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careysub
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Continued from the Phosphorus Thread
Quote: Originally posted by wg48  | I have tried using a cheap welder and mains power via a ballast.
Cheap welders can be used continually but on lower current setting. The main problem I found was the open circuit voltage is only a round 41 volts ac
so only a short arc is supported half an inch at max and easily extinguished when cold material fall into the arcs. That also means the electrodes
need to be advanced frequently. I was using welding electrodes carbon ones may be better but a suspect not.
The mains ballasted version having an oc voltage of 240V was much more forgiving able to sustain several inches of arc or more in conduction mode. I
used the electric cook as the ballast (oven grill and hot plates all on) perfect for quick experiments. Potentially very dangerous unless you know
what your doing and your very careful.
You do not need to rectify the ac but if you do and use an inductive ballast it would be dc and support a longer arc.
The resistance of a water rheostat is adjusted by the concentration of the electrolyte. The required resistance is determined by your required maximum
current under short circuit conditions. But its going to boil eventually unless the container is very large or a cooling arrangement is constructed.
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The key design objective here was how to get a lot of power into the arc furnace so that it can process material in significant quantity and to get
some benefit from scale.
Operating a welder at fractional current does not really do that.
Similarly I am discarding the idea of using a simple resistance load in series with the arc, like the water rheostat, or any similar idea, because,
again, only a fraction of the house circuit capacity ends up in the arc (and these waste much more power than they deliver).
Here is a useful article that taught me some key facts about welders and clarified the issue for me:
http://www.mig-welding.co.uk/arc-welder-types.htm
The article has this to say about low cost air-cooled transformer welders, such as the ones I had been considering:
"AC air cooled welders: Often referred to as buzz boxes. A number of companies made reasonable quality air cooled AC welders in the past. The
technology is now the reserve of DIY stores selling welders to people who don't know any better. They are the least usable of any type of arc welder.
Cons: Very poor duty cycle"
This confirmed the impression I had been forming. Indeed, if I were to get one of these it would be with the objective of converting it into DIY
oil-cooled version to get some decent power out of it.
But there are better options it seems, specifically DC inverter welders which I was not familiar with.
These are based on modern solid state electronics, are highly efficient, and largely as a result of that have very high duty cycles.
This one for example:
http://www.usesmartertools.com/S-200i.html
available at my local hardware superstore for $200 right now, has a 100% duty cycle at 134A / 25V, or 3.35 KW (it is a 220 V model).
It looks like the way to go.
I think the problem you were having with keeping an arc is not 41 V being too low (apparently these inverters run fine with 20-28 V output), but
simply that the power was so low that the arc was easily quenched (that low current thing).
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wg48
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Snip
This one for example:
http://www.usesmartertools.com/S-200i.html
available at my local hardware superstore for $200 right now, has a 100% duty cycle at 134A / 25V, or 3.35 KW (it is a 220 V model).
It looks like the way to go.
I think the problem you were having with keeping an arc is not 41 V being too low (apparently these inverters run fine with 20-28 V output), but
simply that the power was so low that the arc was easily quenched (that low current thing).
[/rquote]
Your 20 to 28V is the voltage across the arc that is not the open circuit voltage. Oc voltages can be 70V or more on professional welders. On a simple
inductively ballasted welder (transformer type) the oc voltage gives an indication of how easy striking the arc will be and how long the arc can be.
Higher oc voltage reduces the arc current variation with arc length and keeps it going at a greater length. An electronic welder will have a similar
characteristic.
Its handy to have a welder available but if your only buying it for the furnace I hope it works better than my very cheap one.
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wg48
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Careysub:
I read the spec on the example of an inverter type welder you gave. It specifies the oc voltage as 65V. So it may be able to keep a submerged arc
going more easily than the one I used. It will be easier to weld with too.
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