A wind turbine is a rotary device that extracts energy from the wind. If the mechanical energy is converted to electricity, the machine is called a wind generator or wind turbine. Small horizontal-axis wind turbines (HAWT) have the main rotor shaft and electrical generator at the top of a tower, and must be pointed into the wind. Small turbines are pointed by a simple wind vane, while large turbines generally use a wind sensor coupled with a servo motor. Larger wind turbines usually have a gearbox, which turns the slow rotation of the blades into a quicker rotation that is more suitable to drive an electrical generator.