When analyzing current signature analysis, like the PdMA EMAX Rotor Evaluation Test, the primary focus starts at the Pole-Pass-Frequency (Fp) sidebands around line frequency. Elevated Fp sidebands can be indicative of a damaged rotor cage or shorting ring. An additional confirming piece of evidence towards a damaged rotor cage or shorting ring is the presence of multiple Fp sidebands with harmonics below the 5th harmonic. While you are looking at the 5th harmonic, don’t forget to look for a split peak as well. A split 5th harmonic peak (i.e., two peaks side-by-side) can be indicative of axial shaft movement resulting from misalignment, improper bearing loading or a magnetic center offset. Using a strobe light for non-intrusive visual correlation is recommended. Marking the shaft and running the motor uncoupled may be required to confirm the magnetic center.
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