Whether you’re using airborne ultrasound or contact ultrasound, the benefit comes from the ability to see an anomaly and understand what it is. Through analysis, a confident and accurate assessment will persuade management to make accurate decisions and take proper actions.
“If you rewind the clock to 10 years ago, electrical analysis was an afterthought,” explained Joseph Gierlach, vice president — technical training and support, ABM Franchising Group — TEGG Service, Canonsburg, Pennsylvania. Gierlach led one of the opening workshops at UE Systems’ Ultrasound World X conference in Clearwater Beach, Florida. “We’re now able to make connections between electrical signatures. This is a misunderstood application. And it’s dynamic and ever-changing.”
While ultrasound’s role in diagnosing mechanical deficiencies is well-known, its use in electrical fault detection is equally valuable, but differentiating between the two can be a fundamental first step. And electrical maintenance carries additional safety concerns. “I want to go home at the end of the day the same way I did when I came to work,” said Gierlach.
“Just as importantly as being able to identify the fault or deficiency, we want to be confident that things are alright,” he said. “When you have competing ultrasound, that’s really where our ears come into play. The unheard can have dire consequences.”
Gierlach reviewed electrical faults such as arcing, tracking, and corona. “Contact chatter is motor contact,” he differentiated. “Contacts that make and break the circuit vibrate. The bad thing is they can burn so often that they won’t open up. They can weld together, or they can create sags or surges in the system. Contact chatter will look like arcing sometimes.” He also reviewed high-frequency noise from variable-frequency drives, EMI/RFI noise on the system, mechanical looseness, and vibration. Rubber mounts deteriorate, and they make an ultrasonic emission.
“Once you begin to have a failure, it never corrects itself,” warned Gierlach. “But not every anomaly is identical in nature. Look at footprints or signatures for similarities to basic faults.” He recommended looking at fault signatures in FFT and in Time Analyzer to validate findings, as well as determining the severity for each fault and generating report images to visualize the findings.
"Trust your analysis,” said Gierlach. “I don’t care what you think you see. Trust the report and analysis. We cannot always open electrical equipment. I don’t want to open the equipment when it’s energized.”
As insulating material breaks down or conducting material breaks down, faults become identifiable. “Arcing and tracking can be present in all voltage classes,” said Gierlach. “For detection, we have to use airborne and contact ultrasound. Arcing is the easiest one to discuss. This is a full-intensity discharge, return to silent, and no clear pattern noted. It’s a burst of energy. How often and frequently helps us to gauge severity. Tracking usually happens before arcing. Arcing is a result of a tracking event that has taken place over time. The higher the amplitude, the more intense.”
For reporting, you have to capture it to visualize it, he said. “It’s easy to put a face on arcing and describe it to some nontechnical person. The byproducts of arcing are carbon deposits. If you can get a shutdown of the system to do a visual inspection, that’s something to look for to validate.”
In the frequency analyzer, for the FFT, the configuration for electrical is 1,000. “Beyond 1,000 in the spectralyzer is not very useful for electrical systems,” advised Gierlach. “Make sure you set this to 1K. And determine if you’re dealing with a 50-Hz or 60-Hz system to determine the low end of your axis.”
The harmonic is a multiple of the base frequency. “In the FFT, we have a harmonic marker,” he explained. “The noise between spikes tells me I have an event that’s electrical in nature.”
The FFT is looking for predictability and repeatability. “When we look at the FFT, arcing and tracking look very similar,” Gierlach said. “But when we look at the time domain, we see two very different signatures. Tracking is a discharge along an insulating surface that’s trying to find a ground. It’s a continuous buildup of these discharges. Tracking has no repeatable event that can be seen over time. There are different points in time. It gets visualized by the different distances between spikes. The more bursts I have, the higher it’s building in intensity. It’s getting worse when you have more of these in the same timeframe. And nothing is symmetrical in amplitude on tracking.” Visualize discharge points or tracking respective to the electrical AC sine wave, he advised.
“We don’t want to overload the system in the analyzer,” added Gierlach. “You won’t be able to differentiate the two most common events, if you use just the frequency tool. Tracking, as opposed to arcing is nonsymmetrical, there’s non-uniformity in the spaces between spikes, and the height of the spikes is non-uniform."
|Sigmond Bush, Liberty Machinery Testing|