Permanent magnet fixation concepts for linear generator

Abstract- Lately, the field of linear generators for wave energy conversion has attracted large attention, [1], [2]. One encouraging way to convert wave energy into electrical through direct drive is by using a linear, synchronous, longitudinal flux permanent magnet (PM) generator where the rotor piston is driven by, for example, a point absorber. As prices decrease, it has only recently become possible to use high remanence NdFeB magnets in technical applications. This opens up for new possibilities for linear generators, as high magnetic excitation can be achieved with smaller magnets. In the present work two concepts of magnet fixation, surface mounted magnets and magnets buried between pole shoes have been studied with NdFeB and ferrite magnets. While the former alternative demands stronger magnets, which are more difficult to handle, the latter yields a somewhat heavier rotor piston. Pole shoes provides better possibilities to shape the magnetic flux curve in the periphery of the air gap, and the magnets are more protected from transients. The surface mounted concept allows for a more efficient use of the magnets, as the leakage is lower than when the pole shoe approach is used. This means that it is possible to reduce the length of the rotor with surface mounted magnets. The different configurations are compared with respect to total magnet mass, efficiency and load angle. Thorough analysis of the generators has been carried out using full physics simulations using a finite element method (FEM) of the time dependent magnetic field. Magnetic intensity distributions, time dependent currents and voltages are also among the parameters that have been studied