These consist of propeller blades turning generators located at the top of pole and tower supports. Capacities of the generators, depending on wind conditions, may vary from a fraction of a megawatt to several megawatts.

The outputs generally are varying with the wind velocity. The electric output increasing as the cube with the wind velocity, i.e. double wind velocity will produce 3 or 8 times as much output. As velocity tends to increase with height above ground, the higher the mounting, the greater the velocity and electric output.

Economics plays a great part. The higher the structure, the greater its cost, and this must be balanced against the greater costs of the generators, until a satisfactory ratio of costs is reached. Also the length of the blades of the propellers must be taken into account, the longer the blades or blades (usually two or three) the greater the electric output, although there is a practical mechanical limit as to their size.

As winds are variable in velocity and direction, propeller blade angles or change in pitch may be varied and the whole turret or nacelle mounted on the top of the structure may be rotated to keep the propeller facing into the wind (known as the yaw). The extent of these maneuvers again influenced by economics.

As the electrical output sought may require more than one such unit, care must be taken in situating these units so that the air flow disturbed by one operating unit does not affect its companion units. Electrically, the output must conform to strict limits of voltage and frequency to interconnect with other units or the utility system, if desired.

Hence the speed and pitch of the propeller blades must also be controlled so that if the wind should increase above or drop below certain limits, the unit is taken out of service (and, of course, when there is no wind).

As winds tend to be greater in open areas, such as off shore or level agricultural areas, costs of associated transmission and distribution systems (if any) involving transformer, switching and protective circuitry must be considered as well as associated maintenance expenses.

On the other hand, areas where such wind farms may be located may still be used for their original purposes or other productive uses. Practically all wind power installations employ propeller driven horizontal shafts. Air turbines driving vertical shafts are sometimes used where appearance and bird safety may be considered  

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