Design, Optimization, Construction and Test of Rare-Earth Permanent-Magnet Electrical Machines with New Topology for Wind Energy Applications

This thesis presents design, optimization, construction and test of radial-flux low-speed rare-earth high-energy permanent-magnet (PM) electrical machines with new topology. Two of the designed machines can be used as directly driven wind energy generators. Due to absence of the field current and field winding, permanent magnet generators exhibit high efficiency in operation, simple and robust structure in construction and high power to weight ratio. The attractiveness of the permanent magnet generators is further enhanced by the availability of high-energy permanent magnet materials like NdFeB. Based on the equivalent magnetic circuit approach and permanent magnet load line characteristics, iterative preliminary design for the proposed generator was firstly carried out. The aim of this simple linear pre-design tool was to have an initial geometry for the detailed investigations of the finite element technique (FET) where the electromagnetic behavior of the machine was optimized. The stator of the machine was slotted. Slotted configuration was chosen as it permits lower effective air gap length and therefore thin magnets can be used. This, in fact, largely decreases the cost of active material as it is dominated by that of the magnets. However, cogging torque comes as a consequence of slots. The cogging torque of the machine was estimated using the flux-MMF technique enhanced by the FET. Cogging torque is an oscillatory torque caused by the variation of the co-energy of the permanent magnets with respect to the rotor position, resulting from the variation of the magnetic circuit seen by the magnets. It is an inherent characteristic of slotted permanent magnet machines. It should be studied and minimized in the applications where minimizing torque ripple, vibration and noise are essential requirements. The configuration of the rotor corresponding to the lowest cogging torque was adopted. The prototype machine was constructed using novel updated high-energy permanent magnets with a remanent of 1.41T. The stator laminations of the machine were cut using laser technology methods. The slots of the machine were flat and the windings were of toroidal type (torus) with short ends. This, in turn, reduced the cost and weight of active material and improved the efficiency. The permanent magnets were rectangular blocks of NdFeB with flux concentration arrangement and magnetized tangentially on the rotor support structure. Soft magnetic material is attached to both poles of the permanent magnets, which not only produced an easy path for the flux penetration but also reduced the leakage flux and therefore a typical ‘rule of thumb’ value for the leakage flux coefficient was achieved. The rotor support structure of the machine was manufactured from nonmagnetic light material (Aluminum). To further reduce the total weight of the machine, longitudinal holes were excavated inside the rotor support structure. The constructed prototype machine was tested as variable low-speed generator with different loading conditions. Good agreement between the theoretically predicted and experimentally obtained results has been achieved. The inertia of the system together with the cogging torque necessitated a peak starting torque of 64% of the rated torque. To reduce this starting torque, the slot openings were filled up with bonded soft magnetic material with a relative permeability r μ =10. The peak value of the starting torque has been reduced to 43% of the rated torque. The cogging torque of the machine was estimated again and the performance of the slot-filled machine was studied and compared with the previous case. The manufactured prototype machine was single phase. Three-phase is also possible. The number of phases is usually determined based on the type of the load and the rated power demand. Finite element analysis and theoretical study were carried out on two threephase machines of the same topology. The first one is a four-pole three-slot machine and the second one is ten-pole six-slot design.