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Using Motor Electrical Signature Analysis to Assess Pump Performance

A power generating facility was doing a performance study on some of their pumps. We were asked to come in and perform electrical signature analysis on a pump as a part of the assessment. The plant was performing a number of measurements including vibration, pressure, and flow, and wanted to include signature analysis in the test.

Electrical signature analysis is performed by connecting current sensors and voltage leads to the motor, and then capturing high resolution current and voltage waveforms, which can then be analyzed as FFT’s as well as raw waveform data. The resulting data gives insight into the entire motor system, both mechanically and electrically, from the incoming power through the driven load. This technology actually uses the motor as a transducer for the mechanical analysis. Any mechanical phenomena will be modulated onto the current waveform where they can be detected and analyzed.

  1. We used the ALL-TEST Pro Online system for this analysis. Signature data is collected in two segments.  A high resolution low fmax (100 Hz) current acquisition of 50 seconds which primarily shows issues at running speed (misalignment, unbalance), and below the synchronous speed (rotor bar or load related issues)
  2. A high frequency capture and FFT of both voltage and current which shows
    1. Electrical issues including power harmonics, power factor issues, voltage and current versus nameplate and balance)
    2. High frequency mechanical faults (stator mechanical and electrical issues, air gap, and even bearing issues)

The system we were connected to was a vertical pump with a 30’ shaft. Driven by a 350 HP, 480V Motor, the system was set to a flow rate of 5600 GPM for the test. In that the motor terminal box was kept opened for the test, we left the handheld data collector near the motor, and collected the data via Bluetooth from a safe distance away

What we found:

  • The motor was electrically perfect with balanced current and voltage, and 90+ power factor.
  • The motor and pump were mechanically excellent with no indications of misalignment or unbalance, bearing issues, rotor, or stator issues.
  • We did however see a significant fluctuation in the current draw, pulsing at about 10 Hz. We determined that this was due to non-laminar flow, or turbulence in the system.
    See the graphics below.

Electric Signature Analysis of AC Motor - waveform

Above is a raw waveform of the current acquisition over 50 seconds. You can see the variation, in this case between about 306 and 313 amps.

Electric Signature Analysis - AC Motor waveform segment

This is an expanded view showing about 2 seconds of the waveform, the pulsations can be clearly seen.

Electric Signature Analysis - AC Motor Demodulation

This is an FFT of the demodulated current spectrum where the pulsation is shown as a broad peak around 10 HZ. This is a typical indication of turbulent flow.

One of the key benefits of ESA is that it can see mechanical issues in the driven load – even in the case of this vertical pump with the impeller 30’ below. This is in addition to detailed information on the entire system, both electrical and mechanical.

For your critical motor driven assets, this is a powerful monitoring and diagnostic tool, and a great companion to vibration measurement. Contact us if you would like to learn more about Electrical Signature Analysis and how it might fit into your program. In select instances we can also provide this as a service.