Fractional-slot concentrated windings for offshore wind turbine generators: opportunities and challenges

Electrical machines equipped with fractional slot concentrated windings (FSCW) have shown great promise at low power levels to improve the electromagnetic performance of PM machines. Principally, they offer shorter end windings, greater manufacturing simplicity, and improved fault tolerance than distributed windings. At high power level, for example, MW level offshore wind power generators, these advantages potentially make them an attractive technology, especially as turbines are deployed further from shore and so require higher reliability. However, the increased winding simplicity leads to additional unwanted armature magneto-motive force (MMF) harmonics in the air-gap which lead to excess rotor and PM eddy current losses. Furthermore, studies have shown that FSCW machines suffer from an inherent lack of machine saliency, making them incompatible with sensorless control strategies presently employed in offshore wind turbine generators. The objective of this paper is to discuss various properties of FSCW and how they serve as benefits or challenges to their application in offshore wind power. Additionally, methods to mitigate the downsides of this winding structure are presented to assist in future work developing this technology. Finally, a summary of existing work on the use of FSCW in offshore wind power is discussed.