a b s t r a c tThe inherent cogging torque component of the PM generators can cause problems at the start-up of wind turbines. To improve the operation of wind turbines, especially at low starting speeds, the wind generator cogging torque should be minimized. In this paper, an effective and practical approach is proposed for cogging torque reduction in the AFPM generators with yokeless and segmented armature (YASA). The proposed approach is based on the dividing magnets radially and shifting the magnet pieces peripherally with an appropriate shifting angle. Using a series of 3-D FEA simulations, the proposed approach is compared with the two conventional approaches, i.e. the magnet skewing approach and the selecting an appropriate magnet pole arc to pole pitch ratio, according to the cogging torque reduction and the negative impacts on the generator loadability. It is shown that using the proposed approach, the generator cogging torque can be greatly reduced (about 87%). Also, it is shown that compared with the other studied cogging torque reduction techniques, the proposed approach has less negative impact on the generator loadability. Some of the simulation results are verified using the experimental tests.

The multi-objective design of an axial-flux permanent magnet (PM) generator with the yokeless and segmented armature is done, for use in the small-scale direct-drive wind turbines. Using the multi-objective genetic algorithm technique, an optimisation process is performed for maximising the efficiency and minimising the cost, size, and mass of active materials, considering the practical constraints of the wind generator. The Pareto optimal fronts are used to find the best solutions. Also, a comparative study is performed to determine the most appropriate configuration among 12 configurations with the different combinations of poles and stator segments. Then, the selected optimal configuration is analysed using the three-dimensional (3D) finite element analysis (FEA). The inherent cogging torque of the PM wind generator affects the cut-in speed and start-up of the turbine. Thus, to improve the turbine performance, especially at low starting speeds, the generator cogging torque should be reduced to an acceptable level. In this study, using a new and practical approach, the peak-to-peak value of the cogging torque is reduced largely (81.4%), without notable reduction in the generator loadability. A prototype of the designed generator is fabricated and tested. The experimental results show good agreement with the 3D FEA simulation results.

Abstract:The inrush current is a transient current that results from a sudden change in the exciting voltage across a transformer's windings. It may cause inadvertent operation of the protective relay system and necessitate strengthening of the transformer's mechanical structure. Many methods were reported in the literatures for reduction and mitigation of transformer inrush currents. This paper represents a study of techniques that have been proposed for transformer inrush current mitigation. A new, simple and low cost technique to reduce inrush currents caused by transformer energization is presented here. In this method, a controlled switching approach with a grounding resistor connected to transformer neutral point and a magnetic flux shunt is used. By energizing each phase of the transformer in sequence, the neutral resistor behaves as a series-inserted resistor and thereby significantly reduces the inrush currents. The dimensions of the magnetic flux shunts are chosen such that the inrush current amplitude is further reduced. The proposed method has been tested by computer simulation using 2-D FEM (two-dimensional finite element method) by Maxwell software. The obtained results show that the proposed method is efficient in reduction of transformer inrush current and is much less expensive since there is only one resistor involved and the resistor carries only a small neutral current in steady-state.