A wind electric generator uses induction and permanent-magnet designs. There are many variations of these two, but in general terms, the induction generator must be spun at about 1,000 rpm or more to produce useful power. It produces current by first generating an electric field by passing alternating current (ac) through a coil. A series of these coils are mounted on a rotating structure (a rotor) that is turned by the wind through the drive train. Surrounding the rotor is a stationary series of coils, a stator. When the electric fields on the rotor pass coils on the stator, the field induces a current in the stator coils which is conducted away as output. Induction generators are not self-exciting. That means they require an external power supply to produce a magnetic flux or field.
PM or permanent magnet wind turbine generators use the high- field strength generated by magnets mounted on a rotor. Variations on this design put magnets on the stator and let the coils rotate. There are advantages to each.
The wind industry prefers magnets made of expensive rare-earth elements. They are worth the expense because the PM generator needs no external power source to initiate a magnetic field, an advantage for wind farms in remote locations. The self-excitation also means a bank of batteries or capacitors for other functions can be smaller.
Other plusses for PM generators are that the high- energy density eliminates some weight associated with copper windings, along with problems of degrading insulation and shorting. The design also reduces electrical losses.
On the downside, rare-earth magnets do not tolerate high temperatures. They can permanently lose magnetic field strength, which demands more from a generator’s cooling equipment. In addition, the cost of rare-earth permanent magnets is a concern because key raw materials are not available in significant quantities in the U.S. Should competition for PM materials increase, lead times and costs will increase.
Because gearboxes are expensive to maintain, wind turbine designers have been experimenting and commissioning turbines with drivetrains that have no gearboxes, which make PM generators essential. Still, the PM generator in such a design calls for a certain circumferential speed to function properly. This means the generator may be 5 to 6-m in diameter.
Filed Under: Generators