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High Voltage Oil Fountain and Effects

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McCluskyCustom

Tue Feb 09 2010, 10:16AM

Registered Member #2538 Joined: Sat Dec 12 2009, 06:56AM
Location: San Luis Obispo, CA, USA
Posts: 10

I currently have a 4-stage full-wave voltage multiplier in an oil bath running at 1/3 voltage (currently up to 20kV), and am experiencing some interesting effects in the oil bath I am using.

I have a 75kV x-ray cable, with the ground braid pulled back about an inch from the HV lead, which is submerged in oil (Shell Diala AX transformer oil). When about 15kV or more is applied, the oil around the end of the cable begins moving, causing turbulence as though there were a small pump nearby. I don't understand the reason for the oil movement. It occurs just by applying high voltage, whether or not any current is flowing to a load.

Another very interesting effect comes from the voltage multiplier. I have a lead coming out of the oil about 1/2" off the DC portion of each stage except the last, simply for probing. When running above 15kV out, I found that more than just oil turbulence, oil actually came up the 3rd stage lead and flew off the end into the air for a few inches, intermittently. The ground of the cable is about 2" away from this lead.

I've uploaded a quick video to demonstrate here:
Oil Fountain

The oil flow can be slightly seen, and the oil fountain is very apparent. The output voltage is about 16kV, so the oil fountain lead is about 12kV.

I didn't think high voltage would affect oil like this, but apparently I was sorely mistaken. Any insight into this would be much appreciated. Anyone seen this kind of behavior, or can explain it?

Thanks!

Luca

Tue Feb 09 2010, 10:40AM

Registered Member #2481 Joined: Mon Nov 23 2009, 03:07PM
Location: ITALY
Posts: 134

Maybe it is a dielectrophoretic effect... Basically, a strong electric field can distort neutral molecules creating dipoles which move in a non-uniform electric field.

Regards,

Luca

klugesmith

Tue Feb 09 2010, 02:50PM

Registered Member #2099 Joined: Wed Apr 29 2009, 12:22AM
Location: Los Altos, California
Posts: 1716

McCluskyCustom wrote ...
I've uploaded a quick video to demonstrate here:
Oil Fountain

A similar effect sometimes happens with male babies on the changing table.

I imagine that dielectric oil in contact with a HV conductor can acquire and, by moving, transport a charge. Just like the belt of a Van de Graaf generator. Ordinary electrostatic Coulomb forces could propel the charged oil.

I also like Luca's explanation.
And finally, there might be surface tension effects -- would the oil circulate in a closed tank with no air space? Surface tension changes play a major role in the motion of thin layers of oil in heated cookware.

[edit] Perhaps it takes both. The free surface of oil is analogous to the Van de Graaf belt, and can be charged by air in a strong electric field. An experiment would be to see if fountain works in a vacuum.
Now since this is the Internet age, we -could- use search engines instead of idle speculation.

[edit ^ 2] Google found this high-voltage-driven fountain, whose mechanism is obvious.

radiotech

Tue Feb 09 2010, 03:07PM

Registered Member #2463 Joined: Wed Nov 11 2009, 03:49AM
Location:
Posts: 1546

A change of the dielectric constant from the cable insulation to the oil would produce a non linear electric field which could be pushing oil and making the turbulence liberating the droplets or atomized particles of oil with unit charges.Somewhat the forces Millikan's team worked with.

Something initiates the movement, when it moves, it leaves a wake, and an AC field, in the oil. Oil in sheets or layers is capable of storing replicas of charging signals applied at one point and detected at another points in moving drums or belts.

Could you very slowly start to apply the voltage to see at which point things begin around the cable ends ? What is the frequency of the AC voltage connected to the multiplier parts under the oil?.

EEYORE

Tue Feb 09 2010, 07:01PM

Registered Member #99 Joined: Thu Feb 09 2006, 06:10PM
Location: florida, usa
Posts: 637

I have seen this too with my HV multiplier and mineral oil. As soon as I power it up, the oil swirls in the oil bath and any wires or sharp points where corona would spray from in open air would spray oil up and out of the oil bath. Its very bothersome because oil goes all over the place. It is so bad that oil will force its way out of the container through the holes drilled for electrodes once sealed. I guess I will need to look into o-rings to stop this.

matt

McCluskyCustom

Tue Feb 09 2010, 08:13PM

Registered Member #2538 Joined: Sat Dec 12 2009, 06:56AM
Location: San Luis Obispo, CA, USA
Posts: 10

Matt - glad to know I'm not the only one experiencing this, but it sounds like it's going to get really annoying at higher voltages!

radio - When I slowly apply voltage, the motion at the cable ends begins slowly at about 10 or 12kV. The voltage on the cable is DC, with very low ripple unloaded. The motion seems to be off of the DC portions of the multiplier and cable, not the AC portion. The input frequency is 70kHz. Good call on the Millikan experiment...

kluge - interesting ideas. Just a note regarding surface tension, the other end of the cable has the HV and GND submerged in oil, and it is clear by surface turbulence that there is significant oil motion below the surface in the oil.

Luca - I'd never heard of that effect before. Interesting! It looks like it's usually on a um or nm scale, though. Maybe impurities, particles, or water droplets in my oil are behaving like this. This paper discusses water droplet motion suspended in oil. It could explain some of my motion, but doubtfully to the extent that it is occurring.

It still remains - how does the oil start to move in the first place? Does the strong non-uniform electric field become so intense that it breaks one of the covalent bonds in an oil molecule, creating an oil ion, which then has a force on it causing motion, and repeating? That would be similar to the ion winds seen from a sharp point, only in oil and at lower voltages. Thoughts?

radiotech

Tue Feb 09 2010, 11:20PM

Registered Member #2463 Joined: Wed Nov 11 2009, 03:49AM
Location:
Posts: 1546

From the world of high voltage utility transformers if somethings arcs under oil it breaks the oil into gasses which causes an overpressure flag to pop up. That along with the electrical trip
tells the maintenance people not to reset it casually. Also the oil is sampled and sent to labs, on a regular schedule, for telltale substances due to breakdown. (but i'm limited to 50/60 Hz stuff.)

70 Khz I'd look for cavitatation by dipping a piezo element from an old depth sounder or an ultrasonic cleaner which might hear something. Those arcs and sparks in high voltage gear create a signal of interest to predicative maintenance folks. ( they were known as the princes of doom at our place)

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