A permanent magnet generator for small scale wind and water turbines

This paper describes the design and development of a 5 kW, 150 rpm, air-cored, axial flux generator for use as a direct drive generator with small wind and water turbines. The generator uses trapezoidal shaped magnets to obtain a greater active length than that provided by circular magnets. The armature coils are also trapezoidal. A prototype generator has been tested and produces up to 5000 W at 150 rpm with an electrical efficiency substantially greater than 90%. The generator performs as predicted by the design process. The paper also describes how the generator can be used to control turbine speed in strong winds. This is achieved by introducing an additional resistive dump load on the electrical output of the generator; this stalls the turbine as it approaches rated speed. If the dumps load is applied and then released, the generator can be kept within its rated power limit. A vertical axis wind turbine and axial flux generator are modeled to examine the speed control system. It is found that when the turbine is subjected to a turbulent wind, the speed control mechanism performs as expected. Practical test has verified the simulation.