Related Products:

EMMK-101, EMMK-102, EMMK-103, EMMC-104, EMMC-105, EMMC-106

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areas of interest, anomalies, failure modes, EMI


Identify possible anomalies and failure modes based on the frequency range of energy spikes and humps

Evaluating Areas of Interest

During the capture of the frequency domain data, look for humps and spikes of energy that stand out against the backdrop of general EMI noise levels. Record the frequency of these areas, and capture the associated demodulated time domain signal. By examining the characteristics of the demodulated waveform and knowing which frequency the demodulated waveform is associated with, you can make some educated guesses about where and what type of defect could be causing the EMI signature.

For generators, four features have been developed to correlate the frequencies associated with the areas of interest to specific generator equipment. These features were developed from case studies and observations of signals over the last 20 to 30 years.


Anomalies related to

Predominant failure modes

30 kHz – 500 kHz

Exciters or excitation components

  • Diode pulse trains (missing diode pulse in the tone pattern)

  • Dirt tracking

  • Loose connection arcing

500 kHz – 5 MHz

Stator slot components

  • Spark erosion

  • Insulation breakdown

  • Core arcing

  • Partial discharge

  • Loose wedging

5 MHz – 30 MHz

End winding structure components, although the region can be affected by anomalies in both the next higher and next lower frequency bands.

  • Loose end basket structure

  • Corona due to end winding spacing issues

  • Connection issues between connection rings

  • Bushings

30 MHz – 100 MHz

Generator output bus components:

  • Isolated Phase bus

  • Segmented bus

  • Non-segmented bus

  • Cable bus components

  • Loose connections at the flex links

  • Cracked insulators

  • Dirty insulators

  • Water or moisture intrusion in the bus

However, sometimes processes (especially with random noise or static) increase the whole level of the spectrum without any specific sub-frequency band increasing any more than another.

This situation often occurs where the contamination levels were very high inside the generator. For example, a cooler leak had been going on for a very long time, and the unit was elevated across the frequency bands. After the cooler leak was fixed and the generator's interior cleaned, all frequency bands decreased in activity.

For large motors, the lower frequency range (30 kHz – 500 kHz) and upper frequency range (30 MHz – 100 MHz) are not relevant unless the motor is a synchronous motor. Induction motors do not have an excitation system, so signals of interest do not occur in the lower frequency area. Also, since there is no output bus from a motor, the high frequency range is not relevant either. In fact, for many years the signals were only monitored up to 10 MHz for motors because nothing had ever been detected above that.

For large transformers, higher frequency areas tend to be relevant to the generator output bus. Unlike with generators, not enough data has been gathered to be able to correlate specific frequency ranges to physical areas of the transformers, so only the power spectrum of the whole frequency band (30 kHz – 100 kHz) is monitored for changes.

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