Magnetic field analysis of asymmetrical machines by finite element method

Magnetic and electric asymmetry have been used by many electrical machine designers to develop useful devices. The main feature of such designs is the nonsinusoidal variation of the magnetic field with time. The effect of harmonics on the overall performance is normally studied analytically either by using the symmetrical components method or the rotating field concept. Both methods lead to a cascade type equivalent circuit which can be used to predict performance and as an aid to optimise the design. For accurate analysis, however, a detailed simulation of the magnetic field distribution is needed. This would normally require the time stepping finite element method. In this paper, a simple finite element technique to solve nonlinear time periodic magnetic field problems in asymmetrical electrical machines is proposed. The approach is based on a steady-state magnetodynamic formulation together with application of harmonic balance to the diffusion equation. The theory is verified on a special low speed generator with asymmetry on both stator and rotor