Advantages of Horizontal Axis Wind Turbines
Disadvantages of Horizontal Axis Wind Turbines
What are Cyclic Stresses and Vibrations?
Gaia-Wind manufactures wind turbines for light industrial, agricultural, commercial, municipal and residential use. Clients include working farms, educational institutions, large home owners, offices and other commercial premises. This article looks at the advantages and disadvantages of horizontal axis wind turbines (HAWT). It also examines the effects of vibration and cyclic stresses on HAWT wind turbines.
Cyclic stresses fatigue the blade, axle and bearing; material failures were a major cause of turbine failure for many years. Because wind velocity often increases at higher altitudes, the backward force and torque on a horizontal-axis wind turbine (HAWT) blade peaks as it turns through the highest point in its circle. The tower hinders the airflow at the lowest point in the circle, which produces a local dip in force and torque. These effects produce a cyclic twist on the main bearings of a HAWT. The combined twist is worst in machines with an even number of blades, where one is straight up when another is straight down. To improve reliability, teetering hubs have been used which allow the main shaft to rock through a few degrees, so that the main bearings do not have to resist the torque peaks.
When the turbine turns to face the wind, the rotating blades act like a gyroscope. As it pivots, gyroscopic precession tries to twist the turbine into a forward or backward somersault. For each blade on a wind generator’s turbine, precessive force is at a minimum when the blade is horizontal and at a maximum when the blade is vertical. This cyclic twisting can quickly fatigue and crack the blade roots, hub and axle of the turbines.