A gearbox is typically used in a wind turbine to increase rotational speed from a low-speed rotor to a higher speed electrical generator. A common ratio is about 90:1, with a rate 16.7 rpm input from the rotor to 1,500 rpm output for the generator. Some multimegawatt wind turbines have dispensed with a gearbox. In these so-called direct-drive machines, the generator rotor turns at the same speed as the turbine rotor. This requires a large and expensive generator. Other wind turbines on the market sit in-between, with gearbox ratios of about 30:1, dispensing with the highest speed stage in a typical gearbox. There is a trade-off between the reliability of gearboxes and gear stages and the cost of slower, higher torque generators.
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