Wind Partnerships for Advanced Component Technology: WindPACT Advanced Wind Turbine Drivetrain Designs

Northern Power Systems, Inc.

Project Description: The WindPACT drivetrain studies explored a range of configurations that 

appear to offer opportunities for reducing overall wind turbine capital cost and improving 

drivetrain efficiencies. Two studies were conducted, one by Global Energy Concepts and 

one by Northern Power Systems. Each team looked at several common configurations and 

proposed some special ones. They completed preliminary designs and produced detailed cost 

and performance estimates to determine the relative cost of energy between competing designs. 

After these alternative designs were thoroughly explored, each subcontractor produced a report 

that detailed the findings. Three configurations—direct drive, single-stage drive with mediumspeed generator, and distributed drive with multiple generators were identified. All appeared to 

offer a significant likelihood of improvement over the baseline design (a three-stage planetary 

drive with a doubly fed induction generator). Each design assumed the use of a rare earth (neodymium-iron-boron), liquid-cooled permanent magnet generator. Several factors contributed to the 

estimated improvements in these designs. The alternative gearbox designs significantly reduced 

the number of gear stages and bearings. The permanent magnet generators reduced mass and 

size by eliminating copper in the generator rotors. The additional flexibility in the generator 

design allowed overall improvements in generator efficiencies. Because of unknowns in technical 

details and differences in cost estimates, no single clear winner was identified. Therefore, each 

subcontractor was directed to proceed with one target design. Clipper Windpower selected the 

multidrive path, multiple generator design for development under a Low Wind Speed Turbine 

Phase I project. Global Energy Concepts chose the single-stage drive, medium-speed generator.

Under its subcontract, Northern Power Systems developed a direct-drive (no gearbox) permanent 

magnet generator because the Phase I study showed that this configuration had the greatest 

potential to decrease the cost of energy. Permanent magnets provided increased efficiency and 

allowed this 1.5-MW generator, with a rotational speed of just 19.65 rpm, to be no larger than 

4.0 m in diameter. The generator design was based on design concepts developed by General 

Dynamics Electric Boat Division for ship propulsion systems. Additional details of these designs can be found in the Northern Power Systems 

WindPACT Drivetrain Alternative Design Study Report. This design is awaiting a test slot in the National Wind Technology Center dynamometer. As part of this program, Northern Power also designed and fabricated a full-power converter that will allow variable-speed operation of 

the WindPACT drivetrain. The converter features state-of-the-art controls and can address the ride-through requirements of the EON Specification. Northern Power is now developing a next-generation advanced power converter under a Low Wind Speed Technology Phase I project. 

Total Project Budget: $5,858,435

Industry Cost Share: $0

DOE Cost Share: $5,858,435

Planned Project Duration: April 2001–December 2005

NREL/Sandia: Northern Power Systems, Inc.:

Alan Laxson, NREL Peter Mattila, NPS

1617 Cole Blvd. 182 Mad River Park

Golden, Colorado 80401 Waitsfield, Vermont 05673

303-384-6944 802-496-2955

alan_laxson@nrel.gov PMattila@northernpower.com

Concept illustration of advanced drivetrains.

Concept illustration of an NPS direct-drive 

permanent-magnet generation.

A Strong Portfolio for a Strong America • Energy efficiency and clean, renewable energy will mean a stronger economy, a cleaner environment, and greater energy 

independence for America. Working with a wide array of state, community, industry, and university partners, the U.S. Department of Energy’s Office of Energy Efficiency 

and Renewable Energy invests in a diverse portfolio of energy technologies.

For more information contact EERE Information Center • 1-877-EERE-INF (1-877-337-3463) • www.eere.energy.gov

Produced for the U.S. Department of Energy by the National Renewable Energy Laboratory, a DOE national laboratory

Printed with a renewable-source ink on paper containing at least 50% wastepaper, including 10% post consumer waste.

DOE/GO-102006-2204

March 2006

Current Status: Project Complete—Northern Power Systems, WindPACT Drivetrain 

Alternative Design Study Report 

www.nrel.gov/docs/ fy05osti/35524.pdf 

1.5-MW Drivetrain Fabricated and Waiting for

Testing in NWTC Dynamometer