Direct drive turbine needs no gearbox

EWT's direct drive wind turbine needs no gearbox or speed increaser. The large disc is the generator.

Most utility-sized wind turbines use a costly speed increaser to drive a generator. Netherlands-based Emergya Wind Technologies (EWT) however, builds wind turbines based on a direct-drive (gearbox free) design. The company says its turbine offers improved performance and reliability over conventional wind turbines.

The company’s main product is a 900 kW, pitch controlled variable speed wind turbine. The unit builds on over ten years of operational experience with the company’s 750-kW turbine, of which over 200 units have been installed. A future challenge, says the company, is to develop large wind turbines in the multi-MW range for onshore and offshore cites.

EWT’s 900-kW design uses a multi-pole ring generator that rotates slower than conventional designs, for long life. An autotransformer, which sits between the generator and a power converter, reduces the voltage to 690V. This is standard in the wind industry.

The generator operates at a varying frequency, directly proportional to the rotor speed. The converter modifies this variable frequency output to a fixed frequency corresponding to the grid frequency, 50 or 60Hz. This arrangement, according to one consulting firm, is now finding favor with many manufacturers of multi-megawatt turbines.

This design replaces a complex high-speed geared drive train common in most conventional wind turbines. To further reduce the number of rotating components, a large single main bearing (three row, cylindrical-roller unit) carries the rotor assembly and generator rotor, to reduce the number of rotating components. A monocoque main chassis gives easy service access to essential rotor hub internals, such as rotor-blade-pitch drives and back-up batteries.

The turbine feeds generator power into the grid by means of a ‘back-to-back’ (ac to dc to ac) type full-converter equipment. This converter has several programmable functions such as a capability to limit output during night hours and a noise reduction feature. Built-in grid-fault-ride-through technology is available on demand. The company says the features make the design well suited to solitary applications, weak grids, and high-turbulence sites. EWT says it operates in compliance with ISO 9001:2000 and is VCA** certified, one that tests safety control systems of servicing companies in an objective and structured way. It is comparable to OHSAS18001. The table lists a few specs.