Design of an axial flux permanent magnet wind power generator

Direct driven variable speed permanent magnet wind power generators (PMWG) have received an increasing amount of attention. The reason for this is the reduced cost of the produced electric power made possible by the elimination of the gearbox and by the use of variable speed increasing the energy capture. Traditionally the gearbox is required to increase the low rotational speed of the turbine (typically 20-40 rpm) up to a speed suitable for a common 4-pole generator (1500 rpm). The removal of the gearbox increases system availability and reduces its weight, losses and the need for maintenance. However, the low rotational speed calls for a generator with a very large number of poles. Furthermore, the generator must naturally be efficient and cost competitive. Due to the variable speed scheme a frequency converter is required to supply the power to a grid. The requirement of a large pole number can be met with permanent magnets which allow small pole pitch. A simple and effective generator structure is realized by the disc type axial flux configuration, the active parts of which are presented. The stator is a toroid wound from iron tape accommodating rectangular coils forming an air gap winding. Rotor discs with attached permanent magnets reside on both sides of the stator. In this paper optimum design based on minimizing the sum of investment and energy loss costs for a 100 kW prototype generator is searched. The investment costs cover only the cost of the active part material while the manufacturing and structural costs are assumed to be constant over the dimensional range studied.