Design of Direct-driven Permanent-magnet Generators for Wind Turbines
Doktorsavhandling, 1996

This thesis presents an investigation of how a direct-driven wind turbine generator should be designed and how small and efficient such a generator will be. Advantages and disadvantages of various types of direct-driven wind turbine generators are discussed, and a radial-flux permanent-magnet generator connected to a forced-commutated rectifier is chosen for a detailed theoretical investigation. Further, a design method is developed for the electromagnetic part of the chosen generator type. The generator is optimized with a simplified cost function which, besides including the cost of the active generator parts and the cost of the structure, also includes the cost of the average losses. Therefore, a method to calculate the average losses is derived. The design method is used to investigate the optimization of a 500 kW generator, and the size, efficiency and active weight of optimized generators from 30 kW to 3 MW are presented. A result of the investigation is that the outer diameters of the direct-driven generators are only slightly larger than the width of conventional wind energy converter nacelles. A comparison of average efficiency shows that direct-driven generators, including the losses in the frequency converters, are more efficient than conventional wind energy converter drive trains. Compared with other direct-driven generators, the proposed generator type is small, mainly because of the forced-commutated rectifier and because the generator is not required to produce a pull-out torque higher than the rated torque.


direct drive

synchronous generator

wind turbine


low speed

permanent magnet

frequency converter


Anders Grauers

Institutionen för elkraftteknik, Elmaskinteknik och kraftelektronik


Elektroteknik och elektronik

Technical report - School of Electrical and Computer Engineering, Chalmers University of Technology, Göteborg, Sweden: 292