Broken Bars - Case History 1
Sea Water Injection Pump on an Offshore Oil Production Platform
Test Objective
In this case history the current spectrum of a motor when in a healthy state is compared to the spectrum after a stall and 'direct-on-line' starts.
Nameplate details
3-phase, star connected, SCIM,
6600 V, 60 Hz, 3.6 MW/4825 h.p.,
376 Amp, 3580 r.p.m
Additional information
Copper fabricated rotor,
Single Cage, 46 rotor Bars,
Nose Joint rotor design of Bars to End-Ring,
Low full-load slip (0.0055, or 0.55%)
Click within this image for in-depth analysis
Current Spectrum
Figure b-1
Original Current Spectrum from healthy unit
Click within this image for in depth analysis
Current Spectrum
Figure b-2
Current Spectrum - high number of broken bars
Test Conditions
MCSA FFT Current Spectrum
Dynamic Range = 80 dB
Frequency Resolution = 15 milliHertz/line

Test Results

Figure b-1 is a current spectrum from a routine survey using MCSA. The motor was operating at full-load current.

The predicted twice slip frequency sidebands for a nameplate frequency of 60 Hz and a full-load speed of 3580 r.p.m. was ±0.66 Hz.

Examination of the spectrum shows that there are sidebands at ±0.6 Hz around f1, the supply frequency, which was at 59.99 Hz and not 60 Hz.

The measured frequency of the sidebands therefore correspond to within less than 10% of the predicted values using full-load nameplate data.

The sidebands are greater than 45 dB down on the supply current.

This corresponds to a perfectly healthy working unit for a 2-pole SCIM with 46 rotor bars.

The ±2sf1 Sidebands are 46dB down on f1, in Figure b-1

Sidebands are due to inherent rotor assymmetry caused by differences in Bar to End-Ring joints.
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