Analysis of wind turbine pitch control bearings---discrete element method and finite element analysis
This study analyzes the bearings used in pitch control system of a wind turbine which enables the blades to have an optimum angle of attack for efficient power generation and to prevent over-speeding of the blades. Large forces and moments act on the pitch control bearings causing significant wear and deformations of the bearings. In this study, the wear pattern in the pitch control bearings has been simulated using discrete element method and deformations in the bearings have been investigated using finite element software ABAQUS. ^ A six degree freedom dynamic bearing model (DBM) was developed for analysis of four point contact bearing which are commonly used in wind turbine pitch control. DBM for four point contact bearings was tested for accuracy under simple loading conditions. The model was also used to demonstrate the effectiveness of four point contact bearings to resist moment and axial loads. The model was validated by comparing velocities of bearing elements obtained from DBM for four point contact bearings with those from kinematic model which considers pure rolling conditions between ball and the races and neglects the effect of external forces. Parametric studies were performed to investigate the effect of changes in bearing geometry and operating conditions on the bearing performance. Further, the ball motion causing mechanical wear in pitch control bearing raceways was simulated using the DBM for four point contact bearings. ^ Finite element analysis was performed on a model of wind turbine hub with three pitch control bearings using ABAQUS. Initially, a model was developed to calculate the loads (forces and moments) acting on pitch control bearings as a function of wind speed, wind turbine blade properties (geometry, weight), angular velocity, pitch angle and angular position of the blade. These loads were applied on the inner race of the bearings modeled in ABAQUS and the deformations and stress in the bearing raceways were investigated. The bearings modeled in ABAQUS were four point contact and double row bearings. ABAQUS / Explicit finite element analysis which is well-suited for non-linear problems such as contacts observed in bearings was used. ^ Due to the large axial loads and moments acting on the pitch control bearings, the bearing races undergo large deformations. The pitch control bearings have large diameter (approximately 2 meters) and as the loads on the outer race vary over its surface, the outer race acquires a shape similar to that of a potato chip. These deformation characteristics of the pitch control bearings were simulated using ABAQUS. The finite element analysis also provides magnitude and locations of maximum stress and deformation in the bearing raceways. Further, the locations of maximum deformation were verified analytically to validate the finite element analysis results. A parametric study was also performed to investigate the effect of wind speed, pitch angle and blade rotation speed on the deformations of the bearings.^
Farshid Sadeghi, Purdue University.
Alternative Energy|Engineering, Mechanical|Energy