Calculating the External Magnetic Field From Permanent Magnets in Permanent-Magnet Motors—An Alternative Method

We present an alternative method for calculating the magnetic field from a set of permanent magnets in a permanent-magnet motor. The method uses a cylindrical coordinate system to model the geometry of the structure enclosing the magnets. A Fourier series expansion yields an alternative to the more familiar multipole expansion given in spherical coordinates. The expansion is developed by using Green's function in cylindrical coordinates. A technique called charge simulation allows computation of an equivalent point charge distribution. Finally, Coulomb's law is applied to express the magnetic scalar potential in a mathematically tractable form.