3-D magnetostatic field calculation for a magnetic circuit with no (a narrow) air gap

Noting that 3-D magnetostatic field calculations for gapless magnetic circuits are strongly affected by the discretization, the author analyzes this effect for an iron torus using a boundary integral equation based on the surface magnetization current method. The main cause of computational error is the imperfect cancellation of the permeability-free terms in the boundary integral equation due to the improper size of the analytical integration region containing a singular point. A method of reducing the computational error is presented and verified to be valid