<P> In conventional wind turbines, the blades spin a shaft that is connected through a gearbox to the generator . The gearbox converts the turning speed of the blades 15 to 20 rotations per minute for a large, one - megawatt turbine into the faster 1,800 revolutions per minute that the generator needs to generate electricity . Analysts from GlobalData estimate that gearbox market grows from $3.2 bn in 2006 to $6.9 bn in 2011, and to $8.1 bn by 2020 . Market leaders were Winergy in 2011 . The use of magnetic gearboxes has also been explored as a way of reducing wind turbine maintenance costs . </P> <P> For large, commercial size horizontal - axis wind turbines, the electrical generator is mounted in a nacelle at the top of a tower, behind the hub of the turbine rotor . Typically wind turbines generate electricity through asynchronous machines that are directly connected with the electricity grid . Usually the rotational speed of the wind turbine is slower than the equivalent rotation speed of the electrical network: typical rotation speeds for wind generators are 5--20 rpm while a directly connected machine will have an electrical speed between 750 and 3600 rpm . Therefore, a gearbox is inserted between the rotor hub and the generator . This also reduces the generator cost and weight . Commercial size generators have a rotor carrying a field winding so that a rotating magnetic field is produced inside a set of windings called the stator . While the rotating field winding consumes a fraction of a percent of the generator output, adjustment of the field current allows good control over the generator output voltage . </P> <P> Older style wind generators rotate at a constant speed, to match power line frequency, which allowed the use of less costly induction generators . Newer wind turbines often turn at whatever speed generates electricity most efficiently . The varying output frequency and voltage can be matched to the fixed values of the grid using multiple technologies such as doubly fed induction generators or full - effect converters where the variable frequency current produced is converted to DC and then back to AC . Although such alternatives require costly equipment and cause power loss, the turbine can capture a significantly larger fraction of the wind energy . In some cases, especially when turbines are sited offshore, the DC energy will be transmitted from the turbine to a central (onshore) inverter for connection to the grid . </P> <P> Gearless wind turbines (also called direct drive) get rid of the gearbox completely . Instead, the rotor shaft is attached directly to the generator, which spins at the same speed as the blades . </P>

Do wind turbines produce ac or dc power