Corona discharge can occur when air is electrically stressed near its break down limit and this will generate broadband RF energy.
If you have small bubbles of air in a insulator with a fairly high dielectric constant in an electrical field it produces an effect known as dielectric focusing and produces a electric field strength in the air in the bubble many times that given by the voltage across the insulator divided by the thickness of the insulator. If this field strength is strong enough to cause any ionised air molecules that are around (and there always a few from the effects of cosmic rays if nothing else) to be accelerated to sufficient velocity before they bump into other molecules for them to ionise them then this can lead to a cascade. Eventually at high enough fields this can lead to full blown arcing but before that there is a region before that where only the statistically rare long free paths will give enough energy to produce many generations and the cascades the result is eventually peter out. The result is a bubble of highly ionised gas that emits RF energy as the electrical current that movement of the ions varies randomly. . Increasing the frequency of the AC supply makes things worse. A shorter time between cycles leaves less time for the ions to be neutralised after the field strength dies before the next peak comes leaving a greater seed population of ions to start the next cycle. This is one of the reasons it is common when impregnating transformers and other electrical equipment to do so in a vacuum Although this effect can take some time to produce complete failure it will very likely do so in the end Nick Rouse ----- Original Message ----- From: "Ken Javor" <ken.ja...@emccompliance.com> To: <emc-p...@majordomo.ieee.org> Sent: Friday, January 25, 2002 6:08 PM Subject: broadband RE from AC induction motors > > Do any forum members have knowledge of a mechanism by which ac induction > motors (two are fan motors, one a compressor motor) can generate broadband > RE from 30 - 600 MHz? This is outside my experience. Are there perhaps > degradation modes that result in arcing? The motors run off three phase 400 > cycle power, 115 Volts rms phase to neutral. The control system is > bang-bang, just mechanical relays making connections/disconnections based on > temperature and pressure inputs. The rep rate of the BB noise is variable > but around 10 milliseconds. > > Thank you. > > ------------------------------------------- > This message is from the IEEE EMC Society Product Safety > Technical Committee emc-pstc discussion list. > > Visit our web site at: http://www.ewh.ieee.org/soc/emcs/pstc/ > > To cancel your subscription, send mail to: > majord...@ieee.org > with the single line: > unsubscribe emc-pstc > > For help, send mail to the list administrators: > Michael Garretson: pstc_ad...@garretson.org > Dave Heald davehe...@mediaone.net > > For policy questions, send mail to: > Richard Nute: ri...@ieee.org > Jim Bacher: j.bac...@ieee.org > > All emc-pstc postings are archived and searchable on the web at: > No longer online until our new server is brought online and the old messages are imported into the new server. > ------------------------------------------- This message is from the IEEE EMC Society Product Safety Technical Committee emc-pstc discussion list. Visit our web site at: http://www.ewh.ieee.org/soc/emcs/pstc/ To cancel your subscription, send mail to: majord...@ieee.org with the single line: unsubscribe emc-pstc For help, send mail to the list administrators: Michael Garretson: pstc_ad...@garretson.org Dave Heald davehe...@mediaone.net For policy questions, send mail to: Richard Nute: ri...@ieee.org Jim Bacher: j.bac...@ieee.org All emc-pstc postings are archived and searchable on the web at: No longer online until our new server is brought online and the old messages are imported into the new server.
