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Windtech: Generator inside out - Siemens direct drive prototype

Siemens Energy took just 18 months to establish that removing the gearbox from a wind turbine is a viable option for commercial operation.

"We are now confident that this technology is a sound base for future Siemens wind turbines," says the chief technology officer of Siemens' wind division, Henrik Stiesdal. Stiesdal has designed wind turbines since leaving school in the mid 1970s and was employed at Denmark's Vestas in the mid 1980s before joining competitor Bonus, which was later acquired by Siemens.

In traditional wind turbines, a gearbox is used to step up the slow speed of the rotor shaft to the input shaft of a high-speed generator, while direct-drive wind turbines require use of a slow-speed generator, not commonly used in the power industry. Siemens first tested two direct-drive models using permanent magnet generators (PMGs) from different companies (Windpower Monthly, September 2008) before recently installing a direct-drive 3MW prototype near its Danish headquarters in Brande.

The SWT-3.0-101 DD prototype has a rotor diameter of 101 metres and is now being tested before its commercial launch this year. While the company is not revealing further details, the prototype uses a principle referred to as "inverted radial-flux" in which the synchronous generator is literally turned inside out, reports Denmark's The Engineer newspaper. The report explains that in an inverted generator the rotor becomes a barrel outside a central stationary stator, allowing Siemens to make the generator more compact. By reducing the diameter of the rotor from about five metres to 4.2 metres, it becomes small enough to be transported into southern Europe through the Elb tunnel in Hamburg, reports The Engineer. The tunnel, which restricts the width of vehicles to 4.6 metres, is a notorious bottleneck for wind turbine shipments out of Denmark and northern Germany.

"Compared to a standard wind turbine with gearboxes, we managed to reduce the number of parts in the new SWT-3.0-101 DD by half," says Stiesdal. "With fewer moving parts, the direct-drive technology has the potential to significantly reduce maintenance time, which could result in even higher turbine availability." The new machine will primarily be targeted for use on land. The main advantage of PMGs is their simple design that requires no external power for excitation, slip rings or excitation control systems. This leads to high efficiency even at low loads (Windpower Monthly, November 2008).

Siemens is not abandoning its traditional technology. "We will continue to develop new wind turbines based on the standard gearbox design as these machines have proven to be exceptionally robust and reliable," says Stiesdal. Last year Siemens launched two new turbines based on the proven concept with gearboxes. The 2.3MW SWT-2.3-101, with a rotor diameter of 101 metres, is already likely to be the top selling Siemens turbine in 2010. Siemens has orders - mostly for the offshore market - for almost 500 of the new SWT-3.6-120 with an even bigger rotor diameter of 120 meters and a rating of 3.6MW.

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