In the Air Gap of Linear Generators for Wave Power

Författare: Anna E. Frost; Mats Leijon; Sandra Eriksson; Irina Temiz; John Ringwood; Uppsala Universitet; []

Nyckelord: ENGINEERING AND TECHNOLOGY; TEKNIK OCH TEKNOLOGIER; ENGINEERING AND TECHNOLOGY; TEKNIK OCH TEKNOLOGIER; TEKNIK OCH TEKNOLOGIER; TEKNIK OCH TEKNOLOGIER; ENGINEERING AND TECHNOLOGY; ENGINEERING AND TECHNOLOGY; Linear generator; Wave Power; Poynting s theorem; Partial stator overlap; Permanent-magnet synchronous generator PMSG ; Linear electrical machines; Engineering Science with specialization in Science of Electricity; Teknisk fysik med inriktning mot elektricitetslära;

Sammanfattning: Wave power conversion is one type of renewable electricity generation. Within wave power, there are many different concepts, whereof some utilizes linear generators for converting the energy in the ocean waves into electricity. A linear generator consists of a translator, which is moving and have magnets of alternating polarity, and a stator, which have conductors sur-rounded by laminated steel. Between the translator and stator is an air gap, which is only a few millimeters wide. All linear generators for wave power, to the author’s knowledge, are permanent-magnet synchronous generators. This thesis looks into the forces and power flow in the air gap of linear generators for wave power, with the purpose of improving their future performance. The studies have focused on permanent magnet synchronous generators for wave power, but several of the results should also be applicable for other applications of linear elec-trical machines.Depending on the design of the linear generators, the translator can move so long that it only partially overlap the stator. This is common among several different wave power concepts with linear generators. When the stator is only partially overlapped by the stator it is denoted as partial stator overlap. It is studied how partial stator overlap affects the generated electric-ity, the absorbed energy, and the tangential and normal force in the air gap. The generated electricity and absorbed energy of a linear generator are quadratically dependent on the partial stator-translator overlap is shown through Faraday’s law and simulations. Experimental data showed that the absorbed energy is both linearly and quadratic depending on partial stator over-lap, where the linear dependence is at least partially due to frictional losses. Simulated results confirm that voltage is linearly dependent on partial stator overlap, which means quadratic de-pendence between generated electric and partial stator overlap. The simulated forces showed a linear dependence.Decades ago, the Poynting vector was used to derive an expression for the power flow in the air gap of rotating electrical machines. In this thesis the equivalent expressions for both flat and tubular linear electrical machines were derived. The analytical results were also compared with results from simulations. Both the analytical expressions and simulations showed that tubular and flat linear electrical machines have slightly different behavior.

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