For determining power extracted from wind by wind turbine we have to assume a air duct as assumed that the velocity of wind at the inlet of the duct is V1 and velocity of air at the outlet of the duct is V2. Say, mass m of the air is passed through this imaginary duct per second.
The wind mill industries uses induction & permanent- magnet designs. There are many variations of these two designs, In general terms, the induction generator must be spun at about 1,000 rpm or more to produce useful power. It produces current by first generating an electric field by passing alternating current (AC) through a coil. A series of these coils are mounted on a Rotating structure (Rotor) that is turned by the wind through the drive train. Surrounding the rotor is a stationary series of coils, a stator. When the electric fields on the rotor pass coils on the stator, the field induces a current in the stator coils which is conducted away as output. Induction generators are not self-exciting. That means they require an external power supply(exciter) to produce a magnetic flux or field.
Modern wind turbines fall into two basic groups: the horizontal-axis variety. (a) vVrtical-Axis design (b) Horizontal-Axis wind turbines typically either have two or three blades. These three-bladed wind turbines are operated "upwind," with the blades facing into the wind.
Wind turbines can be built on land or offshore in large bodies of water like oceans and lakes.
