For years, students have asked, "What is the frying noise I am hearing while listening to motor bearings?" My generic answer has always been, "It could be a number of things, such as badly worn brushes, an arc in the windings or a loose wire in the "pecker head" or junction box."
Not being an electrical guy, I try to avoid too much speculation. However, I can remember a client, from a paper mill, I was training to listen to bearings. We were using an ultrasonic gun and performing a "comparison inspection" of two similar motors. Each motor was at the end of a large spindle. One motor, I remember, had a nice swishing noise; almost fluid sounding. The other motor sounded the same, except it also had an "egg frying" sound. Thinking that this might be brushes or windings, I started checking the motor for temperature variations. I found that at the 4 o'clock position, on the motor with the nice swishing noise, the temperature was around 100 degrees F. The other motor, with the slight egg frying sound, the temperature was at least 12-14 degrees hotter at the same 4 o'clock position. These motors were of similar rpm, size and manufacturer performing the same duties.
The technician I was training decided, several days after I left, to explore this phenomenon further. He brought out an on-line motor tester. The test instrument revealed a short in the windings.
Recently, I held an Ultrasound Level I Class. I had several infrared thermographers in the class that were also electrical technicians. One of the technicians sent me the following statement regarding what he had found on a recent motor installation: I enjoyed your class and learned a lot. I have been able to apply what I have learned on one item so far. We had a 125 hp motor, that had just been rebuilt, fail our resistance to ground portion of an acceptance test. Our site electrical Engineer insisted that the motor was fine. After the motor was started, I was able to record arcing coming from behind the motor j-box. I played the wave file back to the Engineer and he agreed that there was arcing sounds coming from behind the motor j-box. The motor was sent back to the rebuild shop and they installed new leads on it. When the motor returned, it passed our electrical acceptance test and sounded good on UE too. Before completing your class I wouldn't have known how to do this or that UE could be used in this manner for troubleshooting. Thanks, Allan Burnside.
This is not to say that airborne ultrasound will hear every arc, short, or winding problem while inspecting the bearings. It will, however, hear the sound of an arc, if conditions are right. I have always said to listen for an ANOMALY! This would have been an anomaly. Just as corona, tracking or arcing in your switchgear is an anomaly.
When using airborne ultrasound for bearing inspections as well as switchgear or substation inspections, it is a good idea to record a wave or sound file. Then try repairing the motor, cleaning the switchgear or substation and record a "new" wave file to compare the "before" wave file to the wave file "after" the repair is made. Many waveform analysis software will allow you to post multiple files on-screen at the same time for comparisons.
Then build a library of files that you can listen to from time to time.