[FEMM] Solid block vs laminations - does not compare

["robert Macy" <macy@xxxxxxxxxxxxxx>]

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?  

                          - Robert -