GE has incorporated a 6-meter diameter permanent magnet generator in its 4 MW direct drive units. The two main bearings transfer axial and bending loads from the rotor to the bedplate for a higher reliability than the previous generator.
Siemens has also made a change in regards to the generator. In the past, direct-drive machine rotors have been located inside the system. Siemens is now inverting the machine so the rotor is on the outside. As a benchmark, Siemens plans to release its 6 MW direct-drive prototype later this year, which will be one of the largest permanent magnet machines ever built. It could be a game-changing machine when it becomes available in bulk numbers four to five years from now. The 6 MW machine is expected to be popular with offshore wind farms due to its robustness and lower-maintenance requirements.
The direct drive machines were designed for a 20-year lifespan and to be serviced twice a year, most older models are serviced annually. (Power Gen Worldwide, 3/1/2011)