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Optimal Design of a 5-MW Double-Stator Single-Rotor PMSG for Offshore Direct Drive Wind Turbines

This article investigates the optimal design of a double-stator single-rotor permanent magnet synchronous generator (DSSR PMSG) with simulation validation. This scheme of generator is obtained by mechanically combining an interior rotor PMSG and an exterior rotor PMSG. Design of a 5-MW radial flux DSSR surface mounted PMSG is developed with the second generation double layered fractional slot concentrated winding, which reduces large initial investment, size and weight, in contrast to the usual massive direct drive generators for wind turbines. Considering analytically calculated performance of the generator, the genetic algorithm (GA) is applied to minimize weight and cost of the machine with adequate efficiency. Impact of grade selection of PM is analyzed to optimize cost through GA. Moreover, to show advantages of the proposed strategy, methodically computed magnetic flux density distribution in air gaps, synchronous inductance of windings, electromagnetic torque, copper losses, and core losses are compared with the finite element method (FEM) results, and they reveal satisfactory enhanced solutions. Economy of the optimal DSSR PMSG is assured by comparing its total size, weight, and cost with optimized surface mounted inner rotor PMSG and outer rotor PMSG of comparable rated power and rated speed. Furthermore, issue of high cogging torque of this unique topology is resolved by optimizing the relative positioning of both stators’ slots using FEM.