OVERVIEW: Current signature analysis shows decreasing trend in the db difference between 60hz peak and estimated sideband peak... in the neighborhood 45db. Tests are done at low load below 50% (system does not allow loading this machine any higher). No change in vibration.
MACHINE DATA: 500hp, 3600rpm horizontal sleeve bearing motor with cast aluminum rotor, driving centrifugal chiller compressor through shim pack coupling. Compressor has integral gearbox and rotates around 7100rpm.
Slide 1 - Decreasing trend in "db down".
- Note 1A - we are not able to obtain 50% load due to low heat load on chiler
- Note 1b - Not all points have good agreement between sideband frequency and predicted fault frequency, but the last point 04/06/16 shows very good agreement and also agrees with speed deduced from vib spectrum.
Slide 2 thru 6 - Spectra used to determine db down for 12C.
Slides 8 thru 12 - Spectra for sister chillers, all 57db or greater difference
Slide 13 - Vibration overall values remain completely stable
Slides 14/15 - low-level pole-pass frequency sidebands in vibration spectrum
Slide 16 shows expected relationships: load vs current; load vs speed
Slide 17 shows estimates of load from measured current and speed (measured at the same time) on 12C.
- There is not good agreement between estimates from current and from speed However, there is no upward trend. If rotor were significantly degraded, for a given actual load there would be increased current and increased slip. Assuming the average actual load is roughly constant over time, the estimated load based on these two paramters would increase over time (not evident).
- On 4/7/16, observed ammeter. It was oscillating regularly at once per four seconds. Range was about 30-30.5A. Matches pole pass frequency based on speed measured during vibration 4/6/16.
- On 4/18/16, observed ammeter again. It was oscillating regularly at once per 3 seconds. Range was 35-36A. Inspected linkages (inlet vane and hot gas bypass valve) and no oscillation was present. Went back and inspected ammeter and still oscillating the same. Inspected ammeters for the three other running sister chiller compressors and no similar oscillation was present. This suggests the oscillation is related to rotor problems rather than to process oscillation.
The current clamp-on probe in use since 2014 has thumbwheel calibration adjustment which is susceptible to being inadvertantly moved. This may cause errors in current magnitude (which affects prediction of sideband frequency) but will not cause error in db difference. This affects the 2013 reading and January 2016 readings. April 2016 reading was taken in conjunction with vibration so that speed is available for more reliable judging of sideband frequency (this will be continued)
- Retest when available
- Long term -Coordinate current test to occur at same time as vib test to reduce uncertainty in determining pole pass frequency. Look for alternate current probe.
- Is there reasonable confidence in diagnosis of rotor defect.
- What is urgency of repair?
- Any additional in-place tests recommended ? (at low load).