Design and optimization of high torque density generator for direct drive wind turbine applications

Zeinali, Reza
In this thesis, it is aimed to design a high torque density generator for a variable speed, direct drive wind turbine application. Such a generator may reduce the size of the turbine tower and the nacelle and may provide cost advantage. For this purpose, various topologies of the permanent magnet machines in the literature are reviewed. Among the reviewed electrical machines, a magnetically geared machine introduced as a concept with high torque density and high power factor is chosen to be evaluated for the desired application. First, chosen machine is modeled using combination of analytic and Finite Element methods. Finite Element method is utilized to estimate average value of the air gap flux density and analytic method is used for calculating main dimensions and geometrical parameters of the generator. Furthermore, existing analysis methods for this type of machine performance is modified as necessary, to calculate the generator performance including its losses and efficiency. In order to achieve the highest possible torque density and minimize generator mass, an optimization procedure is developed for the proposed design process. The developed model is used to optimize the generator for a 50 kW, 60 rpm wind turbine application. A conventional surface-mounted Radial Flux Permanent Magnet (RFPM) generator is also designed and optimized for the same application as a reference of comparison to understand whether any advantage can be obtained using magnetically geared generator. The magnetically geared and RFPM generators are optimized in the terms of their active materials mass, first. The optimization results reveal that the active materials mass is not the best objective function for comparing relative merits of the two types of generators as the frame contributes significantly to the overall mass of generators. An analytic model is presented to design structural geometry and obtain structural mass of both types of generators. Next the structural mass of both types of generators is taken into account in the optimization procedure to identify relative advantage of each type. The results indicate that by using proposed magnetically geared generator the total mass of the generator of a direct drive wind turbine system can be reduced by half. However, the magnetically geared generator has lower power factor implying that current rating of the power converter is increased. As a consequence while the generator cost is reduced the cost of the converter increases.  
Citation Formats
R. Zeinali, “Design and optimization of high torque density generator for direct drive wind turbine applications,” M.S. - Master of Science, Middle East Technical University, 2016.