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 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.

“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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The role slip rings play in wind turbines depends on their size. In large wind turbines, slip rings transfer power to motors in the blades and data back to the hub from sensors in the blades. This lets the blades pitch into and out of the wind to control the speed of rotation.

The XZERES model 442SR has a rated output of 10.4 kW at 25 mph. It uses the slip ring to transfer power and control signals through the yaw axis and down-tower to power electronics. The alternator, a 3-phase neodymium permanent magnet design, mounts in the tower and is spun by the 23.6-ft diameter rotor. The company also manufactures 3.7 and 50-kW models.

In small wind turbines, such as those with outputs from 3 to 50 kW, cables from the generator located in the nacelle at the top of the tower, typically carry power down to a transformer at the base of the turbine. As the nacelle of the turbine rotates into the wind, a slip ring prevents the cables from twisting into a knot.

XZERES, one of the small turbine manufacturers, sells its products to the global market. When the company’s engineers designed its 10 kW wind turbine, they needed to set it apart in a crowded marketplace. The dependability of slip rings manufactured by UEA has helped its staff do just that. “UEA’s slip rings have been very dependable and have required very little maintenance,” said Ben Fleskes, engineering manager for the turbine manufacturer.

The CAD model of a slip ring from UAE is typical of the physical model used in the XZERES turbine.

In the XZERES model 442SR, slip rings transfer power and control signals through the yaw axis. “The slip ring is critical in transferring power and signal from our generator to our power conditioning system,” he said. “Because this is an up-tower component, a robust design supported by high-quality manufacturing is critical to our company,” said Fleskes.

“The experts at UEA work with us to create custom applications using robust and reliable brush assemblies, and it’s always a pleasure to work with them,” he said.

XZERES is headquartered in Wilsonville, Oregon, and has offices in the UK and Japan. It employs 40 people and exports 90% of its turbines. UEA is based in Waverly, Iowa. It manufactures dependable components that leading OEMs rely on, including custom slip rings, cable reels, shift controls, and hydraulic swivels.

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UEA’s quality department found opportunity in this situation and launched a new nonconformance reporting process and a corrective and preventive action process. By May 2016, UEA’s monthly PPM had dropped to 1,380.

“The significant decrease in defective parts was a true team effort,” said Dhaivat Patel, quality engineer at UEA. “It took the cooperation of all employees and the management team.”

              Progress and a talley on the PPM goal
The company’s goal was to reach 6,000 PPM by April. To achieve this goal, UEA’s quality department first introduced the NCR (nonconformance report) with severity gates (1 to 3) into the process.

Gate 1: Nonconformance reported by same department where part/process is being manufactured

Gate 2: Nonconformance found by other department/by in-process quality inspector

Gate 3: Nonconformance found during final quality inspection or by team leaders

With the introduction of this NCR gate system, a healthy competition was created between departments.

“Because no department wants to receive NCRs, everyone becomes more self-aware and diligent in cross-checking their work and their coworkers’ work,” said Patel.

By using this gate system, UEA developed the internal customer concept between departments. With the help of all employees, UEA quality management’s new nonconformance system has become an integral part of the total quality management system. Each employee is accountable and does everything they can to ensure a quality product reaches customers.

“The key to reducing the PPM is hiding inside the company’s nonconformance report process and supplier quality,” said Patel. “Other companies can follow our lead and experience dramatic results, too.”

For more information about UEA’s new initiative, visit www.uea-inc.com.

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