19. Comparison of Modal Parameters Extracted Using MIMO, SIMO, and Impact Hammer Tests on a Three-Bladed Wind Turbine

As part of a project to predict full-field dynamic strain of rotating structures (e.g. wind turbines or helicopter rotors), a validated numerical model of a structure is required. In this case, a small wind turbine was used. To understand the dynamic characteristics and validate a finite element model of a three-bladed wind turbine, several experimental modal analysis tests were conducted on the turbine attached to a 500-lb steel block. The test structure consisted of three 2.3-m blades mounted to a hub that was attached to the block using a shaft and a lathe chuck. In three separate tests, the structure was excited using a single shaker, multiple shakers, and an impact hammer; the responses of the structure to the excitations were measured using 12 triaxial accelerometers. The results reveal several very closely spaced modes present within the turbine in the test configuration. The natural frequencies and mode shapes obtained by using three different methods were compared to demonstrate the differences (e.g. strengths and weaknesses) between each excitation technique. The paper reports the results obtained and lessons learned during the experimental modal tests of the wind turbine.