Finite element analysis of eddy current losses in steel laminations of inverter-fed electrical machines

Abstract

This thesis investigates the eddy currents and regarding losses in electrical steel sheets. For the purpose, two methods of numerical analysis of electromagnetic fields are developed. Both of those are based on the two-dimensional (2-D) finite element computation of the fields. Since the eddy currents of the sheets are, by definition, excluded from the 2-D formulation they need to be included separately. Within this work, the goal is achieved by modeling the eddy currents via a one-dimensional (1-D) penetration equation in association with the 2-D analysis. Formulations for both, general time variation and sinusoidal time variation are shown. The coupling of the 2-D and 1-D models is investigated from several perspectives. For representing the 1-D eddy current solution within the time-discretized analysis two approaches are considered. Numerous simulations carried out reveal that inappropriate coupling might not affect the losses to be obtained but other electromagnetic quantities such as input power or power factor. A proper coupling method that accords with the computation results is proposed. The combining of those two field formulations is discussed in the case of the time-harmonic approach as well, for which the concept of complex reluctivity is employed. The second objective of the work is to study in detail the losses in rotating electrical machines fed by a frequency converter. The higher harmonics associated with the frequency-converter supply are known to increase to losses in comparison with a sinusoidal voltage supply. According to the simulations, (i) the eddy current losses in the rotor sheets are the most sensitive to the distorted input voltage waveforms and (ii) coincidently can be the most reduced by choosing the switching frequency of the frequency converter suitably. The measurements carried out on the test machine and comparative studies of those and the simulation results verify the methods proposed.