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NREL is operated by Midwest Research Institute ● Battelle Contract No. DE-AC36-99-GO10337 

225-kW Dynamometer for Testing 

Small Wind Turbine Components 

Preprint 

J. Green 

To be presented at the American Wind Energy Association’s 

WindPower 2006 Conference 

Pittsburgh, Pennsylvania 

June 4-7, 2006 

Conference Paper 

NREL/CP-500-40070 

June 2006 
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225-kW Dynamometer for Testing 

Small Wind Turbine Components 

Jim Green 

National Wind Technology Center 

National Renewable Energy Laboratory 

Golden, Colorado 

Abstract 

A 225-kW dynamometer has been commissioned at the National Renewable Energy 

Laboratory’s (NREL’s) National Wind Technology Center (NWTC) near Boulder, 

Colorado. Because both cost and reliability of small wind turbines continue to be barriers 

in the marketplace, new turbine designs are needed for low-wind-speed sites that will 

require development of advanced generators and power electronics. These developments can 

be facilitated and enhanced with this new dynamometer facility. 

NREL’s 225-kW dynamometer is suitable for testing a variety of components and 

subsystems, including generators, gearboxes, mechanical or electro-dynamic brakes, 

power electronics, control systems, and software. Test objectives might include 

performance, system integration (generator, power electronics, and grid), software 

development, or accelerated life testing. The facility is capable of 225-kW peak power 

and shaft speeds up to 4,140 revolutions per minute (rpm). Gear ratios as high as 41:1 can 

deliver maximum shaft torque of 51,000 Newton meters (Nm) or 38,000 foot-pounds 

(ft-lb). NREL is actively seeking industry partners to utilize this facility and advance the 

technology for small, distributed wind turbines. 

This paper will provide an overview of the dynamometer layout and capabilities, including 

photographs, schematics, the torque vs. speed operating envelope, a description of control 

functionality, and an overview of data acquisition capabilities. Opportunities for industry 

partnership with NREL for use of the facility are also presented. 

Introduction 

The U.S. Department of Energy’s (DOE’s) Wind Technology Program is committed to the 

goal of developing cost-effective distributed wind systems for lower wind speed, Class 3, 

wind regimes. Improved technology will be needed to effectively serve a growing market 

for grid-connected wind power for residences, farms, and small businesses. The U.S. Small 

Wind Turbine Industry Roadmap from the American Wind Energy Association (AWEA) 

identifies the high cost and low reliability of small wind turbines as barriers in the current 

market place (AWEA, 2002). The action items identified include new turbine designs for 

low-wind-speed sites and development of advanced generators and power electronics. 

With these technology development needs in view, the National Renewable Energy 

Laboratory’s (NREL’s) National Wind Technology Center (NWTC) near Boulder, 

 1
Colorado, has developed a new 225-kW dynamometer. Upgraded from a former 75-kW 

facility during 2005, the dynamometer is now available for use. 

Test Capabilities 

The 225-kW dynamometer can be used in a wide range of technology testing and 

development activities focusing on drive train components, power electronics, and 

controls used in small wind turbines. These include: 

Generator/Alternator Testing 

• Efficiency 

• Cogging torque 

• Equivalent circuit parameters 

• Thermal characteristics 

• Vibration 

• Defect testing (infant mortality) 

• Insulation life testing 

Gearbox Testing 

• Efficiency 

• Thermal characteristics 

• Lubrication flow 

• Tooth contact patterns 

• Vibration 

• Endurance or fatigue 

Power Electronics Testing 

• Efficiency 

• Thermal characteristics 

• Control and stability 

• Power quality 

• Defect testing (infant mortality)

• Control systems and software testing 

• System integration (generator/power electronics/grid) 

• Software development and testing

Operating Envelope 

The NWTC 225-kW dynamometer will be suitable for testing small wind turbine 

components from the smallest turbines up to turbines with nearly 200-kW ratings. Some 

headroom between the component rating and the dynamometer rating is typically needed 

to adequately cover over-speed and over-torque conditions that turbine components 

encounter in real applications. Maximum available shaft speed is 4,140 revolutions per 

minute (rpm) and the maximum available shaft torque is 37,000 foot-pounds (ft-lb) or 

51,000 Newton meters (Nm). Losses in bearings and gearboxes will somewhat reduce 

the available torque delivered to components under test. The operating envelope for this 

facility is shown in Figure 1. For comparison, this figure also shows the range of 

expected values of power vs. rpm for typical small wind turbines. 

 2
Power (kW)

0

250

0

rpm

Transmission and 26:1 gearbox

Transmission and 9:1 gearbox

Expected range

Current wind turbines

Torque limits Power limits

Speed limits 200

150

100

50

100 200 300 400 500

Power (kW)

0

250

0

rpm

Transmission and 26:1 gearbox

Transmission and 9:1 gearbox

Expected range

Current wind turbines

Torque limits Power limits

Speed limits 200

150

100

50

100 200 300 400 500

0

250

0

rpm

Transmission and 26:1 gearbox

Transmission and 9:1 gearbox

Expected range

Current wind turbines

Torque limits Power limits

Speed limits 200

150

100

50

100 200 300 400 500

Figure 1. NWTC 225-kW Dynamometer 

(Operating Envelope 0-500 rpm) 

Electric Loads 

Three types of electric loads are available for units under test on the dynamometer. The 

first is single-phase service at 120/240 volts alternating current (VAC), 60 Hertz (Hz), 

with up to 50 kilovolt-ampere (kVA) capacity. This load simulates the typical 100- or 

200-ampere (A) utility service available to most homes, small farms, and small 

businesses. The second load is three-phase service at 480 VAC, 60 Hz, with up to 250 

kVA capacity. This simulates utility service for farms and businesses that have larger 

power requirements, up to 250 kW. Other service voltages as well as 50 Hz operation are 

possible but will require enhancements to the facility. For testing off-grid systems 

designed to charge batteries, the dynamometer includes battery bank simulation via a 

voltage-controlled direct current (DC) bus with capacity up to 20 kVA and voltages up to 

216 volts DC. 

Dyno Components 

The drive system for the dynamometer consists of a drive motor coupled to one or more 

speed-reducing gear boxes selected to deliver the requisite torque and shaft speed for a 

given test program. These components are installed on concrete and steel pedestals with 

a drive shaft extending over a rigid steel table as shown in the example configuration in 

Figure 2. This table is on jack screws such that its height can be changed to allow 

alignment of various test article input shafts with the dynamometer drive shaft. At its 

lowest available height, the table is about 5 ft or 1.5 meters (m) below the dynamometer 

drive shaft, providing room for installation of test articles with diameters up to about 8-10 

ft (2.4-3.0 m). 

 3
In-line torque

transducer 26:1 gearbox 13-speed 

transmission

225-kW 

drive motor

Variable-height table

(shown at minimum height)

Concrete pedestal

Unit under test 

(e.g., 100-kW 

direct-drive 

generator)

S. Hughes 

Figure 2. NWTC 225-kW Dynamometer configuration concept 

for a 100-kW direct-drive generator. 

The dynamometer drive motor is a 225-kW (300-horsepower [hp]) rated, variable-speed, 

induction motor. It was obtained from ABB with a matched variable-speed drive and 

harmonic filter. The motor reaches rated power at 1,800 rpm and has a top speed of 

3,600 rpm. The motor installation is shown in Figure 3. 

Figure 3. Dynamometer drive motor and 13-speed transmission. 

 4
Three speed-reducing gearboxes are available for the dynamometer. The first, coupled to 

the drive motor, is a 13-speed transmission with gear ratios ranging from 12.5:1 up to an 

overdrive gear at 0.87:1. It is anticipated that this transmission will be used for all 

dynamometer operations. This transmission will be coupled directly to the unit under test 

or to one of two fixed speed gear boxes to reduce shaft speed and increase torque. These 

gear boxes have ratios of 9:1 and 26:1. Gearbox specifications are shown in Table 1. 

Table 1. NWTC 225-kW Dynamometer Gearbox Specifications 

Device Brand Gear Ratios Torque Limit 

Transmission Fuller 13 ratios 

12.5:1-0.87:1 

1,250 ft-lb 

1,700 Nm 

Fixed-Ratio 

Gearbox 

Byrne 9:1 8,750 ft-lb 

(11,900 Nm) 

Fixed-Ratio 

Gearbox 

Flender 26:1 37,600 ft-lb 

(51,000 Nm) 

Control/Data Acquisition Capability 

Control and data acquisition for the dynamometer will be done using personal computers 

(PCs) connected to National Instruments hardware for signal input and output. The 

concept for system control and data acquisition is shown in Figure 4. Operation will be 

either manual or automated at the discretion of the operators. Either speed and/or torque 

control of the dynamometer drive motor are available in either closed- or open-loop 

modes. Operating modes will include ramps and steady state, simulated wind inputs, and 

programmable cycles. The control PC will also be used to monitor operating 

temperatures and alarm conditions. 

A second computer will be used to record data independent of the dynamometer control 

function. Measurement parameters will be selected to meet the objectives of each test. 

The most commonly measured parameters are likely to be: 

3⁄4 Shaft rpm and torque 

3⁄4 Power, voltage, and current from generator to inverter 

3⁄4 Power, voltage, and current from inverter to grid. 

The dynamometer facility is equipped with four shaft-mounted torque transducers that 

range in capacity from 83-16,700 ft-lb (112-22,600 Nm). Their accuracies are between 

0.1 and 0.5%. The NWTC also has a portable data system for recording power quality in 

compliance with the International Electrotechnical Commission (IEC) standard for 

measuring the power quality of wind turbines (IEC 2001). 

 5
V. Gevorgian 

Figure 4. NWTC 225 kW Dynamometer 

control and data acquisition concept 

Partnerships with NREL

The NWTC is seeking opportunities to collaborate with industry partners in the use of the 

225-kW dynamometer for small wind turbine technology development. For more 

information, refer to the NREL Technology Transfer Web page: 

http://www.nrel.gov/technologytransfer/partnering.html#crada, or contact the author: 

Jim Green 

National Wind Technology Center 

National Renewable Energy Laboratory 

1617 Cole Blvd., MS 3811 

Golden, CO 80401 

jim_green@nrel.gov 

303-384-6913 

 6
References

AWEA (American Wind Energy Association). (June 2002). The U.S. Small Wind 

Turbine Industry Roadmap, A 20-year industry plan for small wind turbine technology, 

http://www.awea.org/smallwind/documents/31958.pdf. Accessed May 2005. 

IEC (International Electrotechnical Commission). (2001). IEC 61400-21:2001. Wind 

Turbine Generator Systems. Part 21: Measurement and Assessment of Power Quality 

Characteristics of Grid Connected Wind Turbines. 

 7
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225-kW Dynamometer for Testing Small Wind Turbine Components: 

Preprint 

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NREL/CP-500-40070 

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WER66002 

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J. Green 

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National Renewable Energy Laboratory 

1617 Cole Blvd. 

Golden, CO 80401-3393 

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14. ABSTRACT (Maximum 200 Words)

This paper describes NREL's new 225-kW dynamometer facility that is suitable for testing a variety of components 

and subsystems for small wind turbines and discusses opportunities for industry partnerships with NREL for use of 

the facility. 

15. SUBJECT TERMS

small wind systems; National Wind Technology Center; dynamometer; wind turbine component testing 

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