June 04, 2008
Electrical Motor Diagnostics: Online vs Offline Testing
Howard W Penrose, Ph.D., CMRP
President, SUCCESS by DESIGN Reliability Services
http://www.motordoc.com
Comments:
An area that can be confusing, when discussing the topic of electrical motor diagnostics, is the difference between online and offline testing. Because Motor Circuit Analysis (MCA), Motor Current Signature Analysis (MCSA), and Electrical Signature Analysis (ESA) are often discussed as part of Electrical Motor Diagnostics (EMD), the layman gets the impression that the technologies are able to produce similar results. In fact, nothing could be further from the truth. Online (MCSA/ESA) tests are similar, but offline (MCA) testing is very different from online in both performance and results, including significant differences between each of the technologies and what they are designed to detect.
Online testing is designed to evaluate the dynamics of the motor system as a whole. While the evaluation involves the ability to trend specific electrical measurements, its power lays in the powerful spectral analysis available to all of the commercially available systems. Similar to vibration analysis, it can detect many of the same mechanical anomalies associated with equipment faults, its primary strength lays in its ability to detect the dynamic electrical components of the motor, including rotor related issues. Unlike vibration analysis, whose transducer is separate from the equipment, MCSA/ESA technologies utilize the electric motor’s airgap magnetic field to detect faults. This allows the analyst to look at any issue that causes variations in the magnetic field, which has a direct impact on the motor’s current waveform. Mechanical variations are detected when the rotor moves side to side within the airgap of the motor, allowing online spectra to detect severe bearing wear and driven equipment faults. This is because of a phenomenon known as the inverse square law. In any field, electrical, magnetic, gravitational, the force of that field varies by the square of the distance between it and an object it is acting upon (the mathematical model varies significantly depending upon the geometry of the field(s) and objects). This means that a slight movement of the rotor will produce a significant effect on the magnetic fields which are generated by current.
However, there are many who are under the mistaken belief that online testing is a solution to replace vibration analysis. MCSA/ESA is a powerful tool when it comes to the detection of components that have a direct impact on the magnetic fields of the motor such as rotor bars, eccentricity, winding movement, power quality, etc., but is relatively weak when it comes to the detection of driven problems and issues such as bearing wear. People with little practical knowledge or experience in the technologies will often misrepresent online testing as a suitable alternative for vibration analysis. Unfortunately, there are those who teach or communicate this as an option, which leads to drastic consequences for the condition based or predictive maintenance program. We will discuss the decisions related to this particular issue in our next edition.
The purpose of offline testing is to evaluate the static components of the insulation system of a machine with the ability, in some instances, to detect the condition of the rotor bars or rotating fields/windings of the machine. Of the three primary players in this technology, one utilizes high voltages and two utilize low voltage. Leaving off standard surge comparison testing, the PD (Partial Discharge) surge detection system of the AWA by SKF (formerly Baker Instruments) is designed to detect insulation WEAKNESS between turns with an electronic package designed to reduce the chance of damaging a winding further when a fault is detected. The instrument also carries the ability to perform low and high voltage insulation to ground tests which detect ground wall insulation weakness and a resistance reading capability to detect loose connections and broken conductors. Of the two low voltage systems, PdMA relies upon insulation to ground testing with an inductive impedance and resistance reading for the detection of winding problems. ALL-TEST Pro focus’s less on the insulation to ground fault detection and instead looks to turn to turn DEGRADATION, which means it looks for variations to the dielectric properties between conductors.
What I have generally found, even when I was working for a technology vendor, is that most programs start out thinking that they need to focus on MCSA/ESA and don’t look to MCA as an option. When it comes down to the program from both a safety and practical standpoint, most programs already have a robust vibration program and no ability to detect static winding problems. In a well operating multi-technology program, we often see it migrate from the concept of online testing to relying upon vibration, utilizing online testing to quickly identify and confirm electrical issues detected by vibration, but the implementation of MCA which usually has quick and dramatic results. The first time through with online testing you will often detect a few anomalies missed by vibration. However, with the first time through using MCA, you can expect to see issues in one out of four to one out of six machines tested.
We will also discuss how to handle the volume of issues detected with MCA in the near future.
For more information on these topics, please see http://www.motordiagnostics.com/presentations.htm for an article archive or ‘Electrical Motor Diagnostics: 2nd Edition,’ which has a robust discussion of the differences between these technologies http://www.bookmasters.com/marktplc/10287.htm.
