Airborne ultrasound inspection is used to monitor the condition of electrical assets and pinpoint the exact location of electrical faults.
Locating and eliminating electrical defects in a timely manner helps manufacturers cut down on unplanned downtime and safety risks. Electrical failures can cause short circuits, ﬁres, transformer explosions, and major damage to integral components within a facility. A common electrical fault that is detected using ultrasound is corona discharge. Corona discharge is caused by ionization of air molecules close to a point of very high voltage potential gradient. The ionization breaks down the air and forms ozone and nitrous oxide. Corona discharge will produce a constant bubbling sound, which will either be at the supply frequency or twice line frequency. Using an acoustic imaging camera makes monitoring the condition of electrical assets with ultrasound even easier. Acoustic Imaging Cameras combine multi-frequency ultrasound sensors with digital imaging to provide reliability practitioners with an acoustic image.
Furthermore, the audio ﬁles from acoustic videos captured can be converted to waveforms and spectra that can be analyzed to determine underlying causes of those frequencies.
The following ﬁgure was taken from the audio ﬁle of an acoustic video captured with a SonaVu™ and then input into SDT’s UAS3 Software for analysis. As seen in the time signal, there is a continuous series of spikes at two times Line Frequency and harmonics.
This same technology can be used to detect defective rolling element bearings. Using an airborne ultrasound spectrum and waveform captured using a SonaVu Acoustic Imaging Camera, and inputting it into SDT’s UAS3, ultrasound technicians can identify various bearing defects by capturing and analyzing data without being in physical contact with the machine, as seen in Figures 2 & 3.
Bearings with race, ball or roller defects will typically produce a high ultrasonic frequency. This frequency is typically non-synchronous and not a multiple of the RPM speed of the shaft, but normally several times the rotating speed of the machine. Thus, an inspection of the rotating shaft using strobe light will not demonstrate a stable image such as for vibration of gears that occur at multiples equal to the RPM speed of the shaft. In many cases, the ultrasonic vibration amplitude of the bearing is not regular. Slight “oscillations” can be observed in live readings.
By Gilles Lanthier, Senior Reliability Analyst, SDT Ultrasound Solutions