This note discusses the cup-anemometer dynamics in a slightly different presentation. Further it contains af presentation of the oversimplified Schrenk model which illustrates in detail how the non-linearity of the anemometer causes overspeeding and also shows that without this property is responsible for the ability for the rotor to start rotating from zero wind speed. There is a short discussion of the spectral analysis, necessary for the understanding of the dynamics. The equations for overspeeding is derived for the neutral atmospheric surface layer and extended to include overspeeding in wind tunnel calibrations, albeit the turbulence here is in general very small.