Magnet topology optimization to reduce harmonics in high speed axial flux generators

Despite the wide-ranging application potentials high-speed permanent magnet generators can offer the literature reveals a scarcity of design studies on these machines. This paper, we concerns with the configuration of PM on the quality of induced voltage in an ironless high-speed Axial Flux PM generator (AFPMG). The generator is a 240V eight-pole disc type machine with rated output of 18kVA at 50,000 rpm. It comprises of two rotor discs, each with eight embedded PM poles, and an ironless stator winding positioned between the rotors. A semi-circular shape is chosen for PM rotor poles, due to the ease of machining and assembly over other possible alternatives, and also their ability to produce near sinusoidal waveforms with various stator-winding shapes relatively easily. The investigation focuses on the effects on the harmonic content of the induced voltage due to varying distance between adjacent magnets. An efficient 3D finite element model (FE) that exploits the symmetry of machine is constructed for the proposed generator. The model then generates the predicted induced back emf over a range of distance between adjacent magnets, while maintaining the volume of the magnets constant. A prototype with the semi-circular magnets was constructed to provide experimental validation to the FE model. The validated model forms the basis for further formulation of design rules, and in particular, on the study of harmonics minimization.