1. INTRODUCTION The conventional electromechanical converter of a wind turbine is composed of a gearbox and an induction generator. A second alternative is to use a direct-drive generator with an AC/AC converter. The direct-drive solution eliminates the maintenance associated to the gearbox, and possibly increases the system reliability. The direct-drive system also provides a fully-variable speed system, which increases the power extracted from the wind, and increases the grid stability. Variable-speed can also be achieved with the gearbox system, if its induction generator is doubly-fed. Then, the induction generator must have a wound rotor connected to an AC/AC converter. The doubly-fed induction generator requires slip rings, which must be regularly inspected. The direct-drive solution eliminates both the inspection of slip rings, and the maintenance of the gearbox. Even though many advantages arise from the use of a direct-drive system, the greatest majority of wind turbines sold today still contain a gearbox system. This can be explained by the cost of the actual direct-drive systems. A study presented in [1] indicates that direct- drive systems using a wound-rotor synchronous machine are substantially more expensive than the geared system. Discussions with a few wind turbine manufacturers also confirmed this idea, that direct-drive is still more expensive than the geared-drive system. If a breakdown of the costs of a direct-drive wind turbine is made, it appears that the generator represents a significant part of the turbine cost. In [1], the wound- rotor synchronous generator studied represents 36% of the total turbine cost, with a rated power of 1.5 MW. The study of [1] is done with a wound-rotor synchronous generator. However, there is a possibility that other machine configurations could bring a lower generator cost. An interesting direction is the replacement of the rotor windings in the synchronous generator by permanent magnets. A study [2] shows that the mass of a radial-flux permanent magnets generator may be 2 times lower than that of a wound-rotor synchronous generator of the same diameter. The costs are also substantially reduced. With respect to permanent magnet generators, some people in the wind turbine industry seem to think that further gains can be obtained if axially-oriented magnets are used in the generator, instead of radially-oriented magnets. In this paper, we compare the Axial-Flux Permanent Magnet (AFPM) machine with slots to the Radial-Flux Permanent Magnet (RFPM) machine. The study investigates if any substantial cost benefits can be obtained if the AFPM machine is used, instead of the RFPM machine. This comparison is made on the basis of the total cost of active material divided by the rated torque of the machine. The designs of both the RFPM and AFPM machines are optimized, and the results show that RFPM generators still have a lower cost/torque than the AFPM machines with slots. 2. AXIAL AND RADIAL-FLUX PERMANENT MAGNET MACHINES Permanent magnet machines allow a great deal of flexibility in their geometry. Among other possibilities, the permanent magnets on the rotor, can be either radially or axially oriented. R li wo lo R R Figure 1: AFPM machine with slots Figure 1 illustrates an AFPM machine with slots. It must be noted that the axial-flux machine can also be built without slots. The result is a TORUS machine, which has the advantage of easy construction, but the disadvantage of large air gaps and thick magnets. The resulting machine design appears to be about 2 times more costly than the RFPM machine of equivalent diameter [3]. In this paper, the TORUS machine is not analyzed further. Only the AFPM machine with slots will be considered. The AFPM machine with slots is compared to the RFPM machine, which also has slots. The RFPM machine is shown in fig. 2. AXIAL AND RADIAL-FLUX PERMANENT MAGNET GENERATORS FOR DIRECT-DRIVE WIND TURBINES. M.R. Dubois, H. Polinder, J. A. Ferreira Delft University of Technology, Lab. Of Electrical Power Processing Mekelweg 4, 2628 CD, Delft, The Netherlands m.dubois@its.tudelft.nl, h.polinder@its.tudelft.nl , j.a.ferreira@its.tudelft.nl ABSTRACT: The application of permanent magnet machines as direct-drive generators for wind turbines is considered. The Axial-Flux Permanent-Magnet (AFPM) machine with slots is compared to the Radial-Flux Permanent-Magnet (RFPM) machine, for this application. Using a computer-based design model, the two machine types are optimized with respect to the lowest Cost/Torque. Design optimization for maximum Torque/Volume is also investigated. The calculations show that the RFPM machine has lower Cost/Torque than the AFPM machine with slots, and lower torque/volume than the AFPM machine with slots. Keywords: Generators, Direct Drive, Cost Analyses.