Factors affecting high voltage arc size / High Voltage / Forums

Forums

4hv.org :: Forums :: High Voltage

« Previous topic | Next topic »

Factors affecting high voltage arc size

Move Thread

LAN_403

Kiwihvguy

Sun Aug 07 2016, 03:41AM

Registered Member #3395 Joined: Thu Nov 04 2010, 08:42AM
Location: Christchurch, New Zealand
Posts: 193

Hi all,

I have accumulated many MOTs over the last few years and have decided to use most of them to construct a large 3 phase MOT stack (bigger than any I've seen on the internet) with a 6 pulse rectifier, leaving a few special MOTs for other stuff like a VTTC or likewise. Fortunately the university I'm studying at has a high voltage laboratory with a 240V 125A 3 phase supply (240V phase to neutral, 415V phase-phase in New Zealand), which should be plenty provided the whole MOT stack will have some form of inductive ballasting.

Ultimately I'm aiming to create the biggest arc size possible for a given power on the input; which will determine what series-parallel or all-parallel arrangement I will connect the MOTs in, bearing in mind MOTs hate being connected in series! However I'm having a little difficulty joining the dots from my own experience experimenting with 2, 3, 4, and 6 MOT stacks in different series/parallel arrangements with series resonance capacitors. There aren't many scholarly articles that have direct research on this question, although I was able to access one through Google Scholar titled 'The other electrical hazard: electric arc blast burns' which had some relevant content.

However, I found that both increasing the voltage and increasing the current allow the arc to be drawn out further. Obviously once the arc has been initiated, the voltage across the arc is less because the path of ionised gas is much more conductive than air that hasn't electrically broken down. Then, it would seem that as the arc is drawn out, increasing in length, the voltage across the arc increases until the power supply no longer supplies enough voltage and then it extinguishes.

But regarding the current, the more current there is in the arc, it tends to be fatter and the plasma discharge path appears to larger in cross-sectional area. This could suggest the resistance would be lower as the surface area is greater, as R = rho*(length/area) and the arc can be modelled as a near-purely resistive load according to the aforementioned article.

Consider this scenario to see the effect of current on the arc: 2 HV PSUs, both 20kVAC out at 50Hz, one is current limited to 1mA and the other is capable of providing 5A (hypotethical!). The current limited one will exhibit sparking when the arc breaks down but will not be able to be drawn out because there isn't enough current to generate a hot plasma pathway. The other 5A PSU will initiate the arc by breaking down the air, but the much hotter plasma path will allow it to be drawn out much, much further.

The question: what is the ratio of importance between voltage and current in the context of a MOT stack that could supply either 5.2kVDC or 10.4kVDC after 6 pulse rectifier? Bearing in mind the higher 10.4kVDC due to the series arrangements means the output current of the whole MOT stack will be less.

Cheers

5.2kVDC is from Vdc = (3*sqrt(3)*sqrt(2)*Average Vrms from MOT)/Pi
= (3*sqrt(3)*sqrt(2)*2200)/Pi = 5199

Sulaiman

Sun Aug 07 2016, 11:00AM

Registered Member #162 Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140

MOTs scare me so this is based on other experiences;

unlike single-phase a.c., don't forget that the output is d.c.
so inductive output ballast for short-circuit protection will not work
(but MOTs are internally ballasted/flux-shunted/current-limited)
and arcs are different

how will you stop the updraft of air due to the heat of the arc blowing out the arc ?

Blackcurrant

Sun Aug 07 2016, 08:02PM

Registered Member #2989 Joined: Sun Jul 11 2010, 12:01AM
Location: UK
Posts: 94

Hi,
I think you will get a slightly longer arc with more voltage, as if everything else is the same you will get more power loss in the electronics (heating) with higher current lower voltage set up. I would think arc length will be a function of the power in the arc. Also the negative electrode on your set up is going to get hotter than the positive.

Kiwihvguy

Mon Aug 08 2016, 02:51AM

Registered Member #3395 Joined: Thu Nov 04 2010, 08:42AM
Location: Christchurch, New Zealand
Posts: 193

Sulaiman wrote ...

unlike single-phase a.c., don't forget that the output is d.c.
so inductive output ballast for short-circuit protection will not work
(but MOTs are internally ballasted/flux-shunted/current-limited)
and arcs are different

how will you stop the updraft of air due to the heat of the arc blowing out the arc ?

Thanks for the reminder, yes I'm aware the output is DC from the 6 pulser so any inductive ballast wouldn't work there.
What I meant to say was I would place an inductive ballast in series with each phase to each set of MOTs - so this in on the mains primary side not the rectified high voltage side.

Can't do a whole lot about the updraft, but I haven't experienced an updraft forceful enough to blow out the arc...

Mads Barnkob

Mon Aug 08 2016, 06:05AM

Registered Member #1403 Joined: Tue Mar 18 2008, 06:05PM
Location: Denmark, Odense C
Posts: 1968

Remember that you can not just infinitely add MOTs secondaries in series for higher voltage. One end of the secondary is tied to the core and at some point you can no longer uphold the insulation from winding to core.

Here is a example of what has to be done:

Kiwihvguy

Wed Aug 10 2016, 01:35AM

Registered Member #3395 Joined: Thu Nov 04 2010, 08:42AM
Location: Christchurch, New Zealand
Posts: 193

Mads Barnkob wrote ...

Remember that you can not just infinitely add MOTs secondaries in series for higher voltage. One end of the secondary is tied to the core and at some point you can no longer uphold the insulation from winding to core.

Here is a example of what has to be done:

Thanks for link Mads, I have no intention of purposefully series 10s of MOTs. I'm well aware that even a bank of 2 in series can cause one to fail if you're unlucky, which I what I experienced last year. I wouldn't put 8 MOTs in series, but rather have 4 pairs, where the 2 MOTs in each pair are in series. Then paralleling those 4 pairs would yield about 4.5kV.

Moderator(s): Chris Russell, Noelle, Alex, Tesladownunder, Dave Marshall, Dave Billington, Bjørn, Steve Conner, Wolfram, Kizmo, Mads Barnkob