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

Background and Test Objective
Four motor-pump drives were used to pump oil from an oil storage tank farm into large tankers in a nearby deep water estuary.

One of the motors had developed a bearing vibration and temperature problem and would only run for 45 minutes since it tripped out on high bearing temperature (95°C / 203°F).

The fundamental cause of the problem could not be determined via vibration analysis.

MCSA was applied to determine the condition of the rotor winding and whether airgap eccentricity was the problem.


Figure e-1
Current Spectrum - healthy rotor cage winding

Nameplate details
3-Phase, SCIM, 11 kV, 103 A,
1.45 MW/1944 h.p., 50 Hz, 742 r.p.m.

Additional Information
8-pole, number of rotor slots = 62
Direct drive coupling
Full-load slip = 0.01 or 1%
Airgap length 2.54 mm (100 thou) ±5% (manufacturer's spec.)


At full-load the twice slip frequency sidebands due to broken rotor bars will be :
±2sf1 = 2 x 0.01 x 50 = ±1 Hz

The motor was operating at slightly less than the full-load current : the input current was 91.3 A compared to the nominal full-load current of 103 A. This means that any sidebands due to broken rotor bars will be slightly less than ±1 Hz around f1.

There are sidebands at ±0.846 Hz around the supply frequency at 50 Hz but they are 60 dB down on f1 and are due to inherent rotor winding asymmetries, for instance in bar-to-end ring joints.


frs = f1((R/p)(1-s) ± nws) ......... (E-1)


Figure e-2
Current Spectrum to show rotor slot passing frequencies.
The frequency resolution is 1.0 Hz/line



fec = f1(((R ± nd)/p)(1-s) ± nws) ...... (E-2)




Figure e-3
Current Spectrum indicitive of an airgap eccentricity problem.
The frequency resolution is 0.1Hz/line




Equation (E-1) is used to predict the rotor slot passing frequencies :

The number of rotor slots is required and as a first step the full-load slip can be used in equation (E-1)

With f1 = 50 Hz, R = 62, p = 4, s = 0.01, nws = 1, 3, 5
we get the following components all spaced 100 Hz (2 f1 apart) : 417.25 Hz, 517.25 Hz, 617.25 Hz, 717.25 Hz, 817.25 Hz, 917.25 Hz, 1017.25 Hz, 1117.25 Hz.

Examination of Figure e-2 shows components spaced at 100 Hz apart (919, 1019, 1119 Hz) and they are very close to the above predicted values.

A more accurate spectrum analysis is now required to detect the components given by equation (E-2) and the current signature pattern indicative of abnormal airgap eccentricity:

Inserting the same values and with nd = ±1, we get components at ±12.367Hz (rotational speed frequency) around the rotor slot passing frequency components.

Examination of Figure e-3 for the faulty motor shows there are components at ±12.4 Hz around the rotor slot passing frequency of 1019 Hz, likewise for the other rotor slot passing frequency components as shown in Figure e-2.

The components at ±12.4 Hz correspond to the rotational speed of the rotor, Nr = 60 x 12.4 = 744 r.p.m. and this is in line with the fact the motor is on a slightly reduced load of 91.3 A compared to the full load current of 103 A
(nominal full-load speed = 742 r.p.m.).

The operating slip is (750-744)/750 = 0.008.
Note the slip from the ±2s f1 components in Figure e-1 = 0.00864 and this further validates the analysis.

The very small difference in these two slips (6.4% using the full-load slip as reference) from the spectrum analysis is that the analysis was done at different times and any small change in speed is reflected in the result.

Note that the magnitudes of the - fr and + fr components are equal, and are only 15 dB down on f1.

This is indicative of an abnormal level of airgap eccentricity

Figure e-4
Current Spectrum - normal airgap eccentricity

Consider one of the healthy motors:

Figure e-4 shows the current spectrum from a healthy motor with a normal level of airgap eccentricity.

Comparison between Figures e-3 and e-4 clearly shows the signature patterns are distinctly different.

There is no component at +fr above 1019 Hz in the healthy motor and the - fr component in the healthy motor is 10dB less compared to the faulty one.

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 - as per the detailed analysis given above.

Airgaps to be firstly checked on site.

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