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PM direct drives for wind turbines

21 November 2012

Wind power is at a critical point of development; they currently account for about 5 percent of the world's installed capacity for electricity generation, providing ample opportunity for growth. A key enabler will be the adoption of technologies that will ultimately cut the cost of wind power electricity generation.

With that aim in mind, Cobham Technical Services is currently helping Colorado-based Boulder Wind Power (BWP) to develop a radical new permanent magnet (PM) generator based on a permanent magnet, direct drive design. BWP's generator uses a novel axial flux, air-core architecture that increases efficiency and reliability, and will ultimately reduce the cost of wind generated electricity to compete on an equal footing with fossil fuels.

In support of the initial engineering studies, BWP chose to use the 3D version of the Opera electromagnetic simulator from Cobham Technical Services’ Vector Fields Software range. Like all permanent magnet direct drive wind turbines, the generator rotor of BWP's 3MW design turns at about 13 revolutions per minute, requiring a high pole count.

While many wind turbine designers employ two-dimensional simulation for the main components in a generator, and only use three-dimensional simulation for elements such as the end turns on windings, BWP's designers must use full 3D simulation at every stage, in order to model the generator's novel architecture as accurately as possible.

The results obtained from prototype testing show a high degree of correlation with BWPs’ analyses. BWP is now working with Cobham to build on these results by enhancing dynamic and transient modelling capabilities, which will enable the company to move forward on the design analysis and optimisation.

Unlike conventional permanent magnet generators, the air-core stator in the BWP design does not use electrical steel laminations wound with conductors, and contains no ferromagnetic materials. This breakthrough has several advantages, including eliminating iron losses associated with flux reversals in the stator and eliminating the magnetic attraction between the generator's rotor and stator. Together these enable higher efficiencies and higher torque levels for a given mass of generator.

With the fundamental design approach for new generator BWP is now in the process of designing units to be incorporated in its clients' multi-megawatt wind turbine designs for prototype testing. These will be available for testing in 2013, with full commercial availability expected in 2014.

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