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# Re: [femm] Material for pole pieces of Magnet

- To: <femm@yahoogroups.com>
- Subject: Re: [femm] Material for pole pieces of Magnet
- From: "Graham Gunderson" <g_jlnlabs@hotmail.com>
- Date: Mon, 27 Aug 2001 18:04:16 -0700
- References: <9mdrmg+6mpf@eGroups.com>

Dear CF Cheuk, The reccommendation for expensive material may be for a reason other than what you assume. It may not involve permeability (or reluctance) considerations at all. Here's how I think of the permeability considerations, by the way: if I have a 1" air gap of a certain cross section, the reluctance of that is equal to half that same air gap filled in with a material of permeability=2 (twice that of air). In other words, the mu=2 filled air gap magnetically appears as a 1/2" gap of air. Likewise, a mu=4 filled gap appears as a 1/4" air gap, all other considerations aside. Now, if you assume that steel has a mu of 1,000 and Vacoflux 50 (which I've never heard of before) has a mu of 10,000 let's say that the magnetic path length from the magnet to the gap is 2". Therefore, t he 1000-mu steel now appears as a 1/500" air gap. The Vacoflux 50 appears as a 1/5,000" air gap. Since the actual air gap is (let's say) 3mm, the difference between 1/500 and 1/5000" air gap additionally (represented by the pole piece material) is indeed insignificant, probably below the physical tolerances of the device. In short, the airgap (where the permeable material is to be placed) is has an actual value of the gap length divided by the gap permeability. (This is, of course, discounting nonlinear considerations like saturation, where the small "air gap length" represented by the permeable material suddenly begins to widen, eventually becoming an actual piece-length "air gap" when fully saturated). Although the air gap analogy (as well as FEMM analysis) shows that material choice is relatively insignificant (at least above a thousand mu or so) the conductivity of expensive, high-permeability mu material is, on average, greater than cheap steel. Do remember that the pole pieces you are using probably represent shorted turns, in transformer relation to the voice coil. As shorted turns, their conductivity determines how "shorted" a "secondary" they represent. As the voice coil conducts amplifier current and exhibits a varying magnetic field external to it, the flux cuts the mu material, causing eddy currents to circulate around the material (which is not exactly something FEMM was built to model). Lenz's Law dictates that these induced currents serve to counter what induces them; therefore, the force between the coil and the mu material (carrying induced current) is repulsive and the impedance of the voice coil is closer to resistive, and less reactive ("imaginary"). This results in a better speaker, performance-wise: the voice coil exhibits additional force from inductive interactions, and the amplifier sees a load that is more resistive: current and voltage have a less convoluted relationship. If I owned a speaker company, I would do the following (which may give you an advantage, if your company manufactures speakers): use cheaper permeable materials, and coat the pole faces of these materials (or any part of the pole piece that the voice-coil flux will cut)in silver, with a final (thin) layer of gold. The silver gives the highest conductivity (inductive repulsion) possible; the gold prevents oxidation. (Polymer coatings could be used as well, provided they are thin enough). This conductive-coating method may let you produce a speaker whose pole pieces are less expensive than exotic permeables, with better performance than those expensive components could offer alone. Regards, Graham Gunderson ----- Original Message ----- From: C F Cheuk To: femm@yahoogroups.com Sent: Monday, August 27, 2001 9:15 AM Subject: [femm] Material for pole pieces of Magnet `Hi All, I am in the process of designing a voice coil actuator (re-inventing the wheel really). I have always been advised to use advanced magnetic material such as Vacoflux 50 for the outer pole piece . The magnet I am using is a NdFeB with Hc of 1000000 A/m. The load line is above the knee to avoid temperature de-mag effect. The air gap need to be of the order of 3 to 4mm to accommodate the coil. Ihave been paying £6000/m for that polepiece material which makes the actuator financially unattractive. I use Dave's program to simulate the magnet/air gap configuration and I evaluate the integral of H along flux lines with the following results: NI drop in inner polepiece (DTD iron) = 99 A NI drop in outer polepiece (the expansive stuff) = 10 A NI dropin air gap = 1210 A What this is telling me is that the B in the air gap is controlled by the air gap itself and the polepiece materials have very little effect!! I then change the Vacoflux 50 in the model with 430 Stainless which has much worse magnetic properties. Dave's program is great in this aspect. It is so easy to change material properties. The resultant B in the air gap has scarcely changed. The conclusion I try to draw is that as long as the polepiece material does not exceedits satuation B, the choice of material is immaterial! So what if the B in the polepiece is above the knee. It does not make any sufficient change in the gap B to justify the high cost. This agrees with most text book when they talk about volume of magnet etc as the iron loss is insignificant when compared with the air gap loss. But this must be wrong because it contradicts with my guru in magnetic (namely the salesman of the expansive stuff). I think I should make a polepiece out of mild steel and test out the hypothesis. Can anyone please enlighten me. What should I watch out for if I change the expansive stuff to mild steel. Temperature effect? demagnetisation? Can't sleep at night? Am I barking up the wrong or the right tree? C F Cheuk England` `Your use of Yahoo! Groups is subject to the Yahoo! Terms of Service.`

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  - Material for pole pieces of Magnet
    - From: "C F Cheuk" <c_f_cheuk@yahoo.com>

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