Unofficial
FEMM
documentation
by Dr Stan Zurek

In FEMM:

If no boundary conditions are explicitly defined, each boundary defaults to a homogeneous Neumann boundary condition. However, a non-derivative boundary condition must be defined somewhere (or the potential must be defined at one reference point in the domain) so that the problem has a unique solution.
For axisymmetric magnetic problems, A = 0 is enforced on the line r = 0. In this case, a valid solution can be obtained without explicitly defining any boundary conditions, as long as part of the boundary of the problem lies along r = 0. This is not the case for electrostatic problems, however. For electrostatic problems, it is valid to have a solution with a non-zero potential along r = 0.

Symmetry of the geometry can be exploited by by using the Dirichlet or Neumann conditions:

• Dirichlet - the condition of A=0 is enforced along the boundary which stops the flux crossing that boundary. It can be thought of as a perfect insulator.
• Neumann - the condition of dA/dn = 0 which means that the flux lines will be forced to cross this boundary at 90 deg angle. It can be though of as a perfect conductor.

Comparison of symmetrical models in FEMM, N = number of turns, N/2 = half of turns
	Fig. 1. Full model, no symmetry, calculated L = 0.117278 H, By(0,0) = 7.02993e-5 T
	Fig. 2. Horizontal symmetry, calculated L = 0.05875 H (so L * 2 = 0.1175 H), By(0,0) = 7.04286e-5 T
	Fig. 3. Vertical symmetry, calculated L = 0.0587631 H (so L * 2 = 0.1175262 H), By(0,0) = 7.04353e-5 T
	Fig. 4. Horizontal and vertical symmetry, calculated L = 0.0294388 H (so L * 4 = 0.1177552), By(0,0) = 7.05736e-5 T
	Fig. 5. Visual comparison of all 4 cases. It should be noted that the actual flux density distribution is the same - as can be judged by the coloured map. The difference is