Motion control for wind, solar and marine renewables

In short:Renewable energy machines run unattended for decades in wind, salt and temperature extremes, and every service visit is expensive. Specify slip rings, drives and actuators on revolutions or cycles over the design life, sealing and corrosion class, cold-start behaviour and maintenance interval, not on catalogue torque alone.

Typical requirements

What renewable energy asks of motion components

Wind turbines, solar trackers and wave and tidal devices share a hard brief. They run unattended for decades, outdoors or under water, in wide temperature ranges, with service visits that need cranes, vessels or rope access. A component that would be fine in a factory, with a fitter on hand, can be the wrong choice here. Life, sealing and maintainability decide the specification as much as torque or speed.

Design life is the starting point. IEC 61400-1 sets the design requirements for wind turbines, including the wind classes and the design lifetime the turbine is built to. Solar trackers are qualified to IEC 62817, and marine energy converters fall under the IEC TS 62600 series. Each motion component should be specified against the same life.

Wind turbines: pitch slip rings and hub electrics

The pitch system rotates each blade about its own axis to control power and to feather the rotor to a stop. Power and signals for the pitch drives in the spinning hub cross from the nacelle through a slip ring, as Windpower Engineering describes. That slip ring is one of the most failure-sensitive parts on the turbine, because if it stops passing signals the turbine shuts down.

What a hub slip ring carries:

• Power for the pitch motors and hub heaters, often hundreds of amps in total. Moog’s wind slip ring range quotes modular power rings up to 500 A continuous and a high-current brush holder up to 1,000 A for lightning protection.
• Fieldbus and safety signals: CAN, CANopen, Ethernet, serial and discrete circuits. Fibre-optic multiplexers can put EtherCAT and diagnostics on a single fibre, and non-contacting links reach 1 Gb/s.
• Earthing and lightning current, which needs its own path so it does not pass through bearings or signal contacts.

The Moog wind power assemblies in our catalogue combine slip rings with hydraulic rotary unions for pitch systems, at up to 48 circuits, 130 A and 575 V with CAN, Ethernet and PROFIBUS data. Smaller Rotoflux rings such as the ESR 110 (gold-on-gold, IP64, up to 32 circuits at 30 A) show the scale of a signal-and-power ring for lower-current hubs. See our page on slip rings for wind turbines and the high-current and fibre optic rotary joint types.

Worked example: slip ring life in revolutions

1. Assume a rotor running at about 12 rpm (check your turbine’s rated speed).
2. At 95% operating time: 12 × 60 × 24 × 365 × 0.95 = about 6.0 million revolutions a year.
3. Over 20 years: about 120 million revolutions.
4. Compare with published ratings: Moog quotes an operational life of 100+ million revolutions for its wind rings, and 200+ million with five years maintenance-free for its F 5784 X.

The example shows why a “100 million revolution” ring may need one mid-life service on a fast rotor, and why maintenance interval belongs in the specification. Moog’s datasheet names wear debris from the power section as a common failure cause: dust bridges insulation between circuits and causes short circuits. Ask how the design keeps power and signal sections apart and how debris is managed.

Pitch drives and backup power

Electric pitch systems give each blade its own drive. The Windpower Engineering example uses a 3-phase, 4-pole asynchronous motor on a pitch gearbox, with position sensors on each blade. Others use permanent-magnet servo motors with absolute multi-turn encoders. All need:

• High-ratio planetary gearing to the blade bearing gear ring. Bonfiglioli’s 700TW range spans 16,000–140,000 Nm stall torque for pitch, 60,000–260,000 Nm for yaw, with ratios from 60:1 to 3,000:1. Browse gearboxes.
• Fail-safe feathering. Each pitch system must turn its blade to about 90° if power is lost. Backup comes from batteries or ultracapacitors on the drive DC link. The published example quotes more than 30 seconds at nominal power and -40 to +65 °C for the ultracapacitor module; batteries need replacing as they age.
• Robust feedback. Absolute encoders that survive vibration, condensation and cold. Harsh-environment capacitive encoders such as the Netzer VLP (-40 to +105 °C, MIL-STD-810 tested) are one option for blade or motor feedback.
• Drives rated for the hub: vibration, wide temperature, and the backup supply’s DC voltage range. See servo drives.

Some turbines use hydraulic pitch instead, which is where the combined slip ring and rotary union assemblies come in.

Yaw drives and yaw slip rings

The yaw system turns the nacelle into the wind using several geared motors with integral pinions on a large gear ring, plus yaw brakes. Yaw moves are slow and occasional but carry very high torque and shock, and the drives must hold against gusts. Specify:

• Stall and holding torque per drive and the load share between drives
• Brake holding torque and whether brakes are on the motor, the ring or both
• Cable twist: most nacelles allow a limited number of turns before untwisting. Where continuous yaw is needed, a large-bore through-bore slip ring such as the 4 in (101.6 mm) bore Moog AC6098 may be used for signals and moderate power.

Solar trackers: slew drives and linear actuators

Single-axis trackers turn rows of PV modules east to west through about ±55–60°. Two drive styles dominate:

What matters when choosing:

• Wind holding. The drive must hold the row against wind and resist being backdriven. Self-locking worms and screw actuators with load-holding nuts do this mechanically. Distributing several drives along the row reduces torsional twist of the torque tube.
• Life in small moves. Trackers move in many short steps every day for decades, so specify life in cycles and starts, not hours at full load.
• Sealing. Dust, sand, rain and washing water. Heavy-duty DC actuators such as the Thomson Electrak HD (12/24/48 V, IP67/IP69K static, -40 to +70 °C) are built for outdoor mobile machinery and show the sealing level to aim for.
• Power and control. Self-powered trackers run from a dedicated PV module or the string with battery backup, so drive efficiency matters.

See linear actuators, how to size a linear actuator and hydraulic to electric actuator for related selection guidance.

Wave and tidal energy

Tidal turbines use pitch and yaw mechanisms, generators and slip rings much like wind turbines, but under water. Wave energy converters use large linear or rotary power take-offs and actuators for control and survival. The design brief is harsher: immersion, biofouling, corrosion, high shock loads from waves, and very expensive maintenance at sea. Expect sealed or pressure-compensated housings, marine slip rings, fibre for data, and robust absolute feedback. The IEC TS 62600 series covers design and testing of marine energy converters. Our subsea ROV and AUV page covers sealing and pressure compensation in more depth.

Environment and maintenance

• Temperature: cold starts at -30 °C and below stiffen grease and seals and raise breakaway torque. Ask for low-temperature data, heaters where needed, and slip ring contact performance at the cold end.
• Corrosion: match coatings and materials to the ISO 12944 corrosivity category of the site; offshore and coastal sites are far more aggressive.
• Condensation: large daily swings pump moist air into housings. Breathers, drains and anti-condensation heaters matter.
• Lightning: give lightning current its own path through the hub and nacelle.
• Condition monitoring: encoder diagnostics, motor temperature, brush wear sensors and drive fault logs let operators plan visits instead of reacting to faults.

Typical pitfalls

1. Specifying slip ring life in years instead of revolutions at your rotor speed.
2. Mixing power and signal sections so brush debris bridges signal circuits.
3. Forgetting cold-start breakaway torque in yaw, pitch and tracker drives.
4. Sizing tracker drives for tracking only, not for holding and survival wind loads.
5. Leaving backup power ageing out of the maintenance plan.

Send us your design life, loads, environment and maintenance targets, and our specialist engineering and supply partners can propose slip rings, drives and actuators built for them.

Standards to have in view

• IEC 61400-1 (wind turbines: design requirements)
• IEC 61400-3-1 (design requirements for fixed offshore wind turbines)
• IEC 62817 (photovoltaic systems: design qualification of solar trackers)
• IEC TS 62600 series (marine energy: wave, tidal and other water current converters)
• IEC 60529 (IP codes)
• ISO 12944 (corrosion protection of steel structures by protective paint systems)

Which standards apply depends on the end product and its market. Your engineer will confirm what the component itself needs to meet.