FEMM Manual - Finite Element Method Magnetics

2. The Mixed boundary condition can used to set the field intensity, H, that flows parallel
to a boundary. This is done by setting c0 to zero, and c1 to the desired value of H in
units of Amp/Meter. Note that this boundary condition can also be used to prescribe
∂A/∂n = 0 at the boundary. However, this is unnecessary–the 1 st order triangle finite
elements give a ∂A/∂n = 0 boundary condition by default.
• Strategic Dual Image This is sort of an “experimental” boundary condition that I have
found useful for my own purposes from time to time. This boundary condition mimics
an “open” boundary by solving the problem twice: once with a homogeneous Dirichlet
boundary condition on the SDI boundary, and once with a homogeneous Neumann condition
on the SDI boundary. The results from each run are then averaged to get the open boundary
result. This boundary condition should only be applied to the outer boundary of a circular
domain in 2-D planar problems. Through a method-of-images argument, it can be shown
that this approach yields the correct open-boundary result for problems with no iron (i.e just
currents or linear magnets with unit permeability in the solution region).
• Periodic This type of boundary condition is applied to either two segments or two arcs to
force the magnetic vector potential to be identical along each boundary. This sort of boundary
is useful in exploiting the symmetry inherent in some problems to reduce the size of
the domain which must be modeled. The domain merely needs to be periodic, as opposed to
obeying more restrictive A = 0 or ∂A/∂n = 0 line of symmetry conditions. Another useful application
of periodic boundary conditions is for the modeling of “open boundary” problems,
as discussed in Appendix A.3.3. Often, a periodic boundary is made up of several different
line or arc segments. A different periodic condition must be defined for each section of the
boundary, since each periodic BC can only be applied to a line or arc and a corresponding
line or arc on the remote periodic boundary.
• Antiperiodic The antiperiodic boundary condition is applied in a similar way as the periodic
boundary condition, but its effect is to force two boundaries to be the negative of one
another. This type of boundary is also typically used to reduce the domain which must be
modeled, e.g. so that an electric machine might be modeled for the purposes of a finite
element analysis with just one pole.
Materials Properties
The Block Property dialog box is used to specify the properties to be associated with block labels.
The properties specified in this dialog have to do with the material that the block is composed
of, as well as some attributes about how the material is put together (laminated). When a new
material property is added or an existing property modified, the Block Property dialog pictured
in Figure 2.9 appears.
As with Point and Boundary properties, the first step is to choose a descriptive name for the
material that is being described. Enter it in the Name edit box in lieu of “New Material.”
Next decide whether the material will have a linear or nonlinear B-H curve by selecting the
appropriate entry in the B-H Curve drop list.
If Linear B-H Relationship was selected from the drop list, the next group of Linear
Material Properties parameters will become enabled. FEMM allows you to specify different
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