Finite element analysis of the magnetic field distribution inside a rotating ferromagnetic bar

Finite element analysis techniques are being used widely to predict the field distribution in magnetic structures having complex boundary geometries and nonlinear B-H characteristics. In situations where two or more magnetic fields interact, care must be taken to ensure that the principle of superposition applies before adding the fields directly. The magnetic Reynolds number can be used to predict linearity, and in those cases where superposition is invalid, the finite element algorithm must be applied repeatedly in an incremental fashion to ensure accuracy. This procedure is illustrated by considering the magnetic field distribution inside a ferromagnetic bar rotating in a magnetic flux set up by a two pole structure. Results obtained by linearly superposing direct and generated cross axis fields at a particular speed of rotation are compared with the magnetic field distributions resulting from repeated application of the finite element algorithm for incremental step changes in angular velocity. Comparison of the neutral axis angle/speed characteristic for the linear and incremental models shows a significant difference between the two approaches.