Power transfer in the air gap of linear generators for wave power

Abstract: An increased need for electric energy and share of renewable energy is important for a more sustainable future, to which wave power can contribute. Some concepts for wave power use linear generators, and the studies presented here focus on the power transfer and forces in the air gap of linear generators.Based on Poynting's theorem, analytical expressions were derived for how much power is transmitted in the air gap of linear electric machines. The equations were derived for flat linear generators as well as for cylindrical linear generators, and for both the radial and the longitudinal power flows. The radial power flow contributes to the electricity that can be taken from the generator, while the longitudinal contributes to unwanted effects such as vibration. The equations show that for flat linear electric machines, vibrations and other unwanted power transmissions can be reduced by increasing the number of poles in the machine, without decreasing the desired power transfer. Otherwise, the same factors that increase the desired power transfer also increase the unwanted.By studying the generator as a damping force and a lumped circuit, it was derived that the absorbed energy of a linear generator is quadratically dependent on the partial stator-translator overlap if the inductance of the generator is not affected by a partial overlap. Consequently, the forces in the air gap also depend quadratically on the relative overlap between the stator and the translator.Data were collected during two experiments in order to determine whether the quadratic dependence between partial stator-translator overlap and energy transfer could be seen experimentally. Linear regression analysis of the experimental data shows that the relationship is somewhere between linear and cubic, but did not clearly show a quadratic relationship. The inductance showed no dependence of the partial stator-translator overlap.