Figure e-5
Additional spectrum analysis around f1 to detect ±fr present
in this case history with abnormal airgap eccentricity

Figure e-6
Additional spectrum analysis around f1 to detect ±fr.
No components found in this motor which was normal and of the same design.

It is known that airgap eccentricity can also induce current components at:

Figures e-5 and e-6 show the current spectra around f1 for the faulty and healthy motor.

There are clearly components at ±12.4 Hz around f1 in the faulty motor but they are not present in the healthy motor.

Note fr = ±12.4 Hz = ± rotational speed frequency of rotor (Nr = 744 r.p.m.).

This is further confirmation that there was an airgap eccentricity problem. However a word of caution in using only equation (E-3) for airgap eccentricity analysis.

Components at f1 ± fr can be induced when the airgap eccentricity is normal and can be due to normal mechanical phenomena from rotor dynamics in complex drive chains (click on Mechanical Influences in main navigation buttons, above).

In this case history there was a direct coupling between the motor and pump and the motor had plain (oil film) bearings. Equations (E-1) and (E-2) should initially be used to determine airgap eccentricity problems in all cases.

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