Comparing Pre- and Post-Maintenance Signals

One of the strengths of EMI Monitoring is the ability to trend the data over time. The signal strength usually increases long before the defect causes the plant to trip. However, this is not always the case, especially if the defect deteriorates rapidly or is caused by some rapid phenomena such as water or rain intrusion.

Comparison of the spectrum over time or trending the Power Spectrum values over time usually gives adequate time to develop an inspection and repair plan.

It is also good to do before and after maintenance comparisons.

Example 1: The demodulated waveforms in the lower frequency band showed that the excitation system might have a loose connection. Although these values were still low, the unit had a Not in Demand period, and the new air-cooled power rectifiers were examined. Loose hardware was found on the disconnect switch associated with rectifiers. The snapshot below shows the before and after maintenance scans.

Example 2: When the motor was sent out for cleaning, it was found to have salt contamination. The snapshot below shows the before and after maintenance scans.

Example 3: Four major water clip leaks were found and repaired. The unit had a wet bar that was dried after the repairs were completed.

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