Original poster: Jim Lux <jimlux@xxxxxxxxxxxxx> At 11:50 AM 12/16/2005, Tesla list wrote:
In an attempt to solve this problem, I have tried to add increasing amounts of dielectric in the space btwn. the 2 coils, only to observe that the problem got worse.People who work with tightly coupled transformers for high voltage know it already, but I have only recently found out: I cannot insulate my way out of a corona problem. The more dielectric I stuff into the gap, the more severe the corona gets in the remaining air.
Ain't that the truth...
The air has to be excluded all together to avoid corona in such strong fields.
And that's the rub. Excluding all bubbles is a real chore, even with vacuum pumps, etc. For instance, the thermal expansion coefficients of everything have to be matched, or as it changes temperature, voids form, and once the void forms, corona follows, and breakdown after that.
People who design small, lightweight, HV stuff (like for space flight) probably spend more time on the potting than on almost anything else. Typical potting compounds are a composite of some silicone or other goop, with solid/powdered filler(s) added to adjust the dielectric and thermal and mechanical properties.
I like to put the primary coil inside the secondary, but the problem also exsists in coils that have visible primary`s.With this example I want to show a couple of things. At first encounter, the interface of air to dielectric is counter intuitive.At 400USD/Gallon of potting compound, it pays to be at least rudimentarily prepaired, before pouring. Doing an electrostatic analysis of this problem paves the road to a successfull design.
Which is why there is an enduring popularity of a liquid or gas dielectric. It doesn't have the outgassing/debubbling/voids problem that solid potting does. And, most of the liquid or gas dielectrics are inexpensive