inject high current in high voltage electric arc

[Quentief]

Hi everyone,

I am opening this new post in the aim to find some answers to my questions.

In the past, I have already built some little projects by using microwave oven transformers.
My very first one was to replace the secondary winding in a way to have a low volt but high current power source, useful to do some little spot welding and melt some screws.

I also built a little circuit to draw some electric arcs. I did not use the high voltage from the MOT though, but I used the main voltage rectified and the inductance from the MOT primary winding instead. The idea was to use the inductive kickback effect to draw the electric arcs without the need to use some kilo Volts. And it worked pretty well, I did a little arc furnace I was able to melt some little quantity of metals.

But now I am wondering: is there a way to combine these two approaches? High voltage to start and maintain the arc, and high current to bring the arc hotter and brighter?

I imagined a circuit where I would use still use my little arc furnace project to start and maintain the electric arc. Then, the idea will be to bring 2 electrodes connected to the low voltage high current transformer, in a way to inject more amps directly inside the electric arc. To give you a better idea, I drew a little schematic of what this circuit would look like (see the attachment).

My assumption is, once the electric arc is started and maintained, it behaves as an electrical conductive wire. I assume that it has a pretty low resistance since plasma is fulfill of free electrons, ripped off ionized gas molecules. Therefore, maybe a little voltage (5 volts, or even less) could be enough to allow high current to go through the plasma and heat it by Joule effect.

What do you think? Could this work?

Thanks for any help, and happy Thanksgiving for some of you who are in North America

[TizianoBll]

Hi,
I think that this video from photonicinduction could help:

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Xenon lamps can be ignited using a high voltage pulse transformer connected in series to a high current DC source. Once the air is ionized the high voltage can be turned off.

Your ignition system is used (trigatron) for high voltage "switching" but I'm not sure if it would work at such low voltages.

Wouldn't be easier to just mechanically lower the electrode (with a lever) to ignite the arc and then pull it away?

As DC current source you can use a DC welder or, without spending too much, you can use AC supply (50 - 220 V) -> Full bridge rectifier -> smoothing cap (mF) -> ballast resistor (tens of ohms) and/or inductor (2 or 3MOT primaries in series would work well).

(BTW: DC arcs are easier to maintain than AC arcs)

[Anders Mikkelsen]

That's basically what a TIG welder with HF start does, use some kilovolts to ionize the air, then current limited low voltage to maintain it. Minimum arc voltage drop is some 10 - 30 V in Argon, but an open circuit voltage of twice that is needed if you use inductive ballasting, in order to maintain arc stability.

[Quentief]

Hi everyone,
thanks for your replies. I am trying to document myself on those devices, the HF arc starters that cant be plugged to the welder. I also found this ineteresting video

The man is coupling an high voltage HF flyback transformer to his arc welder in a way to have voltage and current to match with what a plasma torch needs to work. This seems to be very closed to TIG welder HF devices that you were talking about.



This other schematic is a little bit different than the one from the video, but it has helped to have a better understanding and led me to this question:

If my understanding is correct, by using an HF transformer it is possible to induce high voltage. Then, since the coupled coil is in serie with the welder transformer, the high voltage is added to the welder one and thus we have high voltage between the clamp and the electrode.  This allow the welder to create strike longe arc to to then allow the high current from the welder to make its way through the arc make the plasma hotter. My question is, do you think a such technic could be used if the voltage of the welder is even smaller than the usual 30 volts ? As example, I told you that I have already built a little spot welder from an old MOT transformer. It can produce 5 volts with a max amperage of about 300 amps. Do you think that I could also couple an HF flyback trandformer on its circuit, in a way to make it able strike long arc and inject current as strong as 300 amps inside ?

I dont really know the properties of plasma, I assume that it has the same behaviour than any electricalc onductibe material : low electrical resistance, hence no need of very high voltage to allow high current to get trhough (I mean, once the arc is initiated).

What do you think ?

[TizianoBll]

Hi,
5V is too low. You'll need at least 30V@40A DC. With AC you'll need much higher voltages.

I still think that it would much easier to just ignite the arc by touching the two electrodes (like stick welding), 100V@10A DC should work well.

Have you found an adequate HiFreq transformer? Remember that the HV winding must be able to both withstand HV and carry the high current. It will be tricky...

[Quentief]

Hi again,

Thanks for your replies, this is very interesting. So I understand that a voltage above 20 Volts is better to keep the electric arc stable once striked. I also found an article about solar PV panels arcing faults, where it is mentioned that one ionized, the air breakdown voltage drop from 3000 V/mm to 1.2 V/mm (see https://www.researchgate.net/figure/Voltage-characteristics-of-an-arc-across-a-gap-30-31_fig5_255250101)

So it seems to match with what you are claiming: 5 volts would theorocially allow the arc to keep a length of a few mm. And this is without considering any perturbation (air movements due to the heat from the arc, the current variation which will pass through zero every half cycle if I am using AC,...) that would propbably extinguish the arc. Whereas voltages as big as 20 volts will let the arc have a lenght of a few cm.

But now, if we have another power source with a bigger voltage that runs continuously to maintain the arc, to have the plasma channel constantly maintained. Can the current from the 5 volts power source can get throughout the arc ? I mean with this configuration, even if the voltage of the second power source is as low as 5 volts the arc should not have any reason to extinguish itself, should it ?

Or am I misunderstanding something ?

[Twospoons]

If you read the text you linked to you will see the final voltage across the gap consists of that due to the plasma channel resistance plus the anode and cathode fall.  This is the reason for needing more than 5V to sustain the arc.
So even with your HV ignitor, you will still see more than 5V potential across the gap and no current will flow from your 5V supply.

[Quentief]

Hi all,

So after having read your answers, I tried to find a way to change my cicruit in a way to have enought voltage to sustain the arc. I still have issues to determine the exact voltage value that needs to be provided to keep the arc once striked though. I saw that arc welders tend to have a working voltage at 30 volts (this is also the value that you were suggesting TizianoBll, but do you know what will be the maximum arc length that we can have with this value? In plasma torch this value tends to be more at 60 volts, I don't know if there is way to actually "size the voltage" in function of the arc length that we want to achieve.
I found this equation in that purpose https://www.quora.com/Why-do-long-arc-lengths-in-welding-require-high-voltage-and-low-current : R = 14.4*L/I, where R is the arc resistance (ohms), I the current wihcih is going throught it (A) and L its length (centimeter). I assume that the equation can be rewritten as U = R*I = 14.4*L and thus the voltage required to keep an arc seems to be 14.4 volt/cm. I have not been able to find any measures to compare is this equations really works, and how closed it is from the reallity though.

Moreover, in the case of an electric arc furnace, what would be the optimal solution? A very short arc with a low voltage but high current? Or rather a voltage a little bit bigger to increase the arc length. Even if the current is smaller, I am expecting to have the same dissipated thermic power since the arc will be longer. Is that correct? In addition, less current will maybe preserve the electrode, and if DC is used then I assume that the negative electrode will last even longer since it will be further from the positive electrode, where most of the heat seems to comnig from.

And so as I said I canged the cicuit (see the attachment): so my idea is to use the inductive kick back from a coil (the primary coil of a MOT), powered by the main (at 120 VAC) to start the arc. To limit the current, I am counting on the impedance of the step down transformer (a second MOT rewounded for that puropose). My plan is to reinject the current which is coming from this step down transformer directly through the arc, and thus the output voltage needs to be closer and higher than the voltage at the electric arc once striked. Then once the electric arc is fully established, I was thinking about a switch that I could close in a way to "deactivate" the inductive coil circuit, to then fully power the step down transformer which will continue to inject its high current inside the arc. Again, I am still not sure about the right voltage that the step down transformer needs to build, but do you think this could work?

[Twospoons]

  I found an IEEE paper(https://ieeexplore.ieee.org/document/4181239) that estimates the resistance of a low voltage arc (anything up 600V or so) as 40/I^0.85