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Permanent magnets have been used to provide the magnetic field in small to medium-sized generators for many years. The use of these in large renewable energy (RE)generators may offer performance advantages for future utility-scale generators.
Large (MW-sized) permanent magnet generators (PMGs) are mainly found in the wind turbine sector today but are not confined to this area. PMG offers a number of advantages to the generator field and is being used in applications from standby plants to hydro generators. The PMG is finding particular applications in renewable energy systems, where reduced size and higher efficiency give an advantage. The ability to find applications in both low-speed and high-speed generators is accelerating the use of this type of equipment.
A PMG can be a direct current machine with a rotary collector and brushes or an alternating current synchronous multiphase machine where the rotor and the stator magnetic fields are rotating at a similar speed. This removes the excitation losses in the rotor, which generally amount to between 20 and 30% of the collective generator losses. The reduced losses additionally give a lower temperature rise, which essentially means that a simpler and smaller cooling system can also be used in the generator.
