Encoders play vital roles on wind turbine equipment but the environment is not always kind to them. A few guidelines help get the most from the devices.
Modern wind turbines use a number of controls to maximize operating parameters as well as ensure safety of its equipment and personnel. Encoders play a vital role in both of these operations. Too often, however, they are one of the least understood components in the turbine. From less advanced proximity sensors to state of the art absolute encoders, rotary feedback has a critical role in pitch, yaw, and generator functions.
A few apps
As encoder costs continue to decline, we see more applications that use these durable and precise devices. One common encoder application can be found inside the turbine hub on pitch control units. Encoders are either sandwiched between pitch motors and brakes or on the back ends of motors. You’ll find shafted encoders in these applications as well as hollow-shaft devices. An important element to keep in mind when looking at encoders for these applications is the temperature spec. As you would expect, encoders sandwiched between motors and brakes get hot, so it’s important to make sure the selected device can tolerate the heat. Specs up to 130°C are not unusual. On the other hand, using absolute encoders requires an understanding of field bus protocols, so it is a good idea to partner with a manufacturer with extensive bus communication knowledge.
Encoders are not built to tolerate heat. So when they get hot, a number of things can happen. For one, glued components can come apart and cause the device to fail. Also, signal quality can be affected by extreme heat causing the drive system to misunderstand the position reported by the encoder. Both of these failures can be avoided by understanding the encoder’s capabilities.
Another common wind-turbine application is on the back end of the generator. Encoders are used to regulate speed and, in some cases, determine position. Like the feedback used on pitch systems, generator feedback applications are considered mission critical. If either of these feedback applications fails, the turbine cannot produce power. Also expect encoder problems on those with undersized bearing on a generator. Failed bearings are the number one cause of generator-encoder failures. Electrical and thermal isolation further enhances feedback reliability.
Choosing the right encoder
Selecting the right encoder to use is neither complicated nor risky if you know the right questions to ask. If you understand the fundamentals of your application and you know the important aspects of feedback, then choosing the right path is easy. Each device has a few characteristics that need attention.
What output form will the application require? The two forms are incremental or absolute. These sound complicated, but they are not. If an application is incremental, then its encoder will need an output voltage, usually between 5 and 30 Vdc. Think of this design as a rotary switches, and like a switch, the encoder will either output 0 Vdc or, say, 24 Vdc. The output is either 0 or the supply voltage. So, if supplying an encoder with 12 Vdc, that will be the output. Because an output is either 0 or the input voltage, the output is always a square wave. The low value is always 0 while the high value is that provided on the input.
We describe the resolution of incremental encoders in terms of pulses per revolution or ppr. A common incremental resolution is 1,024 ppr. So for every complete rotation of the encoder you will see 1,024 outputs. Another common resolution, 2,048 ppr, has twice the resolution as 1024. You’ll find incremental encoders with many different resolutions, from 1 all the way to 10,000 ppr. Most applications in the wind turbine will be 1,024 or, on the generator, you’ll see 3,072 ppr.
Absolute encoders are a little different, but operate in much the same way. They rotate and output data is relative to their position. Remember, incremental encoders only output voltage. Each output is exactly the same. Therefore, unless your system is keeping track of the number of outputs (which it is) then you have no way of knowing what the reported position is. Absolute encoders do it a little differently. They output a digital word. That is, they output a unique digital word for each individual position. They also output in several different field bus communication protocols. Instead of using pulses-per-revolution to describe their resolution, they use bits. Rather than say an absolute encoder has a resolution equal to 1,024 ppr, we would describe it as 10 bits. It’s also important to understand what field bus your system uses. It could be DeviceNet, Profibus, BiSS, or others. Other characteristics to consider include:
Protection class and environmental specs. Ask: Will the device be exposed to direct water spray? At this writing, probably not. But, if a maintenance team is supporting turbines in southwest Texas, expect heat and dust. These can kill an encoder quickly so taking time to understand a manufacturer’s capabilities can save time and headaches.
How will it be mounted? Devices have either a shaft or a hollow center section, called a hollow shaft. We’ve seen cases where users had the wrong mounting style and tried to force it to work. This is a good place to get your encoder vendor involved in the application so you can benefit from their application knowledge and experience. And the earlier, the better. This point brings us to one of the most considerations when choosing your feedback: Support.
Wind plants are all over the United States with many in remote locations. Getting the right kind of support in these places can be challenging. Hence, at the top of the list, be sure to partner with a manufacturer that has application and technology knowledge along with a support network available and able to quickly and professionally support you onsite. The difference between turbines being on-line or off-line can be as simple as having the right players on your team. WPE
Timothy C. Cutts, OEM Manager, North America Dynapar, Royse City, Texas
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