Active dampers: Combating Vibrations with Vibrations

In nine years, over 2500 systems have been delivered for more than 15 different wind turbine types from five original equipment manufacturers (OEM). With the active vibration damper ADD.Sound, Wölfel is a technology leader in reducing gearbox tonalities.

The objective evaluation of noise in wind turbine approval procedures is not only about the volume, but about also whether individual tones are audible within the noise – in other words, its tonality. Tonality, perceived as humming, for example, typically increases the disruptive effect of the noise. Therefore, it is considered in the evaluation of sound emissions according to FGW TR1 Rev 19 and IEC 61400-11 through a tonality surcharge.

High relevance in the approval process

This can become relevant in the approval process for wind turbines, because the surcharge can be up to +6 dB in the assessment level at the point of immission. It becomes critical when systems have to bypass particularly noticeable speed ranges to comply with approval values.

In case of wind turbines with gearboxes, periodic meshing forces of the gear stages with the respective meshing frequencies cause tonality; in case of gearless wind turbines, harmonically varying forces of the magnetic field in the generator cause it. The problem of tonality in wind turbines tends to increase because higher power classes of the systems lead to two opposing developments.

For example, optimizing the blade design reduces the overall sound of new systems, making them quieter. However, at the same time, the occurring torques increase with the growing power sizes, and potentially, the tonal excitation level through the gearbox also increases. The ever better integration of the powertrains – such as the direct transfer path from the excitation at the gearbox to radiating elements – also causes an increasing susceptibility to tonality. In addition, the elimination of masking background noises often highlights individual tones more strongly.

Damping tonalities with active dampers

To damp tonality, active dampers have established themselves as a preferred method. They consist of an electronically controlled spring-mass system, which detect the operating vibrations via sensors and generate a counterforce via the controller. This force has the same frequency as the excitation vibration, but is applied in antiphase and thus counteracts the disruptive operating vibration of the respective component (such as the gearbox).

This compensates for the vibration directly at the source and suppresses the tonality. As active dampers also operate at variable speeds, the entire speed range is covered. Depending on the type of issue, several dampers (e.g. three) are installed in a wind turbine. With the ADD.Sound active vibration damper for wind turbines, Wölfel offers a unique solution for all tonality issues and, with more than 2500 systems delivered, is the technology leader in the field of active dampers for reducing gearbox tonalities.

In nine years since the product launch, ADD.Sound active vibration absorbers have already been installed in more than 15 different wind turbine types from five OEMs. Moreover, as a modular retrofit package, ADD.Sound can also be installed in existing systems and is suitable for all types of systems, both with and without gearboxes.

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