A healthy motor draws equal and balanced 3-phase current. A motor with a defect does not do so. Motors can have a variety of problems. The problems cause distortion of wave shape of current.

20% of all the induction motors in use suffer from problems such as high resistance joints, cracked or broken rotor bars or air gap eccentricities.

FFT on a healthy motor current (Current Spectrum) will be a one line spectrum at supply frequency (Time wave form of current being pure sinusoid). FFT of defective motor will have side bands in the spectrum (Time wave form will be a corrupted sinusoid). The side bands, their number, intensity and separation decide type and intensity of problem.

It can be used in coordination with vibration and thermal analysis to confirm key machinery diagnostic decisions.

A number of results have been correlated with actual motor condition and knowledge exists to interpret the current spectrum and know about problems in their incipient stage.