Determination of Rotor Imbalances

During operation, rotor imbalances in wind energy converters (WEC) induce a centrifugal force, which is harmonic with respect to the rotating frequency and has an absolute value proportional to the square of the frequency. Imbalance driven forces cause vibrations of the entire WEC. The amplitude of the vibration also depends on the rotating frequency. If it is close to the bending eigenfrequency of the WEC, the vibration amplitudes increase and might even be visible. With the growing size of new WEC, the structure has become more flexible. As a side effect of this higher flexibility it might be necessary to pass through the critical speed in order to reach the operating frequency, which leads to strong vibrations. However, even if the operating frequency is not close to the eigenfrequency, the load from the imbalance still affects the drive train and might cause damage or early fatigue on other components, e.g., in the gear unit. This is one possible reason why in most cases the expected problem-free lifetime of a WEC of 20 years is not achieved. Therefore, reducing vibrations by removing imbalances is getting more and more attention within the WEC community. Present methods to detect imbalances are mainly based on the processing of measured vibration data. In practice, a Condition Monitoring System (CMS) records the development of the vibration amplitude of the so called 1p vibration, which vibrates at the operating frequency. It generates an alarm if a pre-defined threshold is exceeded. In (Caselitz & Giebhardt, 2005), more advanced signal processing methods were developed and a trend analysis to generate an alarm system was presented. Although signal analysis can detect the presence of imbalances, the task of identify its position and magnitude remains. Another critical case arises when different types of imbalances interfere. The two main types of rotor imbalances are mass and aerodynamic imbalances. A mass imbalance occurs if the center of gravitation does not coincides with the center of the hub. This can be due to various factors, e.g., different mass distributions in the blades that can originate in production inaccuracies, or the inclusion of water in one or more blades. Mass imbalances mainly cause vibrations in radial direction, i.e., within the rotor plane, but also smaller torsional vibrations since the rotor has a certain distance from the tower center, acting as a lever for the centrifugal force. Aerodynamic imbalances reflect different aerodynamic behavior of the blades. As a consequence the wind attacks each blade with different force and moments. This also results in vibrations and displacements of the WEC, here mainly in axial and torsional direction, but also in contributions to radial vibrations. There are multiple causes for aerodynamic imbalances, e.g., errors in the pitch angles or profile changes of the blades. The major differences in the impact of mass and aerodynamic imbalances are the main directions of the induced vibrations and the fact that aerodynamic imbalance loads change with the 7