The power from the rotation of the wind turbine rotor is transferred to the generator through the drive train, i.e. through the main shaft, the gearbox and the high speed shaft.
Why use a gearbox?
If an ordinary generator is used without a gearbox, directly connected to a 50 Hz AC (look in alternating current) three phase grid with two, four or six poles, then an extremely high speed turbine would be necessary with between 1000 and 3000 revolutions per minute, rpm, as will be explained on the pages for generators. With a 43 m 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 in order to connect the generator directly to the grid, it would need 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 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 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 disc brakes. In the background you see the lower part of a nacelle for a 1.5 MW turbine.
- Gearbox: function and requirements
- Type of transmission: parallel and planetary gearbox
- Drive train with gearbox (A): three different topologies
- Drive train with gearbox (B): example of brake on main shaft
- Drive train with gearbox (C): example of brake on fast shaft
- Drive train with gearbox (D): example with rotor mounted on gearbox
- Types of drive train support: three different types
- Drive train with no gearbox: figure
- Compact drive train (1-stage gearbox): Multibrid 5 MW design
- Single bearing (direct drive): Zephyros 2 MW
- Nacelle construction (A): geared nacelle
- Nacelle construction (B): gearless (ABB prototype)
- Nacelle construction (C): gearless (Zephyros 2MW)