There are three main types of slip rings used in wind turbines. The larger utility grade wind turbines require two slip rings; a hub slip ring and a generator slip ring. The smaller private type wind turbines only use a yaw slip ring. All three of these are completely different when it comes to the design and the challenges they present.
The yaw slip rings used in the smaller private type wind turbines are to allow the head of the turbine on top of the tower to rotate with the wind. The generator is located in the head of the turbine and rotates with the head, thus needing a slip ring below it to allow the power cables to go down tower and not rotate and twist. These slip rings are generally 4 power circuits. RPM with this type of turbine is very low but the mounting can sometimes be difficult. Some turbine manufactures require the slip ring to be inside the main vertical shaft which usually does not allow for much room. The other way of doing this is designing a slip ring to mount on the outside of this shaft, but then the slip ring is exposed to the environmental conditions.
The large utility grade wind turbines require both a hub slip ring and a generator slip ring. The hub slip ring is mounted on the back of the gearbox inside the nacelle and provides power along with data/signal to and from the hub. These slip rings can range in size and requirements depending on if an electric or hydraulic pitch system is being used. When an electric pitch system is used the slip ring is providing power to the motors that pitch the blades and also allowing communication to the hub. In the turbines that use a hydraulic pitch system, the slip ring is providing signals to the hub to control the hydraulic valves along with data transfer. The slip ring on the generator is an entirely different product as the gearbox is turning the generator approximately 1800rpm on some turbines. This requires a different brush material to handle the friction and not wear out rapidly. The core must also be perfect at this rpm or it can throw the brush out of contact from the ring.
These three slip rings are all used on the same type of product but all serve different purposes and provide different challenges.