Key to wind turbine design and production
Torque is an important variable to the designers and manufacturers of wind turbine technology. Torque data is especially critical in the evaluation of wind turbine components such as bearings, gears, and braking systems. By providing the true mechanical work being generated by the rotor shaft, torque can be used to determine the true efficiency of the system: mechanical energy in - versus electrical energy out.
In order to collect torque data, a bondable strain gage is typically applied to the shaft. The strain gage is usually a Wheatstone bridge circuit that changes resistance in response to distortion of the shaft surface when rotated under load. The relationship between this distortion (strain) and the mechanical work being generated (torque) is linear in the elastic region and based on the physical properties of the shaft. One of the best methods for collecting strain data from a rotating shaft is via a telemetry system that reads the analog signal from the strain gage and transmits it as a digital signal to a stationary receiver where it is collected for analysis or used for real-time process control.
Many monitoring systems in today’s market provide torque, power (hp or kW), rpm, and direction of rotation data continuously using inductive power and data transfer. These systems have been used successfully for multiple applications in the wind industry, where the typical service life goal for wind turbines is 20 years and meaningful validation trials may last on the order of months or years.
One common application is to test a scaled-up design of a patented vertical wind turbine for 18 months. For example, a 25-100 kW turbine can feature unique stationary stators that funnel and accelerate wind into the rotor blades. The robust, unique combination of the stator and rotor design of such a turbine allows it to operate in Class 6 and 7 wind speeds (25-30 m/s), while many other turbines cut-out at 25 m/s. Design professionals typically use monitoring equipment to capture torque data for a publishable power curve. The data can help validate the full-size models developed from 1/12-scale wind tunnel prototypes. The results can also be used to scale-up designs in the 0.25-1 MW range and scale down designs in the 1-5 kW range.
TMA 25 kW vertical wind turbine
The National Renewable Energy Laboratory has specified over 150 sensors that can measure all dynamic motions and loadings on a generic gearbox typically employed in most large-scale turbine designs. The goal of collaborative efforts with the Department of Energy, wind turbine OEM’s, drivetrain component and lubrication suppliers, and turbine owners and operators is the development of a complete mathematical model of the entire TMA system, available in the public domain for the validation of improved gearbox designs.1
Two gearboxes in the 600-750 kW range were built and instrumented for the investigation. One runs on a 2.5 MW dynamometer at the National Wind Technology Center and the other operates in the field on a turbine in the Ponnequin wind farm, both located in Colorado. Monitoring sensors allow studying of torque loads when braking on high speed shafts. It’s common to find that generators may carry torque for a short period of time.
Test stands applications
Ensuring quality and reliability is a reason why design and maintenance teams invest in test stands for the gearboxes that represent the core of their powertrain. A 2.5 MW wind turbine typically splits the torque from the rotor shaft through a multipath, load-distributing design to drive four generators, reducing component failures and extending the operating life.2
Monitoring systems on gearboxes
To test production gearboxes, a common practice is to combine a test stand with torque sensors. For example, many sensor models monitor torque on up-to four output shafts. The torque signals are fed back to a PLC that controls a hydraulic servo loop regulating rotary actuators that maintain the desired torque load on each shaft. Subsequent endurance tests can oftentimes simulate 20 years of operation.
Utilizing torque as part of a condition-based monitoring (CBM) strategy is not currently standard practice in the wind industry. Bearing temperatures, vibration, and oil particulates are commonly measured variables relied upon to monitor drivetrain health. Measuring and logging torque data is important to many components, especially as it relates to the gearboxes.
The measurement of true mechanical torque is a key parameter for the wind industry in test stand and design validation applications. It may hold promise as a worthy drivetrain CBM variable in the future. As the examples show, torque monitoring systems approved by the National Renewable Energy Laboratory are ideal for continuous measurement.
Jim Schramski is a field engineer for Binsfeld.
Binsfeld | http://www.binsfeld.com
1 W. Musial, S. Butterfield & B. McNiff (2007). Improving Wind Turbine Gearbox Reliability. Presented at the 2007 European Wind Energy Conference, paper preprint, NREL/CP-500-41548: http://www.nrel.gov/wind/pdfs/41548.pdf
2 Clipper Windpower Plc (2006). Liberty. Downloadable product brochure: http://www.clipperwind.com/pdf/liberty_brochure.pdf