Electromagnetic forces in the air gap of a permanent magnet linear generator at no load

The basis for the work is the slow speed energy conversion of ocean wave energy into electricity using a direct-drive three-phase permanent magnetized linear generator. One of several important issues is the normal forces in the air gap, which is critical when designing the support structure of the generator. The electromagnetic forces in the air gap have been analyzed using Maxwell stress tensor method implemented in a two- dimensional finite element code. Simplified analytic calculations are made in order to validate the results from the extensive computer calculations. The normal electromagnetic forces in the air gap, Fδ, are analyzed for a two-sided linear generator at no load. An unstable condition of the global force on the piston occurs due to the fast increasing normal force as the air gap width decreases. A horizontal displacement of the piston from a neutral position with 3mm air gap on both sides produces a resulting horizontal force on the piston, increasing with the displacement. A displacement of 1mm gives a resulting horizontal force on the piston of 5.5kN per pole and meter of core length, which is increased to 9kN per pole and meter of core length for a displacement of 1.5mm. Furthermore, the normal force varies due to cogging as the piston moves vertically. At a constant air gap width of 3mm the normal forces per pole are varying between 9.9 and 11.3kN∕m of core length as the piston is moving from one pole to the next.