Airgap
Eccentricity - Case History 1 Large HV, 3-Phase SCIM - Pump Drive |
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Background
and Test Objective 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). MCSA was applied to determine the condition of the rotor winding and whether airgap eccentricity was the problem.
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Nameplate
details Additional
Information
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At
full-load the twice slip frequency sidebands due to broken rotor
bars will be : ±2sf 1
= 2 x 0.01 x 50 = ±1
HzThe 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 f 1 .
There are
sidebands at ±0.846 Hz around the supply frequency at 50
Hz but they are 60 dB down on f
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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 f 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.
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. This
is indicative of an abnormal level of airgap eccentricity |
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Figure e-4 Current Spectrum - normal airgap eccentricity |
Consider
one of the healthy motors: 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. |
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EXPERT
COMMENT Airgaps to be firstly checked on site. |
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EM
Diagnostics Ltd, |
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