Preliminary Results of Vibration Measurements on a Wind Turbine Test Bench

In this paper some preliminary measurements aimed at determining the dynamics of a test bench are described. The test bench for which measurements were applied, is a Highly Accelerated Life-Time (HALT) tester used for testing wind turbine nacelles. The HALT tester is a large and complex machine with nine actuators that can provide tilt and yaw moments up to 25 MNm to emulate forces from the wind turbine blades. The dynamics of the test bench are unknown, and the measurements that were carried out should provide an understanding of the dynamics, which later should lead to fatigue assessment of vital components of the test benches. Accelerometers and geophones were placed at strategic positions around the HALT tester, both on the steel structure and concrete foundation. Experimental vibration data were obtained both with impact excitation and recordings during operation. From analysis of data obtained during operation, resonances at 1.5 and 3.9 Hz were found. These resonances could potentially be low enough to influence the life time of the test bench. Several resonances from operation measurements could be directly compared to the ones from impact excitation. A comparison between accelerometers and geophones were made, which showed the geophones in general performed best for the tests. For the impact test two sizes of impact hammers were used and the results showed that the larger sledgehammer produced the best results.