Limitations of Electrical Signature Analysis...?

Response to a number of articles and tech tips.

President, SUCCESS by DESIGN Reliability Services
Author: Physical Asset Management for the Executive: Caution Do Not Read This If You Are On An Airplane; and,
Electrical Motor Diagnostics: 2nd Edition

http://www.motordoc.com

I have read some recent articles and comments discussing the limitations of Electrical Signature Analysis (ESA) in terms of horsepower and load.  In a number of circles, the limitations have been given as 50 horsepower and more than 75% of load, and these circles include trainers from different organizations.

Kind of makes me wonder as I have been using ESA on equipment from FRACTIONAL horsepower all the way up to tens of thousands of horsepower and on machines that have been overloaded to machines that have been running uncoupled AT NO LOAD.  There are also a number of additional issues that I can only think are related to a lack of experience with the technology and pattern recognition (covered in ‘Electrical Motor Diagnostics: 2nd Edition’):

1. The good news is that operating at a constant or repeatable load is not as much of an issue as it is with such technologies as vibration analysis.  The pattern recognition of faults relates to peaks that are relative to the peak voltage or current in dB.  The dB level will only vary slightly across a variety of loads.  What you do need to be aware of is anything related to the actual running speed (multipliers) and the pole pass frequency positions on the spectra.  This means that you can trend tests, when using pattern recognition, with the motor at different loads.

2. The use of voltage and current spectra is referred to as Electrical Signature Analysis, the use of current spectra only is referred to as Motor Current Signature Analysis.  Using Motor Circuit Analysis to describe online testing is a sign of ignorance related to the technology.  Motor Circuit Analysis references static (de-energized) testing of electric motors.

3. Demodulated Spectra does not tell you very much with AC induction motors.  Wow… then why do all of the vendors and trainers spend so much time on demod?  Simply because other technologies reference it as a means to detect problems.  In reality, in order to determine the severity of a problem using ESA would require a reference to the peak current or voltage.

4. Such information as trending circuit impedance is not as valuable as pattern recognition, including in predictive maintenance.  Unfortunately, such readings can be misleading depending on the operation of the equipment and the condition of the distribution system.

There are, of course, a myriad of others, and the volume continues to grow.

Through the use of simple pattern recognition techniques, which are not as difficult as many would think, a whole world of trending and troubleshooting becomes available.  This also includes the ability to perform Time to Failure Estimation™ techniques and separate problems from within and without the actual system that you are evaluating.