Re: [FEMM] Solid block vs laminations - does not compare

["robert Macy" <macy@xxxxxxxxxxxxxx>]

Thank you for responding.  

I'm having difficulty getting femm to model an inductor
whose inductance is dropping off at a frequency lower than
expected.  The inductance also drops at half the slope vs
fequency as expected. 

Any ideas of where to adjust my model?

              - Robert -


On Sat, 29 Nov 2003 23:26:23 -0500
 David Meeker <dmeeker@xxxxxxxx> wrote:
> The bulk lamination model only applies when the
> lamination is thin enough so that all of the eddy
> currents are flowing in the plain of the lamination.  The
> eddy current induced in the "usual" direction (i.e. into
> and out of the screen) can be neglected if the
> laminations are thin. In the lamination model, the flux
> tends to get confined to a thin layer adjacent to the
> insulation between laminations, and flux in any skin
> region near the interface of the laminated region with
> air is neglected.  Conversely, a solid block model
> assumes that all of the current is flowing in the into
> and out of the page direction, neglecting the impedance
> of currents that must travel in the plane of the
> simulation to enforce current constraints.  In the solid
> block case, all of the flux flows in a thin skin regions
> near the surface of the material (i.e. the mode which is
> neglected in the bulk lamination model).  If you use the
> solid block model to model sections that are physically
> short in the into-the-page direction, or if you use the
> bulk lamination model to model sections that are thick
> relative to the other dimensions of the problem, you'll
> get nonsense.  Solid and laminated results don't converge
> in the "nonsense" region, because they are making
> different assumptions as to where the eddy currents and
> flux are flowing.
> 
> robert Macy wrote:
> 
> >I really need help on this problem.  Trying to
> understand
> >how femm characterizes eddy current losses in an
> inductor.
> > 
> >
> >The results of the the following analyses suggest that
> it's
> >better to use a solid block than large laminations.
> > Somehow, that does not seem intuitively correct.
> >
> >
> >First, use a huge slab of magnetic material
> >  femm predicts fairly good inductance with fairly high
> >  losses from eddy currents.
> >
> >  BUT! the block is not an infinite length so set the 
> >  core's current to a circuit value of zero.
> >
> >  femm then predicts better inductance and lower eddy 
> >  current losses. 
> >
> >Second, use laminations to simulate limited block length
> >  Make the core's current go zero due to laminations,
> but
> >  make the laminations as thick as the block, or half
> >  the block.  
> >
> >  BUT! the effective permeability has gone down to 11 !!
> >
> >  femm predicts a 50 times drop in inductance!
> >
> >
> >For laminations femm applies Stoll's equation (as shown
> in
> >the femm manual) to predict a "new" effective
> permeability
> >versus frequency versus lamination thickness.
> >
> >Problem is that the difference between analyses using
> >laminations and using block material are *WILDLY*
> >different!
> >
> >As femm uses Stoll's equation, once the laminations are
> >more than 3 skin depths thick; the effective
> permeability
> >becomes simply divided by the ratio of lamination
> thickness
> >to skin depth at an angle of 45 degrees.  It is possible
> to
> >drive the effective permeability arbitrarily to zilch.
>  It
> >seems it should have converged to solid block
> permeability.
> > 
> >
> >
> >Question:  How do you correlate femm's solutions using
> >Stoll's equation for effective permeability to solutions
> >one gets using giant blocks?  
> 
> 
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