General trend on wind energy industry is to pursue higher and more constant winds to achieve higher figures of energy production. In that line, on the last years most companies in the industry have started operating at offshore locations. However, maintenance has become a delicate issue in those sites since locations are chosen precisely where weather conditions are harsh. One of the more maintenance-demanding elements of an electric machine is the gearbox, and that is why the industry is trying to avoid use of such component with hybrid and direct-drive generators. Nevertheless, cost of energy still favors traditional geared generators of inland machines. This document contains a study on the vibration level on the generator due to resonance, that can lead to a significant weight reduction and, as a consequence, a change on the tendency of the cost of energy parameter in relation to direct driven permanent magnet generators.