A method for detecting an evolution of a magnetic state of a permanent magnet rotor in a three phases synchronous motor using a variable speed drive, the method comprising: - during a first time interval: ∘ applying, using the variable speed drive, a first direct current command signal to the motor; characterized by ∘ in response to the applying the first direct current command signal to the motor, estimating a first stator resistance value; ∘ applying, using the variable speed drive, a first alternative voltage command signal to the motor, the first alternative voltage command signal corresponding to a specific direct axis stator current value; ∘ measuring, during the applying of the first alternative voltage command signal to the motor, first stator phase current values; - determining, in function of the measured first stator phase current values and of the estimated first stator resistance value, a first rotor flux parameter; - during a second time interval: ∘ applying, using the variable speed drive, a second direct current command signal to the motor; ∘ in response to the applying the second direct current command signal to the motor, estimating a second stator resistance value; ∘ applying, using the variable speed drive, a second alternative voltage command signal to the motor, the second alternative voltage command signal corresponding to the specific direct axis stator current value; ∘ measuring, during the applying of the second alternative voltage command signal to the motor, second stator phase current values; - determining, in function of the measured second stator phase current values and of the estimated second stator resistance value, a second rotor flux parameter; - comparing the first and the second rotor flux parameters by detecting a difference between the first and the second rotor flux parameter, whereby: ∘ the first time interval has a first interval length; ∘ the second time interval has a second interval length; and ∘ the first and second time intervals are separated by a time period which exceeds at least 100 times any one of the first or second interval length. - during a first time interval: ∘ applying, using the variable speed drive, a first direct current command signal to the motor; characterized by ∘ in response to the applying the first direct current command signal to the motor, estimating a first stator resistance value; ∘ applying, using the variable speed drive, a first alternative voltage command signal to the motor, the first alternative voltage command signal corresponding to a specific direct axis stator current value; ∘ measuring, during the applying of the first alternative voltage command signal to the motor, first stator phase current values; ∘ applying, using the variable speed drive, a first direct current command signal to the motor; characterized by ∘ in response to the applying the first direct current command signal to the motor, estimating a first stator resistance value; ∘ applying, using the variable speed drive, a first alternative voltage command signal to the motor, the first alternative voltage command signal corresponding to a specific direct axis stator current value; ∘ measuring, during the applying of the first alternative voltage command signal to the motor, first stator phase current values; - determining, in function of the measured first stator phase current values and of the estimated first stator resistance value, a first rotor flux parameter; - during a second time interval: ∘ applying, using the variable speed drive, a second direct current command signal to the motor; ∘ in response to the applying the second direct current command signal to the motor, estimating a second stator resistance value; ∘ applying, using the variable speed drive, a second alternative voltage command signal to the motor, the second alternative voltage command signal corresponding to the specific direct axis stator current value; ∘ measuring, during the applying of the second alternative voltage command signal to the motor, second stator phase current values; ∘ applying, using the variable speed drive, a second direct current command signal to the motor; ∘ in response to the applying the second direct current command signal to the motor, estimating a second stator resistance value; ∘ applying, using the variable speed drive, a second alternative voltage command signal to the motor, the second alternative voltage command signal corresponding to the specific direct axis stator current value; ∘ measuring, during the applying of the second alternative voltage command signal to the motor, second stator phase current values; - determining, in function of the measured second stator phase current values and of the estimated second stator resistance value, a second rotor flux parameter; - comparing the first and the second rotor flux parameters by detecting a difference between the first and the second rotor flux parameter, whereby: ∘ the first time interval has a first interval length; ∘ the second time interval has a second interval length; and ∘ the first and second time intervals are separated by a time period which exceeds at least 100 times any one of the first or second interval length. ∘ the first time interval has a first interval length; ∘ the second time interval has a second interval length; and ∘ the first and second time intervals are separated by a time period which exceeds at least 100 times any one of the first or second interval length.