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a turbine’s control system will simplify encoder installation and monitoring.” Extreme temperatures, electrostatic discharge, vibration, and exposure to hydraulic fluids can also impact and shorten the life and performance of turbine components, including encoders. “Reliability is crucial,” says Fell. “Chronic vibration means the general mechanical durability of encoders is a must. But weather and environmental concerns are also an issue. Wind turbines present harsh conditions for instrumentation, including extreme temperatures, moisture, dust, and other contaminants.” He points out that encoders should carry a minimal ingress protection rating of IP67. A higher IP class is recommended for offshore turbines with a salt-spray resistant IP69K housing. Safety is also an important consideration. For yaw positioning, choose encoders with integrated end switches, says Fell. “In a typical design, the nacelle should not turn more than 3.5 revs total range so as not to damage the power or control cables running between the nacelle and tower. Because these end switches are derived from the encoder position, a safety rating of at least SIL2 is necessary.” While maintenance is a concern for most turbine components, Jesse Shearer, Sr., an Application and Design Engineer at UEA, claims that industrial encoders should be made to last. “Most should last the life of the slip ring but if a device fails, they’re typically quite easy to replace. Once accessed, a couple of screws and an electrical connector is all that’s needed,” he says. Granted, this is assuming a wind technician is set and ready to safely climb uptower.
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To prevent encoder failure, Shearer suggests adhering to the manufacturer’s recommendations and not over-taxing the device. “The most common failure mode with an encoder once it’s made it to the field is a bearing or shaft failure,” he says. “So avoid overloading the shaft on the encoder, which will cause the optical disk to break.” Optical encoders, which use an optical disk and a reader, tend to experience the most breakdowns. But the wind industry is slowly beginning to incorporate another option. “The industry is moving more toward magnetic encoders, which are finally trending down in price and are much more robust than optical encoders,” says Shearer. Magnetic encoders are available in incremental and absolute versions. These sensors can detect a change in magnetic field and convert this information to a sine wave. They are ideal for use in wind turbines because of how well they withstand high temperatures and environments with extreme shock and vibration. “The latest generation of compact and highly accurate magnetic encoders are appealing to the wind industry,” agrees Fell, and says they are easy to integrate into new or existing turbine designs. “Some advanced control systems even optimize energy production by making small adjustments to the pitch of blades as they pass in front of the support tower.” This is possible because of the quick response time of magnetic encoders. “These magnetic devices eliminate the need for mechanical contact between sensing elements. This reduces wear and prolongs longevity — the goal for any turbine owner or manufacturer.”
By Michelle Froese Senior editor, WPE&D
WINDPOWER ENGINEERING & DEVELOPMENT — 2017 RENEWABLE ENERGY HANDBOOK
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