Sea Water Injection Pump on an Offshore Oil Production Platform
Figure b-1 Original Spectrum of healthy motor
Figure b-2 Spectrum of the faulty motor
MCSA FFT Current Spectrum
Dynamic Range = 80 dB
Frequency Resolution = 15 milliHertz/line
Comment
Due to operational problems this motor had been stalled
and was subjected to more than normal direct-on-line starts.
It was noted
that the vibration level had doubled but was still within the operational
vibration limits. The motor was considered to be operational based on
the vibration measurements made.
However, due to the motor's operational history and the increase in vibration
it was considered appropriate to use MCSA to re-assess the condition of
the rotor winding.
Test
Results - Figure b-2
Figure b-2 shows the current spectrum and the twice slip frequency
sidebands are at ±0.7 Hz corresponding to approximately full-load
condition.
The dB difference between the sideband magnitudes and the supply component is now 26dB - indicative of a serious broken rotor bar problem.
When compared to the original spectrum the magnitudes of the sidebands have increased by 20dB which is a factor of 10 times larger in the absolute units of amps.
The recommendation based on the spectrum shown in Figure b-2 was that the motor should be immediately removed from service since the rotor cage winding was in an extremely unhealthy state.
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see the faulty rotor and read the
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