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Registered Member #46264
Joined: Sun May 11 2014, 05:27PM
Location: Tucson, Arizona USA
Posts: 61
I was curious what the voltage limit of a salt cap I made was. So I tested it using my Van de Graaff generator and sphere gap with water resistor. The cap measured 2 nF using a DMM. Based on the sphere gap setting I estimate the flash over voltage at 65 KV. The following photos show the voltage flash over.
One thing I noticed was the flash over started at the same point on the electrical tape covering the outer foil plate. The flash over ended at different points on the high voltage top bolt in the bottle top. This flash over was loud. I took the tape off and found the flash over point was a channel formed when the tape overlapped itself. Has anyone here make a HV cap capable of holding 100 KV ?
Registered Member #3343
Joined: Thu Oct 21 2010, 04:06PM
Location: Toronto
Posts: 311
The discharge appears to be running on the surface of the bottle, not through the insulation material.
I am guessing that the distance traveled by the flasing over the surface is about 100mm. If the neck of the bottle can be increase by 60 mm or more the creepage distance will be 160 mm, and may provide insulation for higher voltage. (using a long neck bottle )
Another way to avoid the creepage discharge is providing 1 or 2 shirts to bottle, such way that the creepage distance could be increased 60 mm or more.
What is the bottle dimensions? What material is the bottle ?
Registered Member #46264
Joined: Sun May 11 2014, 05:27PM
Location: Tucson, Arizona USA
Posts: 61
The bottle is 9" tall and 2.25" wide. It's made of PETE plastic. The distance from the outer foil edge to the top terminal is 4". Your guess of 100 mm was right on! I took off the electrical tape and poured melted paraffin wax over the outer foil edge. I tested with the same sphere gap setting and this time the breakdown was through the plastic. Salt water started leaking out the small hole formed by the discharge. The paraffin wax seem to work much better than the electrical tape for prevent the surface discharge.
Registered Member #3343
Joined: Thu Oct 21 2010, 04:06PM
Location: Toronto
Posts: 311
Hi Russ,
Try to add a shirt between the neck and the body of the PETE bottle. This shirt can be made of a cutting of another PETE, with the shape of a lamp canopy.... The contact of the shirt with the bottle to be filled up with paraffin .. I am trying to add a sketch os such arrangement ....
Registered Member #3343
Joined: Thu Oct 21 2010, 04:06PM
Location: Toronto
Posts: 311
Here is the sketch with some options that you have:
]cap_4_model_1.pdf[/file]
The shirt may be a piece of a bottle as shown in the A option, or a inverted shirt as B. Also a flat shirt can increase the creepage distance, allowing higger voltage.
A long neck bottle with 3 shirts appears to be a good design. option D.
All A. B. C, D suggestions requires a paraffin seal between the neck and the shirt (s). The shirt may be 30 mm and initially tacked with small amount of 'crazy glue' and after sealed with paraffin.
You have a better equipment to test the options.. and let us know you results. After the creepage solved, we may verify the corona startig voltage.
Registered Member #46264
Joined: Sun May 11 2014, 05:27PM
Location: Tucson, Arizona USA
Posts: 61
Nice figure Newton. I'll try your skirt idea. The equipment I'm using is very simple here's a photo of it.
A five foot long one inch diameter Aluminum pipe conducts the HV from the Van de Graaff generator to the test capacitor and sphere gap. The horizontal pipe attached to the upper sphere shank is filled with water and has a resistance of about 73 K. The current capacitor I'm testing has a red top and is seen just to the left of the sphere gap. The Van de Graaff generator only produces 6 uA of current so I constantly have to be on the look out for HV leakage.
Registered Member #46264
Joined: Sun May 11 2014, 05:27PM
Location: Tucson, Arizona USA
Posts: 61
Testing a salt water cap made from a PETE tennis ball container. This is a little thicker than the PETE bottle used previously. Salt water is covered with a layer of mineral oil. Flash over at about 70 KV. The distance from outer foil edge to top is 4.5" and radius of top is 1.5". Thus total flash over distance is 6". After the cap is discharged there remains a strong surface charge on the non-foil outside of cap. It's this surface charge that I think is supporting the flash over.
The first photo shows the flash over to the HV conductor.
I'm investigating the cause of this strong outer surface charge.
Registered Member #3343
Joined: Thu Oct 21 2010, 04:06PM
Location: Toronto
Posts: 311
The creepage discharge is common to HV insulators and insultion, It is the spark that moves on the surface of the insulation. For pulse transformers and capacitors that I have made, I used a rate of 600V/mm for fiber, impregnated paper, acrilic, under o potential of 8~10 kV. despite some glass and polyestirene offers higger kV/mm of creepage . The cleaness of the surface is mandatory to get high rate of kV/mm. The book "Electronic Transformers" by Reuben Lee provide some information how to calculate the creepage distance, it is a free book in the intenet. You may see other book, "Dielectric Phenomena" that may be found in the web.
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Over voltage testing of a salt water cap
