On December 1, 2022 I received my usual daily IEEE standards updates for voting which included IEEE P1017.4/D1, “Draft Recommended Practice for Testing and Rating of Electrical Submersible Polyphase Induction Motors.” This would be the complete new draft of such a standard developed by the Petroleum and Chemical Industry Committees of the IEEE Industry Applications Society. One of our responsibilities as voting body members is to review, comment and vote on the work that is developed by concensus by the working committee. What stood out to me as I reviewed it was that motor current signature analysis (also translated to electrical signature analysis) was found as a section under section 6.4, ‘Special Fault-Locating Test Equipment.’ While a majority of the rest of the standard related to details in standard static electrical testing and long instructions on how to do machine vibration measurements, there was a whole two sentences under 6.4.2 on MCSA. The section basically states that it is comparable to vibration data and that it can also be used for predictive failure analytics.
This brought up an interesting point. Motor current signature analysis, a term that has been mixed with electrical signature analysis, was originally developed at Oak Ridge National Labs in the mid to late 1980s (patented in 1988) by Howard Haynes and his team. The purpose was to test the bearings, gears and torsional operation of motor operated valves in contaminated areas of nuclear power plants. There are a number of papers presented afterwords with the Electric Power Research Institute (EPRI) providing a few studies on its application to detect issues with broken rotor bars, which dumbed down the technology to a rotor bar fault detection system. While I lost count on how many PhDs were created based on MCSA studies on rotor bar faults, and a whole book dedicated to it, the original purpose related to electrical and mechanical systems was lost in academia and the industry took a step backwards.
In 2006, the IEEE standard 1415 (IEEE Guide for Induction Machinery Maintenance Testing and Failure Analysis) was issued and in 2021 it was administratively withdrawn. (See How Electric Motor Testing Standards Really Work (Clearing Up A Little Controversy) | THE RAM REVIEW on that definition) In this guide the various methods for MCSA and ESA were outlined, but not fully defined as it was determined by the working group so that specific manufacturers were not called out. The definition was still a bit disappointing as it stated, “Analysis of stator current using frequency spectrum analysis techniques, measured in dB, can yield side band harmonic information to show presence of cracked or broken rotor bars.” I was able to get the following included in the definition for 4.3.10, Current Signature Analysis: “Machine and load mechanical data may also be required (stator slots, rotor bars, fa blades, gear teeth, etc.). Current signature analysis can also be used to detect power quality issues and may be able to identify additional electrical and mechanical faults.” Which was lacking, but did allow us to push through the standard.
This did assist in the development of a working team that was called the Institute of Electrical Motor Diagnostics (IEMD) which had the goal of developing certification programs surrounding on line and off line electric motor testing. The work on an ISO standard was well in process when the economy took a dive in 2009 and the working group disbanded. One of the issues that did occur was that the previous chair of the ISO working group stepped down and a new academic stepped in with his own definitions of MCSA and ESA. He scrapped the work surrounding the concensus work that had been developed by the leading devices of the time, Framatome EMPATH, ALL-TEST PRO OL, PdMA MCE and the Baker Explorer, and introduced untested academic work as the definition. Later a single unknown single-phase current only device was introduced into the standard, which was expanded to include virtually anything related to online testing of motors with an emphasis on partial discharge testing. ISO 20958 was introduced on August 15, 2013 as: “Condition Monitoring and Diagnostics of Machine Systems – Electrical Signature Analysis of Three Phase Induction Motors.” There is a relatively good treatise of current signature analysis under the section 4.2 ‘Stator current analysis,’ which we will cover shortly. The academic’s disparagment of the technology for his own research purposes was relegated to Annex A (informative) Park’s Vector Approach, which as received little practical use.
The spectral analysis section for CSA (current signature analysis) includes:
- Cracked rotor bars
- Cast rotor windings with large internal voids
- Broken bar-to-short circuit ring connections
- Cracked short circuit rings
- Excessive air gap eccentricity
- Rolling element bearing defects
- Coupling misalignment
- Stator winding shorted turns
- Problems in the driven equipment
Noted is that “rolling element bearing defects are included since CSA can identify these defects that may also be identified by vibration monitoring.”
It is also noted that the ‘conventional CSA’ is described as being required at or close to normal full load and ‘The current on one motor phase is analysed for its current frequency content by measuring it with a clip-on current transformer around a supply cable… Newer approaches can analyze all three hases and also look at the relation between current and voltage.’ The description is of a specific device and the ‘newer’ approaches are, of course, the original and existing commercial technologies of ESA. The need to be at or near full load was due to the limitations of the specific device as those who are experienced in the use of the technology, let alone a level one vibration analyst, knows that this would be related to the resolution of the system collecting the data and not a requirement for faults to be apparent with this type of technology. The resolution depends upon the sampling rate (which also provides the maximum frequency for the FFT – FMAX), the time data is collected and the FFT block size (bins). Unlike vibration, where the rotor movement energy must transfer through the bearings and metal to the transducer, ESA/MCSA is measured directly from the airgap magnetic field leaving the peaks relative to the load. This part, of course, is not included in the ISO standard as it would negate the disparaging annex or the individuals involved in the development had limited experience with ESA/MCSA. Instead, as was carefully left out, ESA/MCSA is not load dependent for fault detection and can be trended without operating the equipment at the same load each time data is analyzed, when evaluated in dB as opposed to analysis in linear (ie: straight current values). This fact is well documented ever since Howard Haynes’ first paper in 1989 and numerous IEEE and other technical papers since.
However, the ISO standard is still valuable for the eventual implementation of a general industry certification program. It also allowed the argument to include other than rotor testing into the 2019-2021 EPRI research project on the application of Electrical Signature Analysis for machine prognostics. Other commercial works have allowed industry to understand that ESA has applications that work hand-in-hand and can also rival vibration analysis for continuous monitoring applications.
The wind industry had recognized ESA/MCSA for some time. In the 2017 AWEA (now ACP – cleanpower.org) Operations and Maintenance Recommended Practices manual, ‘Electric Current Analysis’ was cited as: “From up tower generators to substations, electrical current analysis can be widely used for the health indicator of mechanical or non-mechanical components.” Recommended Practice 831, ‘Condition Monitoring of Electrical and Electronic Components of Wind Turbines’ goes into some detail concerning the application of ESA/MCSA, although refers to it as ECA before being corrected in 2018 to ESA. Recommended Practice 831 was originally developed and included ESA in 2013 based upon the work performed by MotorDoc and Acciona Wind in relation to generator and driven component known defects. The upgrade to ESA is cited as relating to the MotorDoc, Shermco and Acciona research into wye ring fault detection using ESA. The section also includes ESA work related to transformer fault detection that were attempted by another organization using the Parks Vector approach, which was not successful, but taken up by joint research projects with MotorDoc in 2015 using high sample rate ESA. The 2019 version of the Operation and Maintenance Best Practices Chapter 8 on Condition Based Maintenance went into far greater detail on ESA applications.
This lead to the AWEA ‘Gearbox Operations Playbook’ which was originally going to be released in 2020 but was delayed due to the AWEA name-change to American Clean Power (ACP – cleanpower.org) with the acquisition of the solar and energy storage trade associations in the United States. The release as an ACP document is planned in the near future. However, it specifically calls out Electrical Signature Analysis with the following definition: “measurements or monitoring of generator voltage and current are used to measure frequency and torque seen in gearboxes. Similar to vibration, frequencies associated with gear defects or wear and bearing rotation frequencies are monitored. Single detection of frequencies may indicate wear and changes over time indicate potential defects that can be visually inspected. Condition is determined based on peak voltage and current with amplitude of defects identified in decibels down to the associated peaks allowing for the detection of change across varying loads and relative torsional peaks.”
As we continue to observe ESA/MCSA technologies enter the marketplace the number of standards adopting the technology will continue to grow. It is important to note that the submersible pump standard and wind standards are of particular importance as the were introduced by the owners of the equipment and not the technology manufacturers, as we’ve identified in the ISO document and the original IEEE 1415. We are seeing an increase in the number of academic papers associated with the technology and there is even an example of current signature for evaluating gears as part of the Predictive Maintenance library in MatLab® and associated papers. Organizations such as the Vibration Institute have started taking a serious look at ESA/MCSA including the increasing number of technical papers at their educational conferences each year. On the market side, an increasing number of new entrants into the market and attention from major electrical manufacturers and equipment manufacturers bodes well for the technology.
For a detailed book on the practical applications of ESA/MCSA, see Practical Electrical and Current Signature Analysis of Electrical Machinery and Systems, Penrose, Howard, eBook – Amazon.com which will be available in hard cover before Christmas.