The present invention relates to a device (20) and a method for sensorless measuring a mechanical rotor frequency of a rotor (6) of an asynchronous machine (40), wherein the rotor (6) has a predetermined defect and the asynchronous machine (40) has a fixed number of pairs of poles. The asynchronous machine (40) comprises a current determination unit (2) for determining a stator current of the stator (7), wherein the stator current has a stator frequency. A processing unit (3) forms a stator current spectrum of the stator current. An analyzing unit (4) analyzes the stator current spectrum and determines an inverse peak (26) and a corresponding inverse frequency in the stator current spectrum, wherein the inverse peak (26) is the peak having the second highest amplitude in the stator current spectrum in the frequency range of the stator frequency. A calculation unit (5) calculates a mechanical rotor frequency of the rotor (6) from the sum of the stator frequency divided by the number of pairs of poles and the inverse frequency, if the slip of the asynchronous machine (40) is lower than 50%, or from the difference of the stator frequency divided by the number of pairs of poles and the inverse frequency, if the slip is higher than 50%.