Diagnostic system for unbalanced motor shafts for high speed compressor

A method for determining whether a rotor shaft of a compressor is unbalanced, said method comprising:

• determining a distance between an end of a displacement sensor and the rotor shaft as it rotates;
• providing a frequency signal representing the distance between the end of the displacement sensor and the rotor shaft wherein the frequency signal is indicative of the balance of the rotor shaft;
• bandpass filtering the frequency signal to provide a single unbalance frequency signal;
• rectifying the bandpass filter signal to provide a positive unbalance frequency signal;
• low pass filtering the positive unbalance frequency signal to convert the unbalance frequency signal to a DC signal whose amplitude is indicative of the balance of the rotor shaft;
• and
• comparing the amplitude of the DC signal to at least one threshold to determine whether the rotor shaft is unbalanced, wherein comparing the amplitude of the DC signal to at least one threshold includes comparing the amplitude of the DC signal to a low threshold and a high threshold, and wherein if the amplitude of the DC signal is greater than the low threshold and less than the high threshold, a warning signal is provided and if the amplitude of the DC signal is greater than the high threshold, the system is shut-down and/or its performance is reduced.