Traversing noises at the azimuth brakes of the wind tracking system make up a significant proportion of the noise emissions from wind turbines and are caused by the grinding of the brake pads on the brake disk when the nacelle turns into the wind. The brakes are released to such an extent that the azimuth motors can turn the nacelle into the optimum position. The brakes are then fully applied again to maintain the position.

Root cause analysis

The investigation of the travel noise has revealed a direct correlation between the noise level and the "glazing" of the brake pads. As long as the nacelle is correctly positioned in the wind, the azimuth brakes are pressurized with approx. 160 bar. As soon as the nacelle is tracked, this pressure is reduced to approx. 30 bar. This is sufficient to allow the azimuth motors to track the nacelle in a controlled manner.

As the brakes are always applied, small amounts of powdery friction material or particles are created between the pads and the disc. At the reduced pressures during tracking, some of the powder sticks to the surface of the pads and gives them a glass-like appearance. This phenomenon does not only occur in wind turbines, but can also be observed in other applications. As "glazing" is a slow process, it does not occur in new wind turbines, but can become a problem over time.

Development of a solution

International brake expert and part of Altra Industrial Motion Corporation Svendborg Brakes has analyzed the phenomenon in depth to support equipment manufacturers and independent maintenance companies. The result is a patented innovation, i.e. a permanent solution that can be implemented by Svendborg Brakes engineers or maintenance companies. By milling a specially shaped groove into the brake disc, existing glazing can be removed and the formation of new glazing prevented. The groove increases brake pad wear insignificantly, but in combination with the simultaneously installed brush it helps to remove adhering powdery material.

The tests have shown that the groove in the brake disk can remove the brake pad glazing, with wear increasing by only 3.5 percent at an operating pressure of 30 bar. These tests were carried out using brake pads from Svendborg Brakes with a slot in the friction material, which also helps to keep the braking surface free of dirt. What became very clear during the tests was the importance of the size and shape of the slot. Therefore, a practical system had to be developed to produce the most efficient groove design.

Provision of the solution

This resulted in a mounting kit for the installation of a total of eight grooves: four on the upper side and four on the underside of the brake disk. The assembly kit contains a template for correctly positioning the grooves in the disk. Reworking takes about three hours per installation. After Svendborg Brakes had developed the necessary equipment and the optimum process for applying the grooves, the company successfully applied for a patent for the solution. Since then, manufacturers and maintenance companies have been able to use this system for noise reduction on the azimuth brake under a license agreement.

In any case, it is important to correctly diagnose the source of the noise during wind tracking, as it does not necessarily have to be a glazing problem on the brake pads. It is particularly important to check for oil leaks from the hydraulic units and the azimuth bearing, as oil or grease on the brake disk is always problematic. The main bearing itself, incorrect engagement of the azimuth gearbox or a geared motor brake that is not being released are also possible sources of noise. Such faults can usually be rectified as part of routine maintenance and without special equipment or knowledge that goes beyond the normal procedures for wind turbines.

Over a period of 18 months, 25 wind turbines from various manufacturers were modified using the Svendborg Brakes groove method. Since then, none of these turbines have experienced any noise from the azimuth brake. In addition, turbines not equipped with Svendborg Brakes components have also been successfully modified using the patented process. This means that a solution is now available for the majority of wind turbines. am