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Gooferking Science
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[*] posted on 29-8-2014 at 07:27
Arc furnace power supply


Recently I have been messing around with arc furnaces again. The power supply I am using right now consists of three MOTs. I have all of the primary coils hooked in series from the wall (120v), and all of the secondaries shorted. This way the inductive load is split between the three transformers so they don't heat up too fast. While this design works, I feel that I need more power. Would removing the secondary coils allow for more power? Would removing the secondary coils make the transformers heat up faster? Do I need to just ditch the MOT method and use something else? Any help is appreciated!



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Metacelsus
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[*] posted on 29-8-2014 at 08:57


If you need more power you could remove a transformer (assuming the remaining two can handle it).

The shorted secondaries are only wasting power in resistive losses, and can be removed.




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[*] posted on 29-8-2014 at 09:15


I used to have a cheap welder that plugged into a 110V outlet. The carbon-arc torch that came with it worked quite well. All it had was a resistor (made from heavy nichrome wire) in series with the power leads. The total resistance was probably 5-10 ohms so the current output would be around 15A. This is an inefficient way to do it but works if you don't care about wasting power.

A better way would be to remove the secondaries from the MOT and replace them with 10-50 turns of heavy wire creating a step-down transformer. How many turns depends on the voltage you want the arc-length and other things. Or buy a cheap welder.
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WChase501
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[*] posted on 9-10-2014 at 10:46


A welder only has a limited amount of time that it can work. If you buy a cheap one, for example the usable time would be lets say 3 mins out of 10 mins, would that be enough time for you to perform your work?
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[*] posted on 9-10-2014 at 10:52


Also Neon sign transformers can be rewound just like a microwave oven transformers. NST though can be used in a shorted out environment for substantial periods of time without overheating just fyi. If you need more information please contact me.
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Dan Vizine
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[*] posted on 9-10-2014 at 11:52


What type of supply is used in industry?

I would have guessed DC may be preferred (with the non-consumable electrode negative)?

High power rectifiers are fairly inexpensive, if this is the case.
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Lambda-Eyde
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[*] posted on 9-10-2014 at 12:59


Overheating can easily be solved by submersing the transformers in an appropriate oil, which, if needed, can be cooled by circulating it through a Al/Cu radiator.



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violet sin
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[*] posted on 9-10-2014 at 13:21


I cam across this the other day: http://blog.modernmechanix.com/mags/PopularScience/5-1933/ar...

it shows how to make your own current limiting reactance from laminate steel pieces and copper wire. says it is an improvement from using a toaster (or two) to limit the current. as only about 3 amperes flows with the toaster method, and only maxed out when the carbon tips are touching. the reactance device upsets the power factor and gets about 10 amperes flowing through the arc, even though only 3.5 amperes are measured as flowing in the circuit.

the inductor was specially designed for the hobby arc melter, not just a re-purposed device.
( all from the article ^^^)

I have never got around to trying it yet, but it looks promising.
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markx
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[*] posted on 9-10-2014 at 14:10


Depending on what kind of output power and duration (duty cycle) is required for the experiment I would suggest opting for an inverter arc welder. The cheaper ones do not cost that much nowadays and one can really get some performance out of them if you run on low power.
The MOT method calls for at least 2 of them connected in series to get a decent inductive load for the primary winding and a custom wound secondary, but the whole assembly becomes bulky, heavy and is still quite ineffective in terms of power conversion.
I did experiment with the MOT's while I tried to find a suitable high amp power supply for electrosynthesis. Connecting two cores with primaries in series and rewinding a common center tap secondary over both cores produced very decent amperage (about 100A at 5V), but man was this setup bulky, ineffective and in need of forced fan cooling. Long story short....investing into a decent switching power supply is the way to go if building the power supply is not the primary objective of the experiment.




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Metacelsus
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[*] posted on 9-10-2014 at 14:23


I'm looking into building an arc furnace of my own. Does anyone know the resistance of a "typical" high current (8-10 A) arc? I would guess it would be only a few ohms.

Edit: Upon further inspection, this question is ill-posed. Arcs take as much current as the ballast allows; the "resistance" (dV/dI) varies widely and is often even negative.

What I really want to know is:

If I have a 120 VAC source and an inductive ballast with an impedance of 14 ohms (varies slightly with current due to nonlinear effects), what will the current of the arc be?

[Edited on 9-10-2014 by Cheddite Cheese]




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Dan Vizine
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[*] posted on 9-10-2014 at 17:46


If the current is determined by the ballast it would be about 8.6 amps, wouldn't it?

I've only recently started thinking about arc furnaces. Has anyone ever achieved practically useful results with anything less than a welder? It's hard to see how, but I'm no expert.

[Edited on 10-10-2014 by Dan Vizine]
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Metacelsus
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[*] posted on 9-10-2014 at 18:19


8.6 amps is assuming the ballast is the only impedance. However, I would assume the arc has some resistance, but it's probably pretty low.

Calculating: If the arc has a resistive impedance of 5 ohms (a high estimate), then that will bring the current down to 8.1 amps (never mind, this difference will probably not matter).




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macckone
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[*] posted on 9-10-2014 at 21:22


For a good electric arc you need at least 2000 watts transformed to the arc with even higher being preferred. Industrial arcs are much higher wattage.
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markx
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[*] posted on 10-10-2014 at 01:47


The resistance of the electrical arc has a negative dynamic profile...meaning as the current increases, the arc resistance decreases. Hence the current limiting has to be done by the power supply. Either by an inductive choke, saturation of the transformer core (electrode welding transformers use this option) or by switching supply current control.



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[*] posted on 10-10-2014 at 08:39


Given the high power needs, I will probably reduce the impedance of my ballast. The highest current I can safely use is 15 amps. This should make my power 1.8 kW (I'm using 120 VAC).



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[*] posted on 10-10-2014 at 19:27


I know this is not exactly on topic but this old article has fascinated me since high school. Some day I'll get around to it.

http://books.google.com/books?id=YigDAAAAMBAJ&pg=PA100&a...
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violet sin
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[*] posted on 10-10-2014 at 20:16


that looks like a smaller article for the same thing I posted up a few. mine was from popular science as well. but gave directions for the balast and 110v operation, but just said to use a couple crucibles or such. here it is again
http://blog.modernmechanix.com/mags/PopularScience/5-1933/ar...

" how to wind a simple coil reactance that controls the current, protects the fuses and cuts down greatly the cost of the electric power "

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metalresearcher
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[*] posted on 11-10-2014 at 00:30


Well, a cheap ($100) hardware store welder of max 150A does work fine. For a bit more ($200) you can get a 150A inverter welder which provides DC. The voltage is a safe value of at most 40V without load and 20V when running. Arcs running over 110V are unsafe.
I can make calcium carbide, melt (small quantities of) MgO, CaO and make Si from quartz sand and charcoal.

http://arcmelt.velp.info
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violet sin
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[*] posted on 11-10-2014 at 02:47


well the way I see it, how much are you going to be processing is probably a big concern. if you just want to dork around with small amounts for the experience and a bit of fun, it seems like the popular science version would be just fine. however, if you want to production run stuff, obviously a welder would be the better call. I try to keep the devices I make smaller unless I need the big version specifically. storage space( especially), funding and tools are pretty limiting factors for most.

small scale works just fine for me, as nothing I do makes money and I don't actually NEED to be doing it. technically I have to stay up late to just have the spare time for projects. I just wish the reactance coil plans I found were written in number of turns, not overall dimensions of the finished spool size. its not exactly common to find cotton braided shielding on enameled wire( much less hope it is the same diameter as in 1933. no length was given, and no rating for the finished coil :(
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[*] posted on 11-10-2014 at 06:24


The best are arc welders with DC around 20V,cheap one with fans on the back cut out after a short time because of heat, but they produce upto 180A.
The way around the temperature problem is to build a non conductive enclosure, aim for around 30 litres in size, disconnect the fan and plonk the whole thing in the vessel (set our current first). Fill the vessel with mineral oil.....REPEAT MINERAL OIL.............
It is non conductive, if you run the welder for more than 20 mins then add a pump and copper coil, place the coil in a bucket of water (or whatever) connect one end to the top of vessel and the other end to the pump, then from the pump short bit of copper to the bottom of the vessel.
We have run our cheapo welder (for welding) for 5 years now and it happily welds for 2-3 hours (it might do more but I get bored after a couple of hours welding lol)
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[*] posted on 11-10-2014 at 06:26


Forgot to add MOT are not safe to mess with, if your new to that kind of thing treat them like you would nitro glycerine or hydrofluoric acid, because they take no prisoners and especially in the uk with 240V going in you dont get a second chance
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[*] posted on 23-12-2014 at 08:51


Wow, a lot of great information on this thread. Thank you everyone for your input. Another method would be to buy a good ($200-$300) AC welder that has an excellent duty cycle. One that can run near continously at ~100 amps. These are easy to find, and usually run on 240v. This could be rectified to DC with some high current diodes and large capacitors. I think this would be a great power supply for an arc furnace.

A lincoln 225 amp "tombstone" welder has 20% duty cycle at max amperage. It would run for a long time at lower amperages.

[Edited on 23-12-2014 by Gooferking Science]




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markx
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[*] posted on 28-12-2014 at 16:57


Quote: Originally posted by Gooferking Science  
Wow, a lot of great information on this thread. Thank you everyone for your input. Another method would be to buy a good ($200-$300) AC welder that has an excellent duty cycle. One that can run near continously at ~100 amps. These are easy to find, and usually run on 240v. This could be rectified to DC with some high current diodes and large capacitors. I think this would be a great power supply for an arc furnace.

A lincoln 225 amp "tombstone" welder has 20% duty cycle at max amperage. It would run for a long time at lower amperages.

[Edited on 23-12-2014 by Gooferking Science]


Or the more contemporary solution fitting the 21st century....an inverter type welding apparatus. The cheaper ones go for about 150$ these days and are quite appropriate for the task at hand.
If one has the desire to fiddle with the challenge of building one from scratch then some reference can be found here:
http://danyk.cz/svar_en.html

Although if the final objective is to run an arc furnace experiment I would not reccomend to go through the trouble of building the power supply for it. It is really not worth the pain if one has no inclination towards the inner workings of power electronics.




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[*] posted on 22-10-2015 at 18:57


A friend and I are working at building a carbon arc furnace like the one shown in this video: https://youtu.be/VTzKIs19eZE

We got a couple of free microwaves and removed the useful stuff, including two transformers. I have a couple questions about how to best balance the transformers when rewinding them. The two transformers are from different wattage ovens, I intend to hook them up in series and am unsure what the ultimate output goal should be. Do I want to just balance the output voltages to be the same or should I attempt to balance (VA) to minimize the load differences causing one transformer to overheat sooner? I have removed the secondary (HV windings) from both transformers and re-wrapped the cores with #8 gauge wire. I have wrapped the smaller transformer with the maximum number of wraps I can fit (15turns). The larger transformer can take more wraps, (12.5 wraps outputs the same voltage as the smaller transformer, although I could fit several more if needful)

With these in series should I leave the heavier transformer with less wraps or add as many as I can fit?

detail on the the transformers are as follows:

Small transformer: Primary Resistance 0.7 Ohms, Secondary 15 wraps, 0.3 Ohms, voltage output 14.6v. (120V in)

Larger Transformer: Primary resistance 0.4 Ohms, Secondary 12.5 - ? wraps, voltage output 14.6v to ~20v.

Thanks
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WGTR
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[*] posted on 22-10-2015 at 22:01


If the secondaries are connected in series, they will have the same current going through both of them at the same time. Calculate the number of turns such that each respective core stays within its VA ratings at the expected load current. The voltage output for the smaller core will be less than the larger one in this case. Wire gauge size may need to increase or decrease depending upon the given number of turns needed.

My $0.02.

Also, I think you'll find the arc to be a bit unstable with a 30-40V supply, due to the arc's strong negative resistance characteristic. It will want to go between the two extremes of pulling too much current, or not enough to keep the arc going. Graphite electrodes do allow for a stabler arc than other materials, however.

I think you'll have better success with a 60-80V output, with a large ballast resistor or inductor in series to help stabilize the current. It's like calculating the series resistor to bias an LED. The arc voltage will probably run between 15-30V, depending on current. That leaves 30-65V across the resistor/reactor. Using Ohm's Law will allow you to calculate the needed ballast for a given range of amperage.

[Edited on 10-23-2015 by WGTR]
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