19 Comparison of Modal Parameters Extracted Using MIMO, SIMO, and Impact Hammer Tests on a Three-Bladed Wind Turbine 191 1- 5.3 (Hz) 2- 5.4 (Hz) 3- 5.6 (Hz) 4- 7.69 (Hz) 5- 7.74 (Hz) 6- 8.9 (Hz) 7- 14.9 (Hz) 8- 15.4 (Hz) 9- 16.1 (Hz) 10- 18.3 (Hz) 11- 21.1 (Hz) 12- 23.2 (Hz) 13- 25.5 (Hz) 14- 29.7 (Hz) 15- 30.2 (Hz) 16- 36.3 (Hz) 17- 37.3 (Hz) 18- 37.9 (Hz) 19- 51.3 (Hz) 20- 58.3 (Hz) Flapwise Differential Flapwise Collective Edgewise Differential Edgewise Collective Fixture 21- 58.9 (Hz) 22- 62.1 (Hz) 23- 67.6 (Hz) 24- 69.1 (Hz) Fig. 19.6 Natural frequencies and mode shapes of the wind turbine attached to the 500-lb block The modes of the assembled structure that come from the modes of the test setup are called fixture modes in Fig. 19.6. These modes were identified by comparing their shapes and frequencies to the modes shown in Fig. 19.5. Comparing the results in Figs. 19.5 and 19.6, the natural frequencies for the modes of the test rig that were related only to the mass of the structure (modes 4, 5, and 10 in Fig. 19.6 that are respectively equivalent to lateral, longitudinal, and bouncing modes shown in Fig. 19.5) have slightly changed. However, the modes of the structure that were related to the rotational inertia of the assembly (modes 6, 12, and 13 in Fig. 19.6 that are respectively equivalent to rotating, rolling, and pitching modes shown in Fig. 19.5) show a significant change. This can be attributed to the fact that in the model shown in Fig. 19.2, the effect of the distributed mass of the turbine that would influence the rotational inertia of the structure was not considered. The modes of a turbine can be categorized as either collective or differential modes. The collective modes are the modes with the same phase on three blades; the differential modes are the modes that the blades do not have the same phase or deflection. 19.4.3 Case 3: SIMO Test on the Wind Turbine Assembly In the SIMO test, the test set up shown in Fig. 19.1 was used; however, only Shaker 1 was exciting the structure. The data acquisition system used the same set-up as previous measurement in Case 2. Most of the modes of the assembled structure that were shown in Fig. 19.6 could be extracted by the SIMO measurement. A comparison between the modal parameters extracted using MIMO, and SIMO measurement techniques is performed in the Sect. 19.5 of the paper.
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