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Title-Wind as alternative energy source
  Why Use a Gearbox?

Camwheel movie The power from the rotation of the wind turbine rotor is transferred to the generator through the power train, i.e. through the main shaft, the gearbox and the high speed shaft.

But why use a gearbox? Couldn't we just drive the generator directly with the power from the main shaft?

If we used an ordinary generator, directly connected to a 50 Hz AC ( alternating current ) three phase grid with two, four, or six poles, we would have to have an extremely high speed turbine with between 1000 and 3000 revolutions per minute (rpm), as we can see in the page on Changing Generator Rotational Speed. With a 43 metre rotor diameter that would imply a tip speed of the rotor of far more than twice the speed of sound, so we might as well forget it.

Another possibility is to build a slow-moving AC generator with many poles. But if you wanted to connect the generator directly to the grid, you would end up with a 200 pole generator (i.e. 300 magnets) to arrive at a reasonable rotational speed of 30 rpm.

Another problem is, that the mass of the rotor of the generator has to be roughly proportional to the amount of torque (moment, or turning force) it has to handle. So a directly driven generator will be very heavy (and expensive) in any case.

    Less Torque, More Speed

The practical solution, which is used in the opposite direction in lots of industrial machinery, and in connection with car engines is to use a gearbox. With a gearbox you convert between slowly rotating, high torque power which you get from the wind turbine rotor - and high speed, low torque power, which you use for the generator.

The gearbox in a wind turbine does not "change gears". It normally has a single gear ratio between the rotation of the rotor and the generator. For a 600 or 750 kW machine, the gear ratio is typically approximately 1 to 50.

The picture below shows a 1.5 MW gearbox for a wind turbine. This particular gearbox is somewhat unusual, since it has flanges for two generators on the high speed side (to the right). The orange gadgets just below the generator attachments to the right are the hydraulically operated emergency 1.5 MW Gearbox (NEG Micon)disc brakes. In the background you see the lower part of a nacelle for a 1.5 MW turbine.