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# Material for pole pieces of Magnet

- To: femm@yahoogroups.com
- Subject: Material for pole pieces of Magnet
- From: "C F Cheuk" <c_f_cheuk@yahoo.com>
- Date: Mon, 27 Aug 2001 16:15:12 -0000
- User-agent: eGroups-EW/0.82

    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.

    I have 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 drop in 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 exceed its 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

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