed that the equations do not work for Alnico, apparently because it is not linearly organized. That seems like it would line up with your explanation. I do not understand what you mean.
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asher
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Post by asher on Feb 17, 2023 19:15:16 GMT -5
And if you use the magnet core also as the bobbin, instead of an air core?
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Post by ms on Feb 17, 2023 19:25:17 GMT -5
And if you use the magnet core also as the bobbin, instead of an air core? You cannot get more than a few times no matter how high the permeability is.
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asher
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Post by asher on Feb 17, 2023 19:53:36 GMT -5
Why not?
I have no commitment to the idea that you can, but what makes you say so?
>>
What you are saying sounds like reasoning for gapped-core chokes rather than a permeable bobbin as I describe. The limit makes sense with respect to core losses when current passes through the coil, but that's an extremely different scenario than we are dealing with in a guitar.
In a guitar, the magnetic induction H (the actual vector of MMF) is what couples the string movement to the pickup coil. The goal is induced voltage, which is opposite to the goal with the transformer (where induced voltage opposes current flow).
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Post by ms on Feb 18, 2023 5:43:22 GMT -5
Why not? I have no commitment to the idea that you can, but what makes you say so? Measurements, modeling, and theory.
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Post by aquin43 on Feb 19, 2023 9:30:32 GMT -5
Asher, the flux through the coil always has to form a continuous loop of some sort. No one has ever seen a magnetic monopole so everything is dipoles and flux loops. That means that the flux coming out of one end of your high permeability core has to find its way back to the other end via the air surrounding the core. The air only has a permeability of one and the path length is at least as long as the core, so no matter how magnetically "conductive" the core is, the magnetic circuit always includes this air path which sets a limit to the overall effective permeability.
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asher
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Post by asher on Feb 19, 2023 21:30:38 GMT -5
Uh huh. So what theory?
That seems like a reasonable explanation.
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nickk
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Post by nickk on Apr 9, 2023 14:17:05 GMT -5
A lot depends on what is expected from the mathematics. If you want an analytical solution of the flux distribution in a real string, you will be disappointed. There is no simple algebraic solution. It has either to be measured or computed numerically. The mathematics provides an insight but the problem is too complex for simple analysis.
Zollner and ms have carried out the measurement in different ways. Zollner has used a finite element analysis to calculate the flux distribution in three dimensions. I have used the results to create an approximate model of the string. Nobody here seems to have access to a full 3D magnetic modeller.
If you read Zollner's book, you will find measurements of the absolute sensitivity of a Strat pickup and even an approximate equation for the variation in sensitivity with distance from the pickup. It is this variation with distance that makes the pickup non-linear. I think that elsewhere on this forum I have modeled the expected distortion, using Zollner's sensitivity equation.
Have you tried OpenEMS? I’ve tried EMI modelling for ~25Mhz but not magnetic although it is used for MRI modelling with ParaView.
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