Airgap Eccentricity - Case History 1
Large HV, 3-Phase SCIM - Pump Drive

EXPERT ANALYSIS & RECOMMENDATION

Analysis of the motor current spectra shown in Figure e-1 shows that there are twice slip frequency sidebands due to inherent rotor asymmetry but they are 60 dB down on f1 - the rotor winding is healthy.

The current spectrum in Figure e-3 is indicative of abnormal airgap eccentricity.

Airgaps to be firstly checked on site.

Inspection Results

The motor's nominal airgap length is 2.54 mm (100 thou/mils) and the manufacturer allowed a tolerance of ±5% (total of static and dynamic eccentricity).

The rotor was manufactured to have a total run out of 0.05 mm (2 mils) giving a dynamic eccentricity of 2%.

The motor airgaps were checked on-site and the airgap eccentricity was found to be 35% and 20% at the drive-end and non-drive end respectively.

For a large motor a 35% level of airgap eccentricity is considered to be severe.

The motor was removed to a repair workshop and the rotor run out was checked and was found to be as per the original specification.

The fundamental problem was therefore static eccentricity.

The running airgap eccentricity could have been higher and the high static eccentricity would produce a high U.M.P.with a possible increase in dynamic airgap eccentricity due to any rotor bow as a result of high U.M.P.

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