Additive Manufacturing: A New Paradigm for the Next Generation of High-Power-Density Direct-Drive Electric Generators

In recent years, there has been a growing demand for highpower-density direct-drive generators in the wind industry owing to their high reliability, torque per unit volume, and conversion efficiencies. However, direct-drive wind turbine generators are very large, low-speed electric machines, which pose remarkable design and manufacturing issues that challenge theirupscaling potential and cost of implementation. With air-gaptolerance as the main design driver, the need for high stiffnessshifts the focus toward support-structure design that forms a significant portion of the generator's total mass. Existing manufacturing processes allow the use of segmented-steel-weldmentdisk or spoke-arm assemblies that yield stiffer structures per unitmass but tend to be heavier and more expensive to build. Asa result, there is a need for a transformative approach to realize lightweight designs that can also facilitate series production at competitive costs. Inspired by recent developments inmetal additive manufacturing (AM), we explore a new freedomin the structural design space with a high potential for weightsavings in direct-drive generators. This includes the feasibility of using nonconventional complex geometries, such as latticebased structures as structurally efficient options. Powder-binderjetting of a sand-cast mold was identified as the most feasibleAM technology to produce large-scale generator rotor structureswith complex geometry. A parametric optimization study wasperformed and optimized results within deformation and massconstraints were found for each design. The response to the maximum Maxwell stress due to unbalanced magnetic pull was alsoexplored for each design. Further, a topology optimization wasapplied for each parameter-optimized design to validate resultsand provide insights into further mass reduction. These novel designs catered for AM are compared in both deflection and massto conventional rotor designs using NREL's systems engineeringdesign tool, GeneratorSE. The optimized lattice design with a Ubeam truss resulted in a 24% reduction in structural mass of therotor and 60% reduction in radial deflection. It is demonstratedthat additive manufacturing shifts the focus from manufacturability constraints toward lower mass.