Structural integrity of a direct-drive generator for a floating wind turbine

In this work, the suitability of a direct-drive radial flux permanent magnet generator is examined as a probable drive-train candidate for a stepped-spar floating wind turbine system that supports a 2MW downwind turbine. The suitability of the generator is assessed based on the structural integrity of its design (i.e., the stability of the air-gap between the rotor and stator) in response to the nacelle motions and its possible design implications on the overall system. Air gap deflections due to structural deflection and bearing tolerances were examined independently. The nacelle motions are obtained from experimental and numerical investigations on a 1:100 scale model. ANSYS suite is used to estimate the structural deformations of the generator and the changes in the air-gap distribution. Also, a simplified analytical model is used to compute the resulting changes in flux density and force distribution along the rotor periphery. The analytical model is also validated by 2D magneto-static simulations by utilising Finite Element Methods Magnetics software (FEMM).Preliminary results suggest that, if the nacelle accelerations are limited to 0.3 times the acceleration due to gravity (. g) and the motion response cycles are below the fatigue limit, the air-gap stability of the generator is more sensitive to magnetic forces. Contributions to air-gap eccentricity from shaft displacements can be limited if the bearing supports can be designed for high stiffness. This also confirmed the adequacy of the platform design. The results also emphasise the need for air-gap management when designing direct-drive generators for floating wind turbines. Two methods are investigated as potential solutions to limit the maximum air-gap deflection to 10% level. The method of increasing structural stiffness led to a structurally unfavourable design that could potentially affect the stability and resonance properties of the system. The method of increasing the design air-gap led to a structurally more favourable design, although this meant an increase in magnetic material and hence the costs. Thus, implementing direct-drive radial flux permanent magnet generators for floating wind turbines is challenged by the difficulty in achieving optimal weight and costs at acceptable performance without compromising the air-gap tolerances. There is a need for an amendment to design standards to recognise the design challenges of Floating wind turbines. © 2013 Elsevier Ltd.