From: Bert Hickman
Date: Tue, 30 Apr 2002 07:37:45 -0500
Subject: Re: [TSSP] Secondary voltage stress factor
John and all, I think you're right in terms of helping to reduce cumulative damage to the windings - holes developed in the insulation of adjacent turns may then induce turn-turn flashovers and coil death. A dielectric overcoat may create a single point weakness in an area, protecting nearby turns from subsequent hits. However, there may also be a downside to a thicker dielectric overcoat. As all of us are (painfully!) aware, thick dielectric coatings seem to acquire substantial surface charge during operation. It's not clear if these surface charges are unipolar (via corona rectification?) or spread out in patches with a mix of polarities (via small partial discharges along the surface?). In any event, creeping leader surface discharges "hug" the surface of a dielectric, and are enhanced when there's a conductive plane on the opposite side at a different potential. As the leader propagates along the dielectric surface, streamers fan out and collect higher displacement current than if the leader was propagating in free air, since the leader is "charging" the comparatively higher capacitance of the leader-streamer region and the nearby plane underneath. This is why the overall discharge "creepage length" can be as much as 4-5X that of simple breakdown in air. Stranded surface charges will likely further aid the propagation of a surface discharge when the propagating leader is of one polarity and the surface charge in a nearby region is of opposite charge. The additional contribution of displacement current as the leader discharges the previously charged region will likely further enhance leader growth and distance traveled. If the leader approaches a region with stranded charge of like polarity, it may halt further progress, alter its direction, or even become airborne over the offending region. A number of studies in the literature confirm that residual negative surface charges have significantly greater impact on the propagation of positive leaders than the opposite case. Other names for "creeping leaders" are "creeping flashovers", "creeping discharges", and "surface discharges". Interesting stuff! :^) Best regards, -- Bert -- -- Bert Hickman Stoneridge Engineering Coins Shrunk Electromagnetically! http://www.teslamania.com FutureT@aol.com wrote: > In a message dated 4/28/02 6:43:30 PM Eastern Daylight Time, > bert.hickman@aquila.net writes: > > Bert, > > I think the windings may need a thick coating to protect them from > burning up due to racing sparks. I usually just put a couple of > latex water-based > brush-on coatings on the winding. Whenever I consistently get > racing sparks on my coils, the winding eventually is destroyed > and shorts out, or the wires burn in two. Maybe the water based > coating causes the trouble, I don't know. The large bang sizes > I use vs. the thin wire diameter, maybe contribute to failure due to > the thin insulation of the thin wire. > > Cheers, > John > > > >> Unlike power arcs, >> "racing sparks" seem to be of comparatively low energy, and seldom >> seem >> to do permanent damage. > > >
Maintainer Paul Nicholson, paul@abelian.demon.co.uk.