Analysis of train lighting brushless alternator with skewed rotor for harmonic minimization

Harmonics in generated voltage waveforms of brushless alternators of inductor type, used in railway coaches are often the cause for excessive heating and tooth tip saturation and compel de-rating of such machines. This paper discusses a novel technique to overcome this problem at the design level itself, by predetermining the generated voltage waveform and analyzing it for the harmonic content for improving the output power quality. Analysis of three-dimensional feature like rotor slot skew is performed with two-dimensional electromagnetic field formulation, which results in significant reduction in computational effort. A modified time stepping finite element method is proposed for shape optimisation of certain design parameters and a multislice technique is employed to take into account the axial variations of the field, introduced by rotor skew. Comparisons of computational and experimental results obtained from a 4.5 kW alternator, confirm the great potential and usefulness of the proposed methodology