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I have a question about magnetics and particularly about inductance per turn, generally indicated as follows $$A_L = \frac{L}{N²} = \frac{µ_{core}A}{l}$$

where A is the cross section of the core and l is the magnetic path length. So the inductance per turn depends on the permeability (µcore) of the core.

When I take a look on materials on Magnetics, I compare "High flux" core and "Kool µ" core (powder core).

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The composition of the two cores is different so it seems logical to me that if the core is geometrically the same, the inductance per turn cannot be the same, isn't it? Otherwise I do not understand why it still exist Kool µ core as the saturation flux density Bsat is lower than for the High Flux core.

However, it is the same as the technical documentation show it:

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Is it possible to have the same inductance per turn whereas the composition of the two powder core is different?

Have a nice day! :D

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    \$\begingroup\$ It appears that their relative permeabilities are similar. The reason why they offer the lower Bsat material could be that it shows lower losses in high frequency applications. \$\endgroup\$
    – tobalt
    May 20 at 13:59
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    \$\begingroup\$ They say it has lower magnetostriction, so less audible noise when used within the audio range. Both come in a range of permeabilities, so for the same permabilaity you'd expect the same AL. As that's tunable by altering the ratio of ferrite to binder, it's not really an issue. 'Kool u' sounds like a very marketing-led name, who knows how they're trying to push it commercially. The question 'why' often has the answer 'money'. It might be cheaper to make? The most significant parameter is loss per hysteresis cycle, that can't be tuned easily, you need a good basic material. \$\endgroup\$
    – Neil_UK
    May 20 at 14:46
  • \$\begingroup\$ Thank you for your comments :) Where do you find the information about lower magnetostriction ? \$\endgroup\$
    – Jess
    May 20 at 15:34
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If two cores are made with different materials and the same geometry and both have the same AL, the only way this is possible is that their permeability is the same, you can see it in the formula you wrote above,

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and the manufacturer declares that the permeabilities they can be the same, look at the table of "key characteristics". Refers to relative permeability for example "14u" is equal to 14 times the permeability of vacuum.

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  • \$\begingroup\$ 14µ doesnt mean that the permeability is \$14 µ_0\$. It means that one turn of wire creates 14 µH. Usual relative permeabilities for such cores are in the 1000s up to 100,000s \$\endgroup\$
    – tobalt
    May 20 at 14:15
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    \$\begingroup\$ @tobalt You are confused, look at the data sheet, here you can see for the material we are talking about, it has a u = 14 with an AL = 19. (mag-inc.com/Media/Magnetics/Datasheets/0077257A7.pdf) What you mean is AL here you can learn more about it: e-magnetica.pl/doku.php/al_value \$\endgroup\$ May 20 at 14:27
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    \$\begingroup\$ @tobalt Remember that these are powder cores, not solid ones. The permeability is indeed in units of μ₀. If you look at their selection table (for kool mμ cores, at least), you'll see that parts with permeability listed as 14μ have \$A_L\$ ranging from 19 to 37 nH/T², nowhere near the 14 μH/T² you're implying. \$\endgroup\$
    – Hearth
    May 20 at 14:28
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    \$\begingroup\$ @tobalt If you're unfamiliar with powder cores, they consist of a controlled amount of powdered magnetic material (in the case of kool mμ, some type of electrical steel), mixed with and bonded together by a magnetically inert epoxy. The overall effect is to make cores that are exceptional for use with high DC bias, as they have a distributed air gap (or epoxy gap!) throughout the material, dramatically raising its saturation flux density (while also lowering its effective permeability). \$\endgroup\$
    – Hearth
    May 20 at 14:31
  • \$\begingroup\$ you're both right. I am confused :( However, I blame this on the datasheet using the µ without the 0, which gets them awfully close (numerically and visually) to the usual notation of single turn inductance that I am used to. For the toroid cores I have used, numbers in the 10s of µH/T² are actually usual. I will remove my downvote as soon as possible (looks like SE demands an edit to be done) \$\endgroup\$
    – tobalt
    May 20 at 14:45
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As I recall (though it's been a while), Magnetics' high flux cores (which are made of permalloy, a fairly expensive material due to the high price of nickel, which makes up the majority of the alloy) are significantly more expensive than their kool mμ cores (made of some type of electrical steel, which is mostly iron, a very cheap metal, plus some small amounts of silicon and aluminum, also cheap). If you don't need the higher Bsat provided by the high flux line, I would think it not worth the extra expense.

As mentioned in the comments by tobalt, the kool mμ may also have lower high-frequency losses, and looking at their datasheets (high flux, kool mμ) that does indeed seem to be the case; the section here shows as much:

High flux:

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Kool mμ:

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