On Design and Analysis of a Novel Transverse Flux Generator for Direct-driven Wind Application

University dissertation from Stockholm : KTH

Abstract: This thesis deals with the analysis of a permanent magnet synchronous generator suited for direct-drivenwind turbines inmegawatt class. The higher specific torque and power density of a transverse flux permanent magnet machine in comparison to conventional radial-flux machines make it a promising solution for direct-driven wind turbine generators. The novel transverse flux generator investigated in this work would allow a better utilization of the available nacelle space due to its more compact construction.The major part of the thesis deals with the finite element analysis and analytical calculations of transverse flux generators. The computations are performed for single units of the basic transverse flux topology (BTFM) and the one utilizing iron bridges (IBTFM). As the selection of the pole length in a transverse flux machine affects the pole-to-pole flux leakage and thus its performance, the topologies have been analyzed with respect to the varying dimensions in the direction of movement. The topologies utilizing IBTFM have been found to be superior to the BTFM with respect to the flux linkage (by 110%) and utilization of the magnets (by 84%). The machines with longest magnets gave the largest flux linkage, while machines with short magnets should be preferred for better magnet utilization. The four sets of dimensions have been selected for a dynamic finite element analysis.The power factor is evaluated for the topologies with the varying dimensions in the peripheral plane in static finite element analysis. The performance of the topologies with the best power factor in the studied range (0.62 in the BTFM and 0.57 in the IBTFM), as well as the topologies that give the highest power factor to magnet volume ratio, is compared with the dynamic simulations.The electromagnetic and cogging forces of the transverse-flux generator are estimated. The IBTFM is superior to the BTFM with respect to the force production, where the three-phase electromagnetic force is twice as large as in the BTFM. The force ripples of the three-phase electromagnetic force are found to be insignificant in both topologies.An analytical procedure based on the results from the finite element simulations is applied for evaluation of the transverse flux generators with different shapes and topologies. The effectiveness of each topology is investigated based on the estimation of the torque production in a certain nacelle volume. A toroidal generator with the iron-bridge topology is the most compact alternativefor a wind turbine as it has the highest torque-per-volume ratio.Furthermore, the analyticalmodel, including evaluation of the synchronous inductance, is developed and compared with the results obtained in the threedimensional finite element analysis. Themodel provides a good agreement for the studied set of dimensions.