Trends in sliprings

Sliprings work in the turbine’s hub where they transmit power and signals from stationary cables in the nacelle to rotating equipment in the hub. A slip-ring assembly is often a graphite or precious metal brush that contacts the outside diameter of a rotating metal ring. As the ring turns, electrical current or signal is conducted through the stationary brush to the metal ring making the connection. Additional ring-brush assemblies are stacked along the rotating axis for more electrical circuits.

In pitch slip rings, the trend in new designs is on two fronts. “First, as turbine designs increase in power output and size, the demand on the pitch slip ring is higher power transfer,” says Moog’s Senior Business Development Manager Steve Black. “Second, data handling and signal demands are increasing to provide additional condition monitoring in the hub.  It is critical that the contact technologies be durable enough to handle power peaks at the extremes of operation.”  Equally important is a need to protect data lines from potential cross talk with power lines.

Another trend is toward more complete pitch controls. “These would include hub controls, pitch drive motors, power back-up, blade monitoring, and the slip ring to transfer the power and control signals through the rotary interface,” says Black. The company supplies complete systems to the OEM and also offers aftermarket pitch slip rings to improve system performance.

Also look too for new pitch control slip rings in the aftermarket. “The wind turbine service market is growing tremendously as the turbine population ages and asset owners are look for ways to control maintenance costs and maximize turbine availability and output. Products should be able to replace slip rings in older turbines with long-life, high-reliability design to provide pitch slip rings that can operate for years without maintenance.

“Ideal pitch slip rings should be installed and forgotten. Many designs in use today require periodic maintenance to clean away wear debris, flush out oil and dirt, and relubricate. Often replacement of the entire slip ring is done too frequently.  Keeping the turbine technicians out of the nacelle will keep costs down and outputs up.  That is the goal for asset owners and the focus of efforts in the aftermarket portion of our wind energy business.

The high-reliability quest has let some companies devise non-contacting slip rings. One is an inductively coupled power transfer device. Inductive-power transfer applies the scientific principle of electromagnetic inductance. The manufacturer of one says its sliprings work at high efficiencies by using wireless power transfer that can harmonize itself in the field. This makes the systems resistant to changes in real world environmental conditions and load variations, says developer New Zeeland-base PowerbyProxi.

For wireless power, transmission distances can reach to 6 to 8 ft, but efficiency levels between transmitter and receiver are low. Commercial applications in which transmission distances are less than 8 in. allow efficiencies over 90%.

Power transmits as a single load while a built-in data-communications system switches power, for example, to meet different device requirements of wireless receivers. In this way, the slip rings provide wireless power and data to pitch control in wind turbines, improving wind turbine efficiency by reducing the cost of maintaining and replacing existing mechanical slip rings and their carbon brushes. Any size turbine can be supported, says the company.