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WINDPOWER
GEARS & GEARBOXES
Gears & Gearboxes What is the purpose of a gearbox in a wind turbine? In a nutshell, a turbine works this way: The energy in the wind turns the rotor which is connected to the main shaft, which turns the gearbox, which spins the generator to create electricity. The gearbox essentially connects the rotor to the generator but more importantly, increases the rotational speed of the rotor from a low speed – about 5 to 20 revolutions per minute (rpm) – to a more useful speed around 1,000 rpm. Before describing how a wind-turbine gearbox works, here’s a brief word on power. Mechanical power, such as that transmitted by a rotating shaft, has two components: torque (the twisting force in a shaft measured in foot-pounds) and speed, usually in revolutions per minute or rpm. At constant power, mechanically increasing the shaft speed must lower its torque. The opposite is also true: Increasing shaft torque reduces its speed. Now, if you could remove the case from a conventional wind turbine gearbox, you would see that the input rotation speed from the rotor is increased at three stages. The first stage is almost always a planetary speed increaser because the design can handle the enormous torque or twist produced by the rotor. This first stage increases speed about four fold, so it is said to have a 1:4 ratio. For example, 10 rpm from the rotor is turned into 40 rpm which is fed into the second stage. The middle stage could be another planetary speed increaser when torque is high or a simpler big gear driving a smaller gear, usually helical spur gears. Rotational energy from the first stage goes to a large gear that turns a smaller gear for a speed increase. The ratios here are around 1:5, so the incoming 40 rpm is turned into (5 x 40 =) 200 rpm. Likewise, the third stage has a ratio of around 1:5 with the small gear of the
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middle stage driving the large gear of the third stage. Hence its output is around (5 x 200 =) 1,000 rpm which is a useful speed for what’s called an induction generator. Of course, there are variations to the gear ratios and this description. Wind turbine gearboxes differ from other industrial gearboxes in a few fundamental ways: • Wind turbine gearboxes have much tighter tolerances between gearing elements. • Most industrial gearboxes are speed reducers so they turn a high input speed from a motor, into lower speed and high torque. Wind turbines gearboxes do the exact opposite. • Most industrial gearboxes operate at constant speed and torque. In a wind turbine gearbox, loads are anything but steady, so the input torque is constantly changing. • Industrial gearboxes in temperaturecontrolled facilities are usually anchored to solid concrete which allows for accurate shaft alignments. Wind turbines gearboxes are mounted on steel frames or cast platforms, and sit atop towers that vibrate. This also exposes wind-turbine gearboxes to environmental extremes. This section is provided by Broadwind Energy and WPE&D Editor Paul Dvorak.
WINDPOWER ENGINEERING & DEVELOPMENT — 2016 RENEWABLE ENERGY HANDBOOK
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AeroTorque Corporation 1441 Wolf Creek Trail PO Box 305 Sharon Center, OH 44274 United States www.aerotorque.com
Winergy Drive Systems Corporation 1401 Madeline Lane Elgin, IL 60124 United States www.winergy-group.com/cms/website.php
ZF SERVICES 777 Hickory Hill Drive Vernon Hills, IL 60061 United States www.zf.com
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12/14/15 11:32 AM