What criteria should be considered when selecting and maintaining encoders for wind turbines?

Wind turbines require accurate and reliable blade pitch-control to optimize wind generation and safeguard the asset from extreme conditions that could cause damage. Turbine generators also rely on precise feedback to properly control and synchronize energy output with line frequency. Disruption of these processes can lead to poor turbine performance, downtime, or costly repairs.

Encoders are small devices that play a big role in productive wind-turbine operations. These sensors provide reliable feedback for wind speed, control, over-speed protection, and position.

Here an encoder has been mounted on a slip ring for installation in a wind turbine. Key attributes to consider when choosing encoders include electrical characteristics (such as operating voltage and output configuration), shock and vibration resistance, operating temperature, lightning protection, and maximum shaft load. (Photo: UEA)

“Encoders or rotation sensors, can be used in a number of places on wind turbines,” explains Christian Fell, COO of FRABA-POSITAL North America, an industrial sensor engineering company. “Incremental encoders are ideal for monitoring the rotation speed of the rotor. They transmit a stream of electrical pulses at a frequency that is proportional to the speed of rotation of the encoder’s shaft.”

“On the other hand, absolute encoders measure the absolute rotation angle, and are typically used to provide feedback on blade pitch and the azimuth or yaw angle of a nacelle,” he adds.

Application encoders can be located deep within a turbine’s nacelle and hundreds of feet off the ground so reliability is key, according to Fell. “Devices with communications interfaces that are compatible with 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 United Equipment Accessories, 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.

To prevent encoder failure, Shearer recommends 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.”

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