The design and manufacturing of a slip ring component is imperative to the overall function and efficiency of a wind turbine.
Wind turbines require reliable transmission of power and data signals from the nacelle to the control system for the rotary blades – none of which happens without the slip ring. A slip ring is used to transfer electric current from a stationary to a rotating unit. It’s a small component compared to gearbox bearings or generators, but if it’s defective, it can make or break the performance of a turbine.
When a slip ring fails, power and communication data cannot pass through to pitch mechanisms and other controls in the hub and top box; this can cause a wind turbine to shut down. This is also why manufacturers have devoted a great deal of engineering and research to developing wind turbine slip rings that last longer and require less maintenance.
Over the years, engineers have seen many different types of failures in hub slip rings, as it is a difficult environment. Once such problem they have encountered is under-designed circuits for the amperage they are carrying. This is particularly common in “wire brushes” where there have been burnt back due to excess current. Over time, excess current will greatly weaken the contacts, and eventually lead to failure.
From external factors (such as harsh environments) to internal mechanisms (like using solid brushes and rings stacked to save space), there are many factors to consider when choosing a slip ring for your wind turbine.
Regardless of the design, it is important to consider the quality of the materials used, the annual maintenance required, and the overall life expectancy.
A wide selection of circuitry is available depending on the power requirements, with many combinations of amperage and voltage (ac or dc). Today’s advanced designs can transfer higher wattage with decreasing power loss. High quality slip rings have handled over 55 kW for pitch control motor use with circuits rated over 100 amps and 690 VAC.
Wattage transfer capacity and power losses may be affected by various factors, which is why custom designed slip rings are often recommended for wind turbine applications to ensure proper capacity and function.
Reducing Operations & Maintenance (O&M)
Most wind-turbine operators would agree that the fewer up-tower maintenance trips, the better. Replacing slip ring brushes is time consuming and costly. Extending the maintenance intervals saves money in the long run.
Traditional slip rings need frequent maintenance to avoid degradation of the rotating electrical connection, caused by regular wear and debris. In some cases, failures occur for simple reasons such as technicians not cleaning and lubricating some designs.
By using solid metal brush slip rings, manual cleaning is minimal and lubrication is eliminated. A higher spring pressure than that on conventional slip rings can also help clean the ring as it rotates.
Fortunately, there are more advanced designs that allow for slip ring assemblies to achieve 100+ million revolutions before brush replacement, again reducing maintenance and downtime. These designs typically come with built-in, lifelong lubrication, reducing the amount of maintenance required to about five minutes per year. Some turbine operators say that they have eliminated annual maintenance altogether.
Material considerations are important for reducing O&M. In a hostile environment, such as that common to remote wind turbine locations, high-grade slip ring materials are important to reduce surface degradation. For instance, you may think that gold automatically translates into a better product, yet some wire-brush slip rings wear down their gold plating, resulting in lost conductivity and transfer capacity. That’s why it’s important to understand this essential component in your overall wind turbine performance. If you try to save money by cutting down on quality, you risk losing long-term productivity of the whole unit.
Jesse Shearer is Sr. Application/Design Engineer at United Equipment Accessories, Inc., which designs and manufacturers slip rings, hydraulic swivels, shift controls and cable reels for a wide variety of industries and customers around the globe.