LMS Test.Lab simplifies wind turbine gearbox testing

According to the World Wind Energy Association (WWEA), the world market for wind turbine installations reached a new record in 2011—a total of 420,000 kW, compared to 376,000 kW in 2010. And according to preliminary data, the total installed capacity around the world has reached about 2.39 GW—accounting for 3% of global electricity demand.

To meet this growing demand, more—and larger—wind turbines will soon be needed. But the accelerated development also presents challenges, as turbine designs are becoming increasingly complex and they are required to operate for decades in harsh weather conditions. This means that each custom project design requires more testing.

Moventas, a first-tier supplier of wind turbine gearboxes, has learned that the number of noise and vibration tests it performs has quadrupled in the past five years, and many projects require them.

Jari Toikkanen, manager of Moventas' research and testing group, explains that these tests are mainly carried out to improve product reliability and meet the strict requirements of organizations such as the American Gear Manufacturers Association (AGMA) and European ISO standards. He adds: “In addition to their best efforts in product development, wind turbine original equipment manufacturers (OEMs) are demanding more and more detailed vibration tests to test product performance than ever before.”

Gearbox resonances

One area of ​​particular concern is the vibration of a wind turbine’s large gearbox, which combines the planetary and helical rotors to drive the generator. Another important component is the torque arm that connects the gearbox to the turbine. For example, in a large 3MW model from Moventas, the gearbox weighs 30 tons and is 2 meters in diameter; it is 2.5 meters long. The torque arm is 4 meters wide from sleeve to sleeve, 0.5 meters thick, and weighs 5 tons.

Engineers perform a variety of model impact tests to test the resonant response of these components. Why? To check for and avoid resonant response.

Vibrations from surrounding structures or gear mesh frequencies can damage the generator’s frame, blades, drive shaft and towers, which can be more than 120 meters tall.

“Generally, we aim to avoid vibrations in the torque arm in the 80-250 Hz range and the rest of the structure in the 400-800 Hz range,” says Toikkanen. “If the resonant frequencies are in or near this range, engineers can change the modal frequencies by adjusting the geometry of the gearbox components and the torque arm—often changing the thickness and shape of some components to optimize performance.”

Toikkanen notes that this process is complicated by the fact that the gearbox torque arm vibrates differently at different rotor blade speeds, and the vibration frequencies of the gears can also vary—from input rotational revolutions per minute (RPM) to as much as 10 times in light or high winds.

Improving Testing Efficiency

To meet the challenges of various tests, Moventas has turned to LMS Test.Lab software—with LMS SCADAS 8 dynamic data acquisition system—to perform various mode analysis measurements in a very short time. The system includes all the integrated tools needed for modal analysis:

● Test setup;
● Control;
● Measurement;
● Signal debugging;
● Result analysis;
● Data management;
● Report generation.

According to Mr. Toikkanen, built-in workbooks and step-by-step instructions prompt engineers where to enter parameters and show how the entire process is performed. This simplified operation allows measurements to be made faster.

The system's online monitoring function also improves testing efficiency. Mr. Toikkanen said: "We can use real-time visualization to see the results of the measurement immediately while measuring. We can verify the test on the spot, get first-hand information on how the structure deforms after each hammer, and understand the cause of any unexpected resonance." The application of

this visualization function is very important to Moventas engineers. The animated simulation status is displayed on the same screen as the frequency response function (FRFs) - showing the vibration amplitude of key parts of the gearbox. This allows engineers to immediately understand the structural torsion and bending of the gearbox at different vibration frequencies, so they can identify which bearings can transmit vibrations - and determine the key gear mesh harmonics.

Fast-response engineering projects

Toikkanen says that the different configurations of the software packages allow routine tests to be run “in days rather than weeks.” When a faster cycle is required, modal tests can be run in the morning, with the results of the analysis available and documented the same day.

In addition to using the LMS Test.Lab system, the company has been working closely with LMS Engineering Services on projects that require additional computing resources—such as faster response projects and those that need to meet key customer requirements. Defining the scope and level of detail of durability analysis calculations, coordinating projects with Petri Lahtinen, lead structural analyst at Moventas.

In one project, LMS Engineering Services provided a critical fatigue life analysis required by a wind turbine manufacturer. The analysis required examining two major wind turbine cylindrical gearbox components—the torque arm and the gear planet carrier—that were expected to withstand more than 20 years of operational load life. LMS engineers created a finite element model of the components and applied unit loads to determine the stress time series for each part. This stress time series together with the complete component load history is used in the durability simulation software (LMS Virtual.Lab) to predict the fatigue life of the base material.

In a later project, LMS measures the gearbox shaft rotational vibration at low input speed and high output speed. The accelerometer signal mounted on the low-speed shaft is used directly in the system analysis. The signal of the high-speed shaft is directly acquired by measuring the rotational speed with a laser vibrometer. A series of operating responses can show the rotational vibrations generated by all shafts and the related resonances.

Mr. Toikkanen believes that as wind turbine manufacturers place more stringent requirements on component suppliers, relevant testing - and reducing the cost of energy production in projects - will be crucial to driving innovation in turbine development. (end)