The generalized Maxwell-slip friction model applied to starting of small wind turbines

Starting of small turbines is significantly affected by friction resistance of drivetrains. When the effect of drivetrain is not considered in small turbine design, the rotor starting may be overestimated, becoming resistance assessment of small turbines relevant in the literature. The present work develops a novel modeling to estimate starting of small wind turbines using an extension of the generalized Maxwell-slip (GMS) friction model. The friction torque of a small wind turbine is analyzed and compared with experimental data available in the literature. Static and dynamic torques are evaluated through the use of the extended GMS, in which a procedure to estimate the aerodynamic starting torque of the turbine is performed. The results show that the proposed approach is a generalization of Stribeck model applicable to both regimes unsteady and steady-state, demonstrating good agreement with measurements from the literature. For a constant wind velocity, the aerodynamic torque for a stationary blade always decreases along the time, and only lift force and chord distribution can generate aerodynamic torque at starting. However, it is interesting whether the wind velocity is variant, starting from zero to a constant value, this variation increases the aerodynamic torque a little soon after starting. The estimated powertrain axial load and torque are close to the measurements with maximum errors of 9.4 and 6.7%, respectively, demonstrating the good behavior of GMS for starting of small turbines.