Review of direct-drive radial flux wind turbine generator mechanical design


Johan N. Stander, Department of Mechanical and Mechatronic Engineering, Stellenbosch University, Private Bag X1, Matieland 7602 South Africa.



The direct-drive radial flux synchronous generator is considered as the modern wind turbine drive train. Both the electrically (e.g., Enercon) and permanent magnet (PM; e.g., Siemens) excited direct-drive generators are gaining popularity on the market today. Compared with the matured geared counterpart, the electrically excited direct-drive generator is heavier and more expensive but more reliable per unit capacity. The PM-excited generator is expensive, is simpler in electromechanical design, has a high power-to-weight ratio, and yields a higher energy conversion efficiency than its electrically excited equivalent. The PM generator technology has the potential to yield the highest energy-to-cost ratio. However, standardization of this direct-drive generator parts/subassemblies may overcome the existing cost barrier. Most current literature focuses on PM generator wind turbine technology, specifically on generator energy conversion optimization, and the scalability of technologies to capacities in access of 5 MW. Strangely, PM generator's mass and cost reductions through optimized structural design incorporating manufacturing, transportation, and installation constraints are less studied. This paper solely focuses on the mechanical and structural design aspects of large radial flux synchronous PM generators specific to direct-drive wind turbines. Generator topologies such as the common iron-cored and unconventional air-cored generator are discussed. However, design considerations specific to the iron-cored generator topology are studied. The design considerations investigated involve the geometries and the configurations of rotor/stator active and inactive structures, the interfaces, and the conductor/PM mounting methods. Copyright © 2011 John Wiley & Sons, Ltd.