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Author: Subject: Carbon arc Furnace
lvjrf
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[*] posted on 13-10-2004 at 23:56
Carbon arc Furnace


Hi All
I known The Theory Of Carbon arc for lighting welding

but are thereany body show me how made electric furnace (carbon arc)
to melting metal ?

thanks
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Oxydro
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[*] posted on 14-10-2004 at 17:23


Try this site for a little bit of info, it was the first thing I found.... bond with google for more information

http://members.misty.com/don/carbarc.html
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lvjrf
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[*] posted on 17-10-2004 at 03:21
thank


thank Oxydro
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Oxydro
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[*] posted on 17-10-2004 at 06:25


No problem, it's an interest of mine :).

There was also an article in (IIRC) PopSci by Theodore Gray (guy who collects elements, owner of "The periodic table table";) on making a simple arc furnace for melting various metals, but it no longer seems to be available online.

There is also a section in "How to make and use a small chemical laboratory" about making an electric arc furnace. If you can't get that book or file (it's on the ftp), let me know and I'll post pics of the relevent pages on my website.

I always wanted to build one of these, but I never got around to getting the proper eye protection -- I don't want to risk my eyes, they're bad enough already. If you try it, be sure to keep us informed on how it goes.

Edit: http://walkermetalsmith.com/reprints/Portable-Arc-Furnace.pd... too

[Edited on 18-10-2004 by Oxydro]
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ignaro
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[*] posted on 20-10-2004 at 08:46


Hi!

I was thinking (and procrastinating) about to build an electric furnace also.

The most obvious way was (from my viewpoint) to use an arc welder: Low voltage, easy current regulation, isolated secondary, etc.

I was reading some of the Oxydro links, an I saw that they use a direct connection to distribution lines (shock hazards) and resistors (energy waste) or reactances as current limiters.

Is there some cause (other than low cost of equipment) for such methods?




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[*] posted on 20-10-2004 at 13:58


Simplicity. Robustness.

I like it!
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Oxydro
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[*] posted on 20-10-2004 at 19:26


Oh, an arc welder would be wonderful... if I happened to have one. Although it's probably the best current source, you do have to go out and buy the welder, if you don't already have one... a tub of salt water as a resistor is a wee bit easier on the wallet.

What might be the best alternative is to rewire one or more microwave oven transformers to provide low voltage high current.
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[*] posted on 21-10-2004 at 02:02


Quote:

What might be the best alternative is to rewire one or more microwave oven transformers to provide low voltage high current.


Dumb question: How are you going to sustain an arc of reasonable current with low voltage? Resistance of air is something like 10kOhm/cm IIRC.




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[*] posted on 21-10-2004 at 03:06


The easiest way is a salt-water rheostat. A description how to build one and an arc-furnace was already posted here by me, do a search for "DIY".

There is a electrical misconception here too, you dont need a transformer but a ballast for an arc furnace. thats not the same.

[Edited on 21-10-2004 by Organikum]




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[*] posted on 21-10-2004 at 04:14


Vulture, an arc of low voltage/high current is absolutely fine. I got my arc to work with alternating DC current, 42 V and 120 Amps. The reason being? As soon as the electrodes touch, you create superheated carbon, which evaporates (and burns). The ionised gasses conduct the current.
Nonetheless of course, the higher the voltage, the further apart the electrodes can be. So with very low voltages (i.e. 10 V) you get an arc that is extremely short - so short that the electrodes virtually touch. Therefore it's harder to keep the electrodes at the correct distance, i.e. where they don't touch each other, but also where the arc can sustain itself.
I remember with my 42 V arc, every dozens of seconds or so I had to move up the electrodes. I am sure this wouldn't be quite as frequent had I used a higher voltage.

[Edited on 21-10-2004 by chemoleo]




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[*] posted on 21-10-2004 at 04:30


70V to 120V is the preferred voltage for arc furnaces, DC is preferred over AC.

A salt-water rheostat with an rectifier after the rheostat works just fine.

For small furnaces is understood.




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[*] posted on 21-10-2004 at 08:24


Quote:

What might be the best alternative is to rewire one or more microwave oven transformers to provide low voltage high current.

A search for "arc welder" microwave brings a lot of pages.

I like this: http://www.backyardmetalcasting.com/guest_timwelder.html




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[*] posted on 21-10-2004 at 09:06


Quote:

70V to 120V is the preferred voltage for arc furnaces, DC is preferred over AC.
A salt-water rheostat with an rectifier after the rheostat works just fine.


By my (only) experience, a rectifier can be made using the lowest-cost-per-amper-diodes, conected in paralell.

I read that such thing must not be done, because there are not two equal diodes, then a diode starts conduction before the others diodes, it conducts the entire current and it dies in a Jeanne d'Arc way.
Maybe my diodes never read such argument, because they worked. I used the rectifier in a very old DC motor (220 V, 0.25 HP).
They worked a couple of years (4-5 hours per week) then the motor jammed and expelled some smoke; then, the diodes were sorrow because the motor death, or they tried to emulate their coworker, and they died also.




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[*] posted on 21-10-2004 at 15:49


I think, with the parallel diodes, in practice the resistance of the diodes and their leads creates enough of a resistance to overcome the slight differences in the forward voltage drop of the diodes.... if that makes any sense -- just my best guess.

So parallel arrays *can* certainly screw up, it's just that real life situations usually have balancing factors.

Edit: Ignaro, that is in fact where I found the idea of a rewired transformer... More accurately first on http://www.dansworkshop.com/Homebuilt%20arc%20welder.shtml

Regarding Chemoleo's comments on arc length:
I think that the voltage doesn't have much effect on the ultimate length of the arc. Only the current affects the maximum length (of course you have to have a high enough voltage to overcome resistance, but the resistance of an arc is tiny).

What the voltage affects, however, is the maximum gap the arc will start at. With air's breakdown voltage of approximately 1100v /mm, at voltages under a few hundred you essentially have to touch the electrodes together. That is how the arc is stated in an arc welder, by eithertapping or scraping the electrode briefly on the surface of the work. Tapping for DC and a scraping motion (I don't know how to describe it better) for AC, as far as I recall.

[Edited on 21-10-2004 by Oxydro]
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[*] posted on 21-10-2004 at 17:15


Parallel diode arrays work better if the diodes are in close thermal contact. As diodes heat up their forward voltage drops, so if one is hotter than the others it takes more current and so gets hotter, and you get thermal runaway leading to destruction.
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[*] posted on 21-10-2004 at 21:13


We worked on an arc furnace about a year ago in lab. Used a 220 stick welder and copperclad carbon arc gouging rods. we made it work, but it was far more trouble than it was worth. problems:
-The arc is hot. 10,000 F. hot. everything melted, or burned.
-Even with the current turned down considerably (if you turn it down too much like below 100 amps, you lose your arc.) the carbon rods would burn up quickly, and would have to be ajdusted frequently. On the up side, it didnt take long for anything in the middle to melt. problem, the stuff in the mold would melt whereever the arc was, so you have to heat it longer... and this deformed the mold.

you have to keep the mold under whatever it is you are melting, becuase otherwise the load cools too quickly to cast it. Also, We tried using a graphite crucible in doing so, we destroyed the graphite.
-Every fume coming out of there is toxic. You are ionizing our air which contains Nitrogen and Oxygen. Needs to be well ventilated. (where as normal welding operation is either done with an inert gas, or hydrogen gas(for stick welding) and thus doesnt need nearly the same precausions that you need when arcing regular air)

-Did I mention it was hot? Kevlar gloves and a #11 lense on your helmet. trouble is you have to stand there, and watch your puddle, and so you have to be looking at it.
-Oh yea, your refractory or whatever you build it out of will experience thermal shock, decompose, or both.

Good things:
-Able to make a casting of mildsteel (thats pretty cool, since you typcaly cast ... cast iron, not mild steel)
-Tried to cast stainless... destroyed it.
-Was able to melt #7 Tungsten electrodes (pure)
-Was able to melt 2% thoriated tungsten electrodes (what a waste)
-Aluminum... was meant to be funny, but it turned to dust/white smoke instead of melting like we thought it would.
-castings were crude, and had silica from the melted sand stuck to the outside in the gaps, but you could generaly see their shapes.

Now im not saying all arc-furnaces are not practical, just ones made from welders. If you looking to cast high carbon iron, copper, pewter or aluminum, just stick with a gas burning furnace. takes a little longer, but I think its far less of a hazard and a little more fun.
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[*] posted on 22-10-2004 at 08:04


Quote:

Ignaro, that is in fact where I found the idea of a rewired transformer... More accurately first on http://www.dansworkshop.com/Homebuilt%20arc%20welder.shtml

Yes, I saw it. I preferred the other design because its quick-and-dirtyness, very similar to my creations. I have some kind of neatness disability :)

Quote:

Parallel diode arrays work better if the diodes are in close thermal contact. As diodes heat up their forward voltage drops, so if one is hotter than the others it takes more current and so gets hotter, and you get thermal runaway leading to destruction.

Maybe the best method is to bound parallel diodes with a cooper sheet or similar, I think that I'll do next time, if there is a next time.
For info only, I don't want to disagree: according to this criterium, I made the rectifier in the worst way.
I used 20 or 24 diodes, I dont remember it. Four copper wires nailed perpendicular to a wood piece. Wires length and distance nearly 5 cm long. I soldered diodes trying to maintain them apart, some of them were near the wood.

Uber luminal:
It was a rod to rod arc, or a rod to metal one?
The rod to metal arc transfers a little more energy from arc to metal, and the current in the metal develops a little heath in it... maybe the sum of the two little quantities can be helpful, at least while the starting, if the metal is a set of pieces: the contact resistance can increase the heat development in metal.
I read, I dont remember where, that primitive laboratory arc furnaces were made from CaO.

An (maybe crazy) alternative to the resistor for current control

A half value resistor and a single diode (or parallel diodes equivalent to a single diode), then we obtain the same average current, and a capacitor parallel to arc, for to maintain the arc in the no conduction intervals. Less energy waste.




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[*] posted on 22-10-2004 at 22:31


sorry ignaro, but the capacitor has to be a BIG one. And you will not achieve current regulation either. Only way to do this is by putting an IMPENDANCE in series with the circuit,causing a small voltage drop, depening on current.
This impendance can be made by a resistor (high power loss, hot, bulky) or a inductor
( bulky, heavy, possibly expensive if purchased, difficult to design if homemade from old transformer) or a capacitor (not realistic).

This said, many transformers used for welding are a special design with a magnetic shunt in the core, which is adjustable. This way the transformer itself is current limited somewhat (A MOT sometimes has a magnetic shunt built in, aswell as neon light transformers)

Using half wave rectifying:

Ripple voltage over a capacitor is
U = I *20/C for your halv wave rectifier , for a full wave it is I*10/C

where I = current (mA) , C capacitance (uF)

if 20 A is drawn, and ripple voltage < 5V (about 10 % of arc voltage) gives a capacitance of 80000 !!!uF . That is double the size of a coke can! Also, the ripple currents charging and discharging such capacitor will heat it extensivly , so it has to be first grade (expensive, about 50-100 US$) for a voltage rating of about 63V. (use rating = double working voltage)

This said, it can be realized by using multiple parrallell connected caps of smaller values, that can be found through various surplus for cheap. This way the wiring is important so that the current is shared equally through all caps, since this type of surplus caps usually are of less sturdy quality.

/rickard

[Edited on 23-10-2004 by rikkitikkitavi]
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[*] posted on 25-10-2004 at 06:38


80000 uF? That is fine!
I have a lot of ceramic capacitors, I can get more.
Voltage rating is low, but I can do serial connections (equal capacity in each serial group), and afterward connect groups in parallel.
Soldering cost could be a little high, I guess; and space consumed a little high also :D

Seriously:
Maybe a short arc (i.e. low voltage) could be more stable for a half wave rectifier if using thick electrodes and high current: The high temperature of electrodes point could help the arc restart, by heating the air in the gap.
But high current excludes the use of direct connection to mains...

And I'll never try to make an arc with the mains voltage. That position comes from local environment conditions: We have 220-380 volts here, I prefer other forms of suicide :). Better is to use transformers.

More Seriously: I am reading "The Electic Furnange" (Sic). From http://www.archive.org/texts/collection.php?collection=milli... ;
Really, it is The Electric Furnace by Alfred Stansfield, there are some typos in the books list.
better is to download the file TheElecticFurnange_orig.tif (148.8M), not TheElecticFurnange.tif (67.3M) The second one is easy to download but hard to read.
It is a good book, date is 1914. It don't refer material and devices by brand names, it use substance names, or describe the devices.




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[*] posted on 5-12-2005 at 19:49
iron core laminations


I'm planning on building a current limiting inductor and start doing some carbon arc testing using 120VAC/60Hz as input power. I'm shooting for a current draw of around 10 amps with the electrodes touching.

The subject here could equally be called "lamentations." I'm trying to find a source of sheet metal or strips of the silicon iron used for iron cores for transformers, inductors, etc, and I'm not having any luck. :(

Because I can't find a source I thought I'd build my own laminations. But you still have to find out what kind of sheet metal to buy and how thick. I found a very old book today that said the thickness is usually 14 mils (0.014"). Material is "stovepipe iron" or "Russia iron." My guess is that this is just silicon iron.

If I can't find a source for the right material I will likely just buy some thin sheet steel and cut out my own strips. I will then coat them with insulating varnish.

If anyone can tell me where to get these laminations I will be grateful.




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[*] posted on 5-12-2005 at 20:18


You need to talk to a transformer manufacturer. The material is grain oriented silicon iron, and the manufacturer will have it already cut into E and I sections. Better still is to ask them to wind the inductor for you - which they will do, with the right size core,wire, insulation for your job. If you don't want to do that you can scavenge the laminations from an old, dead, or surplus tranny. Cutting from raw sheet stock would be a LOT of work. I assume you don't have a CNC Turret punch?



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[*] posted on 5-12-2005 at 20:42


Thanks twospoons.

I understand what you are saying about making the laminations being a lot of work. I only have a jigsaw. I won't be using E and I configuration. I'll just wrap an "I" piece. This is going to be fairly crude.

I do have a phone number for a transformer manufacturer. I'll give him a call. (I also read that the grain was oriented and the metal annealed.) If I make my own I'm assuming that the thinner the metal the better (within strength limitations).

Where would I find an old transformer? A TV? A microwave oven? City landfills don't allow much salvaging anymore. I don't have a TV or microwave that needs destruction right now. ;)

Am I being too fussy? I'm not trying to optimize this design. Overkill by 200% won't break my budget. Could I just get by with say 0.031" (22 ga) steel as long as the core doesn't heat up too much? I'm planning on winding the core with insulated solid wire #12 (AWG). Building it myself is most of the fun.

[Edited on 6-12-2005 by Magpie]




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[*] posted on 5-12-2005 at 22:25


Your best bet, as far as easily available materials would be the transformer from a dead microwave oven. They are built to handle 900 to 1000 watts (look on the ratings sticker). This guy shows how it's done:
http://www.dansworkshop.com/Homebuilt%20arc%20welder.shtml
The microwaves are a dime a dozen at junkyards, swap meets, next to the trash cans on trash day. The transformers are rarely bad, if they are it's usually the high voltage side, which you will remove anyway. What out for those High Voltage capacitors, they can bite, even after the thing is unplugged.

Edit to add: Google microwave transformer welder to get additional ideas and info. The first site mentioned had a great page, but he's selling his plans now and isn't as detailed as he used to be.

[Edited on 6-12-2005 by Mr. Wizard]
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[*] posted on 6-12-2005 at 09:48


Thanks for the good info Mr. Wizard. It looks like I just need to be a little more aggressive in my scavenging.



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[*] posted on 6-12-2005 at 19:51


With a bit of dumb luck I picked up a 1600w microwave oven today at a junk yard for $5. I don't think they usually have them. It has an E-I type core transformer which should work out nicely for my inductor. :D



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