Optimized Permanent Magnet Generator Topologies for Direct-Drive Wind Turbines

The thesis deals with the issue of cost reduction in direct-drive generators for wind turbines. Today, the combination gearbox-medium-speed (1000-2000 rpm) induction generator largely dominates the market of MW-scale wind turbines. This is due to the lower costs of the gearbox option compared to the costs of gearless systems. Nevertheless, there is an acute interest among researchers and wind turbine suppliers in the possibility of removing gears and slip rings from the drive train, leading to lower maintenance (no oil is required and slip rings can be avoided) and higher reliability due to the absence of wear between gears. The direction followed by the thesis is the investigation and comparison of various permanent magnet (PM) machine topologies. The aim is to identify the topology(ies) with the lowest cost/torque and highest torque/ mass. A new TFPM geometry, called TFPM machine with toothed rotor, is derived in the. The new machine topology has the following characteristics: the stator is single-sided, the rotor PM and flux concentrators are independent from the build-up of mechanical tolerances, the installation of rotor parts (PM and flux concentrators) can be automated and the flux circulation allows laminated steel to be used in the stator core. A prototype of the TFPM machine with toothed rotor is presented. The comparison between TFPM machines with toothed rotor and conventional PM synchronous machines is discussed. Comparison of the cost/torque and torque/mass of the two machine topologies for diameters ranging between 0.5 m and 3.0 m showed favorable expected performances of the TFPM machine with toothed rotor for diameters of 0.5 m and 1.0 m. However, diameters larger than 1.0 m favored the conventional PM synchronous machine with/without flux-concentration. Using the results of the optimization process, the costs of active material are computed for a 1.5 MW wind turbine. It is found that active material represents about 5% of the total turbine cost, while previous estimates indicated that the generator costs are rather between 30% and 40% of a complete direct-drive wind turbine. Therefore, further optimiza¬tion of direct-drive machines should also include the costs of manufacturing and the costs of the mechanical structure.