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Electrical Spectrum Printout ExplanationBy Mark Goodman, VP Engineering, UE Systems, Inc., Elmsford, NYELECTRICAL INSPECTIONWe are often asked at which voltages and on what type of equipment is ultrasound most effective. The answer is not simple since it often depends on the individual asking the question. First of all, determining the definition of low, mid and high voltages is relative. Those in the power distribution arena will look at 12 kV as low voltage, those who work in a typical manufacturing plant will look on 440 Volts as high voltage. The main electric problems detected ultrasonically are partial discharge, arcing, tracking and corona.
Ultrasound detects the ionization of air, as it produces turbulence. This can be from Corona, Tracking or Arcing. Heat is produced when there is either Resistance due to corrosion or from Tracking and Arcing. Since Heat is produced by the flow of current through corrosion or resistance, you can have a “hot spot” detectable with IR, and have no Ultrasound because there would be no ionization.
LOW VOLTAGES The main concern in low voltage equipment is arcing. Typically 110, 220 and 440-volt systems are inspected with infrared imaging and/or spot radiometers for temperature changes. Hot spots, usually an indication of resistance, can be indicative of a potential for equipment failure or it could indicate a possible fire hazard. When arcing occurs, it is often accompanied by heat. However, it is not always possible to detect a hot spot if the equipment is covered.
Ultrasound will hear arcing in circuit breakers, switches, contacts and relays. In most instances, a quick scan of a door seal or vent will detect the ultrasound emission. Listening for internal arcing in circuit breakers and switches can be accomplished with the contact probe. For example, touch a circuit breaker switch with the contact probe to listen for internally generated arcing. The most effective method of low voltage inspection will be to combine infrared imaging with an ultrasonic probe. Please keep in mind that since air cannot be a conductor of electricity below 1000 volts, corona cannot exist. Any “buzzing” sounds are either loose components vibrating at 60 (or 50) Hz or tracking.
Mid and High Voltage
inspection Higher voltages often produce more
potential for equipment outage. Problems such as
arcing, destructive corona or tracking (sometimes
referred to as "baby arcing") and corona as well as
partial discharges and mechanical looseness all
produce detectable ultrasound that warn of impending
failure. Detecting these emissions is relatively
easy with ultrasound. The acoustic difference among
these potentially destructive events is the sound
pattern. Arcing produces erratic
Destructive corona has a build up and drop-off of energy resulting in a buzzing sound accompanied by subtle popping noises. While scanning for these emissions, use a parabolic reflector such as the UE UWC (Ultrasonic Wave Form concentrator) or the UE LRM (Long Range Module). These accessories more than double the detection distance of the standard scanning modules. Partial discharge (PD) which occurs inside electrical components such as in transformers and insulated buss bars, is another problem that can be detected with ultrasound.
Partial discharge can be quite destructive. It is effected by and causes deterioration of insulation. This is heard as a combination of buzzing and popping noises. The contact probe is employed for PD detection. If your Ultraprobe has frequency tuning, try 20 kHz. To learn more about our Ultrasonic Wave Form Concentrator and our Long Range Module, visit our web site: www.uesystems.com.
While it is relatively easy to determine arcing, tracking or corona by the sound pattern, there can be occasions where it may prove confusing. It maybe possible that a strong buzzing sound related to corona might in fact be nothing more than mechanical looseness. Spectral analysis can be a useful tool in analyzing electric emissions. Since all Ultraprobe instruments heterodyne ultrasound down into the audible range, the headphone jack may be used to record sounds. These sounds can then be downloaded to a PC with a sound card and viewed as a spectrum or time series for analysis.
Some preliminary experimentation has demonstrated that the main harmonic of an electrical emission (60 Hz in the USA, 50 Hz elsewhere) will be most prevalent in corona. As the condition becomes more severe, there will be fewer and fewer 60 Hz harmonics observed. As an example, arcing has very few 60-cycle components. Mechanical looseness will be “rich” in 60 Hz Harmonics and will have little frequency content between the 60 Hz peaks and will also demonstrate harmonics other than 60 Hz. The following spectrums will demonstrate the characteristics explained previously.
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