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# Re: hysteresis losses in enclosures

- To: femm@egroups.com
- Subject: Re: hysteresis losses in enclosures
- From: erik_t_viking@my-deja.com
- Date: Tue, 31 Oct 2000 08:54:27 -0000
- In-reply-to: <39F98DB3.2F98FEFE@foster-miller.com>
- User-agent: eGroups-EW/0.82

    --- In femm@egroups.com, David Meeker <dmeeker@f...> wrote:
    >
    >
    > erik_t_viking@m... wrote:

    ...

    > > And now my questions, related to FEMM:
    > >
    > > 1. Why is the electrical conductivity of ALL steels in FEMM set
    to 0?
    > > I'm using 9.5 MS/m as a normal value, about 1/3 of the 34.4 MS/m
    of
    > > Al.
    >
    > I didn't have information on the conductivities of all of the
    materials when
    > I made up this library. The point of most of those materials is to
    get a
    > B-H curve for magnetostatic analyses. I've recently obtained some
    > manufacturer's data sheets on a lot of these materials, so I'll try
    to
    > update the library as soon as I can. The value that you used may be
    > reasonable for "cheap" iron. The conductivity of steel varies from
    about 2
    > MS/m for good silicon steel laminations to about 10 MS/m for rolled
    steel
    > that's not particularly intended for EM applications, with
    different types
    > of steel falling everywhere within this range.

    >
    > > 2. Can I model various phase-shifted currents by setting the real
    and
    > > imaginary parts of the current density? I'm using 0.72 MA/m2
    > > (resulting in a total peak value of 4000xsrt(2) Amps for the given
    > > conductors).
    >
    > Yes, that exactly right. However, some sort of subtle things
    happen when
    > you do this. The current density that you assign via the material
    > properties dialog is really just a _source_ current density--that
    is, the
    > current that would be there at 0 Hz. At frequency, eddy currents
    are
    > induced that knock down the total current carried in your
    conductor. To
    > actually fix the total current in a conductor in a harmonic
    problem, you
    > need to get ahold of the femm30 beta version (available at
    > http://www.egroups.com/files/femm/). You can then specify total
    currents
    > carried by sections of the model via the new "Circuit" properties.
    This is
    > what I've done in the attached examples.

    I've downloaded the beta version, but since it was accompanied by the
    old manual, I was unaware of this new feature.
    Furthermore, FEMM freezes if the frequency is set to 0 Hz in this
    example. Logically, even I couldn't interprete phase shifts at zero
    frequency:-)

    >
    > > 3. What boundary conditions to specify for this kind of problems?
    >
    > Well, the skin depth of your problem is really small. Skin depth =
    > Sqrt[1/(sigma*mu*Pi*freq)] where sigma is the conductivity in MS/m,
    mu is
    > the absolute permeability, and freq is the frequency in Hz. The
    result is in
    > meters. For the properties you described, the skin depth is about
    0.58 mm.
    > This means that the field doesn't get through to the outside part
    of the
    > iron, and it is sufficient to specify A=0 on the outside of the
    iron. This
    > is what I've done in the attached "viking1.fem" example. Note that
    in this
    > problem, I have defined a very thin region about 1 mm width the
    inner
    > surface of the iron that is finely meshed in order to capture the
    skin
    > effects in the iron in "enough" detail. The result is a mesh with
    a _lot_
    > of nodes that takes a long time to solve.

    This geometry comes awkwardly close to the one I've used. It must
    have been quite some work and calculation time.

    >
    > An alternative is to specify a "small skin depth" impedance boundary
    > condition on the _inside_ of the iron, making that the edge of your
    domain.
    > This greatly decreases the size of the problem, since you don't
    have to
    > worry about explicitly modeling the skin depth region in the
    enclosure--this
    > is taken care of by the boundary condition. The
    attached "viking2.fem"
    > takes this approach. There are two down sides to this approach--it
    is more
    > difficult to back out the losses in the enclosure. This requires a
    surface
    > integral of |A|^2 that the postprocessor doesn't currently perform,
    but
    > which you can do by exporting the profile of A along the surface.
    After you
    > do this integral, you need to multiply the result by (1/4) *
    (2*Pi*freq)^2
    > * sigma * (skin depth) to get total losses. Also, femm currently
    doesn't
    > allow you to define a hysteresis lag in the boundary material.
    I'll add
    > this in eventually, because it is really a small modification.

    this particular behaviour of this boundary condition and the
    inability to calculate the surface integral made me hesitate to use
    it. I even experimented with it on the outside of the conductors,
    with weird results.

    >
    > > 4. Along the inner surface of the enclosure, I've specified a
    mesh of
    > > 0.02 cm. Is this fine enough given the skin dept of slightly less
    > > than 1 mm? More nodes seems to crash this machine :-(
    >
    > Yes, this ought to be good enough. See answer to (2)
    >
    > > 5. Does anyone know a steel with a relative permeability of around
    > > 1600 with 'worse' hysteresis properties as the M-36 steel?
    > >
    > > 6. If |H| and |B| are known, then how do I estimate the resulting
    > > heat losess for this situation. I only have detailed curves for
    > > laminated transformer steels for |B| > 1 T, which of course are
    > > softer ferrites.
    >
    > You can obtain losses directly from the program with block
    integrals.
    > Switch to the block integral mode by cliking on the green square on
    the
    > femmview toolbar. Then, select the region of interest by clicking
    somewhere
    > inside it with the left mouse button. The area selected with light
    up in
    > green. When you've selected all the areas of interest, click the
    integral
    > sign on the toolbar. Then, choose the "total losses" integral. It
    will add
    > up all of the predicted hysteresis and eddy current losses.
    >
    > Note--to get hysteresis losses, you have to specify some nonzero
    number for
    > the phi_h parmameter in the material property dialog. This
    parameter
    > defines a frequency-independent phase lag between B and H in a
    material,
    > which is a relatively crude but easy to handle model of hysteresis
    effects.
    > To get really good results, you ought to do some tests on a toroid
    made out
    > of the material of interests to identify a parameter that makes the
    losses
    > matched the observed. Failing that, Stoll's "Analysis of Eddy
    Currents"
    > says that the hysteresis lag is "usually less than 20 degrees", so
    for a
    > cheap steel not specifically intended for magnetic applications, 20
    degrees
    > is probably a reasonable value to assume.

    OOOOOPS. This is the main problem for my unearthly results. As a
    matter of fact, I should have this curve somewhere.

    > Anyhow, the losses in the attached example of the example problems
    loss
    > numbers in the end sections average out to like 760 W/m^2 for the
    surface
    > impedance model w/out hysteresis, and more like 960 W/m^2 for the
    > brute-force one that also includes hysteresis losses. These losses
    are
    > highly dependent on the material parameters that you use. The
    brute-force
    > model also shows that the skin region in the enclosure is highly
    > saturated--however, the harmonic analyses are all linear, so you
    have to
    > take the results with a bit of a grain of salt....

    A somewhat accurate prediction of the B-values involved is good
    enough for me. Anyway, this test prototype should be saturated, so
    this calculations make sense.

    Thanks and keep up the good work!

    Erik Evertz

- References:
  - Re: [femm] hysteresis losses in enclosures
    - From: David Meeker <dmeeker@foster-miller.com>

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