An operating method is used for a compressor of a refrigerating machine, where the compressor includes an electric motor- and a power supply device configured to modulate a supply voltage and/or current and/or frequency for driving an electric motor. The method includes (a) setting a stator resistance calibration value and at least one threshold value for a control parameter, the threshold value being correlated to the calibration value; (b) injecting, continuously during operation of the electric motor, a disturbance signal into the electric motor (c) detecting a resulting voltage signal and a resulting current signal, corresponding to the perturbation signal; (d) associating with the control parameter a value calculated as a function of the resulting voltage signal and a resulting current signal; and (e) regulating or interrupting the supply voltage and/or current and/or frequency depending on the control parameter with respect to the at least one threshold value.