Investigating the effect of variation of coefficient of friction on electrical machines designed for wave energy

Wave energy is a currently untapped source of renewable electricity, where the technology to convert this energy into a useable form is still currently in its infancy. It is, however, being developed at a rapid rate and worldwide a number of pre-commercial full-scale devices have been demonstrated. Due to the inherent low speed high force nature of ocean waves, in many wave energy converters the optimal electrical machine requires a high torque and to operate at a low speed. These requirements naturally favour a large radius and generator topologies that exploit magnetic gearing to allow for direct drive power take off. The bearing friction losses in these larger machines can be a significant portion of losses and can have an influence on the optimal machine geometry, such as the shape and size of the stator face. Bearing degradation increases the coefficient of friction, causing both a direct reduction in the output of the system and the also machine face to no longer be the optimal design for the operating conditions. There is the potential to alter the design of the machine to reduce the effect of degraded bearings. By varying the assumed frication coefficient during the electrical machine design optimisations, the importance of this effect on lifetime performance can be investigated and used to inform maintenance schedules in this new and expanding industry. It is shown that optimising for the correct value of friction coefficient can improve overall efficiency by 2%.