| Since air has mass (1.23Kg/cubic meter), its motion can exert sizeable forces as its momentum is stopped or slowed down. The momentum transfer can be used to rotate propeller style wind sails, often called wind turbines. The rotating turbine is connected to an electrical generator through a gear box: |
| Most generator systems are synchronized to the utility grid so the generators spin at a constant speed. To compensate for different wind conditions most systems have variable pitch propellers that can capture more or less wind forces. Some wind generator also have gearboxes that have variable input vs output gear ratios, as another way to deal with different wind speed conditions. The better machines carefully measure the wind speed and make corrections to the propeller blades and the gearbox to maintain maximum mechanical to electrical conversion efficiency. |
| The minimum speed that a wind turbine can begin producing useful electricity is often called the cut-in speed. Many systems have a typical 10 mph cut-in speed requirement. When the wind speed exceeds a certain level, many wind turbines disconnect their generators from the power grid and rotate the propellers to a feather position to prevent damage to the turbine. The cut-out speed of many systems is around 50 or 60 mph. |
