Patent ID: 7911170

Claim:
Asynchronous machine ( 40 ) including a rotor ( 6 ) and a stator ( 7 ) and having a fixed number of pairs of poles, said rotor ( 6 ) having a predetermined defect, said asynchronous machine ( 40 ) comprising a current determination unit ( 2 ) for determining a stator current of said stator ( 7 ), said stator current having a stator frequency, a processing unit ( 3 ) for forming a stator current spectrum of said stator current, an analyzing unit ( 4 ) for analyzing said stator current spectrum and determining an inverse peak ( 26 ) and a corresponding inverse frequency in said stator current spectrum, said inverse peak ( 26 ) being the peak having the second highest amplitude in said stator current spectrum in the frequency range of said stator frequency, and a calculation unit ( 5 ) for calculating a mechanical rotor frequency of said rotor ( 6 ) from the sum of said stator frequency divided by the number of pairs of poles and said inverse frequency, if the slip of said asynchronous machine ( 40 ) is lower than 50%, or from the difference of said stator frequency divided by the number of pairs of poles and said inverse frequency, if said slip is higher than 50%.