Sciencemadness Discussion Board - DIY 3000'C homemade electric Arc-for P,... production

DIY 3000'C homemade electric Arc-for P,... production

halogenstruck - 24-9-2009 at 11:07

very dangerous.be very careful and if you don`t know it completely,simply forget it.do it in your own risk.

all electric wires must be capable of at least 3kw,20Amper.
here i want to explain you how to have an easy homebuilt high temperature source of heat for your needs

ver 3000 degree:eletric arch,without need of transformer:
it can be used for regular preparation of Phosphorus, calcium carbide, silicon carbide,...
100% practial and tested.
you should just buy an nichrome[nikel-chromium alloy] 3000Watts coil[like the one used in hot plates,heaters,...] and u should secure it on a fire clay frame or base and then in serie you should connect two clips that keep the
graphite rods therefore when u just touch the graphite rods its like that electricity is passing simply through nichrome wire but when u take rods back,electricity current get disconnected.in practice i used 220V and i did
not do that with 110V but it seems to me that it should work too but little bit weaker.
all the copper wiring should be from thick wires to be able to work with high current otherwise they get warm then insulator will burn and shortcut!!!
how to run it:simply connect the rods,it sparks,then gradually take it back,electric arch will be formed.don`t watch to arch with naked eyes.use UV-lens or very-dark eye glasses,wear gloves!!!electric shock!!!
mixtures like phosphorus making mixture(C+Ca3(PO4)2+SiO2)contain C and are conductor.therefore if put the rods inside the retort made of fire clay-fire cement,...in contact with each other,after some seconds they have spark then the mixture get warmed then rod contacts will be cut but
electricity will run through carbon content of mixture then because of created high temperature,mixture melts and pass electricity much better till all the mixture is liquid.
i used this simple device for melting silver(100Gr),palladium ,platinum and even rhodium although not easily about the latter.
it can be used simply for preparation of sulphides from sulfate and charcoal too!!!
=============
before i had a powerful transformer and powerful diode(high amper)[can be made by paralleling diodes with lower current limit and making a rectifier] and i used 50v electricity as DC for electric arch making with graphite rods and i melted NaCl and made Na althogh i did it in open air and it burnt with yellow flame but some of its globules were surronded
inside salt and i made metallic sodium when i was just 14!
==============
a question:I want to use the reference&Whimsy
section of this forum but it is restricted.how i can access them?i couldn`t find any access method mentioned in site.thx

[Edited on 24-9-2009 by halogenstruck]

electric arch.bmp - 352kB

[Edited on 25-9-2009 by halogenstruck]

blogfast25 - 24-9-2009 at 12:33

Quote: Originally posted by halogenstruck

you should just buy an nichrome[nikel-chromium alloy] 3000Watts coil[like the one used in hot plates,heaters,...] and u should secure it on a fire clay frame or base and then in serie you should connect two clips that keep the
graphite rods therefore when u just touch the graphite rods its like that electricity is passing simply through nichrome wire but when u take rods back,electricity current get disconnected.in practice i used 220V and i did

Please explain that part better...

hissingnoise - 24-9-2009 at 12:46

Blogfast25, I think it's about wasting 3kilowatts per hour in order to get some resistive heating or arcing going in a fire-clay pot, and doing it in a way that will surely set fire to your house and home. . .
It sounds too dangerous and inefficient to be of any use.

[Edited on 24-9-2009 by hissingnoise]

halogenstruck - 24-9-2009 at 12:58

it uses less than 3kw for sure as in direct contact it is 3kw but when we have arch,it`s less than that.
if u don`t serie a resistor,then your home fuse box cut the electricity as a huge current want to pass.in fact it limits electricity current.
i did it many many times.
i tried to illustrate it on a jpg pic.sorry for low quality design.

electric arch.bmp - 352kB

hissingnoise - 24-9-2009 at 13:10

Even so, it sounds wasteful, at best.
A welding transformer would be better. . .surely?

blogfast25 - 24-9-2009 at 13:15

Thanks for the clarification, Halogenstruck. The 3 kW resistor reduces current strength and if you can safely (by insulating yourself) manipulate the graphite electrodes, this should be relatively safe to use for an AC arc. Max. current about 14 A.

halogenstruck - 24-9-2009 at 13:43

you are welcome blogfast25.
in fact in practice you should put the arch on a heat resistant board.in past i fixed one rod inside the crucible from clay and just moved one electrode.i used glove and well insulated plier[handle had a thick plastic insulator][600v safe].the distance between electrodes can rich between 1-2cm i think.

12AX7 - 24-9-2009 at 18:48

Well, it's all heat, so it's all 100% inefficient. But maybe you count heat as efficiency? In which case, it's 100% efficient. Well, not with the resistive ballast, then maybe only 50% tops, but hey, it's cheap as hell, and it's not going to be running for days, who cares.

FYI, "arch" is an architectural feature. "Arc" is a thermoelectric discharge. ("Spark" is either a short duration arc, or an ionizing discharge not at thermal equilibrium.)

Tim

Sedit - 24-9-2009 at 20:36

If hes speaking of what I think he is(Hard to tell due to the confusing opening post) I have also done something simular using an old electric stove top burner as the resistor. This is nothing more then a thin piece of Nichrome wraped in ceramic, wraped in a metal sheth. It prevents blowing the breaker but I always found it hard to sustain an arc. I only toyed with it for a while but Im sure I could get something up and running if I continued to work with it.

halogenstruck - 24-9-2009 at 22:13

thx for correcting me,u r absolutely right.arc
the resistor element does not emit hit a lot,it`s only there to avoid shortcut and power out,when we have arc then R of arc is much higher than nichrome wire 3KW as arc is a 1 cm distance of air,therefore heat loss from 3kw coil is low

barbs09 - 25-9-2009 at 00:56

Well arc furnaces (and arc lights) have been around for a while and may still have a place in a DIY lab for making Phosphorus or carbide. The link to the old article below is a good read. For a resistive load they use a conductive solution. Perhaps their method of striking an arc is better/safer than using pliers and gloves.

http://blog.modernmechanix.com/2007/08/02/arc-furnace-incase...

Another

http://blog.modernmechanix.com/2007/03/19/experimental-arc-f...

The guy in the above article can't have any eyes left!!

Lots of other good stuff an the Modern Mechanics web site as well

Halogenstruck once the arc is struck do you have to constantly move the electrode in to make up for carbon erosion? I guess gouging rods are adequate since they are designed to withstand high-temperature arcs?

[Edited on 25-9-2009 by barbs09]

garage chemist - 25-9-2009 at 05:19

Phosphorus from phosphate, silica and carbon needs just 1500°C and is therefore one of the least temperature-demanding applications of an arc furnace. Temperatures above 2000°C are needed e.g. for calcium carbide production and melting down alumina for abrasives production.
Go to 4:20 to see alumina being melted and poured:
http://www.youtube.com/watch?v=fopD-QA1YY0

Using an inductive current limiter is a good improvement over a resistor, as it doesn't waste power and limits the current just as well. A transformer that produces about 40-60V secondary would be even better- it's mostly the current in the arc that decides how powerful the furnace is.

The difficulty in building such a furnace lies in catching the phosphorus vapor. The whole thing must be gastight and the outer shell must be kept above 280°C to avoid the phosphorus condensing in the furnace.

blogfast25 - 25-9-2009 at 06:41

barbs09 and garage chemist:

Thanks for the links. Now I know all about hot dogs too and am even less inclined to eat them!

I suppose the carbon electrodes used in most ordinary batteries are out of the question, huh? I imagine these to fritter away in record time. What type of carbon to use and where to get it?

garage chemist - 25-9-2009 at 08:06

The carbon "gouging rods" for arc welding would probably be a good choice here, as they are specially made to be used as arc electrodes.

Magpie - 25-9-2009 at 08:21

Ace Hardware has gouging rods.

http://www.sciencemadness.org/talk/viewthread.php?tid=2680&a...

halogenstruck - 25-9-2009 at 08:39

graphite rod inside spent dry cells work well.yes as arc run graphite rod burn very very slowly and evaporate,for some period electricity still keep the arc running as distance grows but after sometime you need to aproach them a little bit,but in practice when arc starts it create heat then mixture melts and by conductivity heat is made in following.in fact mostly arc is needed till mixture melt then molten lava conduct electricity and create enough heat to stay molten and warm up even more as for sure they conduct electricity better than hot air.
in fact if container is insulated very well smallest heat can keep the temperature constant just to compensate lost heat from insulant.because of this fact,even with a nichrome wire coil alone you can make a 1000'c furnace and even 2000 if use a very good insulator but in fact over 1000 it melts and does not work and u need SiC resistor.
for leakage you can simply put an inert stream of neutral gas like CO2 or burnt gas from a gas burner.
for inductive oven,u can use a reactor or retort made from mixture of fire clay and graphite powder inside a microwave.as graphite absorb electromagtetic waves it warms up and get red.

halogenstruck - 25-9-2009 at 08:45

yes possibly if u use strong electrodes and in an inert atmosphere,it should work the best.
an idea:
how if we make a coil from thick copper and at end we connect it to graphite road and put it inside microwave.
first electricity get induced on coil and makes arc then mixture is getting warm and melt.when melted and red,it`s conductor and absorb electromagnetic waves directly from microwave and stays warm and melted and no need to arc anymore.therefore don`t care to move rods anymore.just as an innitiator[like catalyst!!]

blogfast25 - 25-9-2009 at 09:54

Quote: Originally posted by halogenstruck

before i had a powerful transformer and powerful diode(high amper)[can be made by paralleling diodes with lower current limit and making a rectifier] and i used 50v electricity as DC for electric arch making with graphite rods and i melted NaCl and made Na althogh i did it in open air and it burnt with yellow flame but some of its globules were surronded
inside salt and i made metallic sodium when i was just 14!

And you made sodium with an arc, how?

Oh, I see: DC arc! (must get those reading glasses...)

[Edited on 25-9-2009 by blogfast25]

S.C. Wack - 25-9-2009 at 11:43

Quote: Originally posted by blogfast25

And you made sodium with an arc, how?

And I thought that the rhodium, etc. would be the most questionable part.

It's all out there, from dead people I trust a little more.
http://books.google.com/books?id=H_RUAAAAMAAJ
http://books.google.com/books?id=WDUKAAAAIAAJ
http://books.google.com/books?id=ah9DAAAAIAAJ
http://books.google.com/books?id=Lx9DAAAAIAAJ

http://pubs.acs.org/search/advanced
title search furnace, or don't.

User - 26-9-2009 at 02:03

@halogenstruck
It's some nice DIY stuff.
Can you explain to me how you constructed the heat transfer between the arc and the reactor.
And do you have pictures of youre device.

I have 2 neon trans. laying around doing nothing.
It is quite easy to draw long and hot sparks with it.
Might be uesfull stuff.

halogenstruck - 26-9-2009 at 02:46

in fact i did the arc in a graphite crucible or in normal crucible.
in case of silver i fixed one rod inside crucible in contact with silver powder and arced between graphite rods.i arced between rods many times to melt it.in case of Rh i melted around 1 or 2 gr with some difficulties as its melting point is very high.
in case of salt i did it on the brick.u can do it inside a normal crucible.
i think if u use a graphite crucible as - electrode,cathode and a rod as anode and use CO2 current around it when u work u can prepare good amount of Na especially if u use NaOH.
i did not use it for P preparation but surely it`s the commercial process for making P and works very well.
in P case like many other cases u don`t need arc always,remember, ARC IS NEEDED FIRST WHEN THE MIXTURE IS COLD,AFTER ARC MELTED A LITTLE BIT OF MIXTURE,THEN CURRENT WILL PASS THROUGH THE MELT AND CREATE ENOUGH HEAT EVEN MORE THAN ARC TO MELT WHOLE MIX AND KEEP IT IN MOLTEN STATUS.
sorrily it was a long time ago and i did not take any picture that time.it`s a simple thing u can master it easily but be a little careful and regard safety issues like goggle/glove/mask/heat insulators/don`t allow wires even with insulator touch each other as they warm up then insulator may melt and a shortcut is possible.

halogenstruck - 26-9-2009 at 02:59

about neon trans i don`t know it exactly but it think it won`t work in this way as it increase the voltage but u don`t need it.u need only current.

chloric1 - 26-9-2009 at 05:51

Quote: Originally posted by User

@halogenstruck
It's some nice DIY stuff.
Can you explain to me how you constructed the heat transfer between the arc and the reactor.
And do you have pictures of youre device.

I have 2 neon trans. laying around doing nothing.
It is quite easy to draw long and hot sparks with it.
Might be uesfull stuff.

Neon sign tranformers are better designed for building Tesla Coils not arc furnaces. There are several books on building thoughs. What you ideally need is a step down transformer. One that produces 30 to 40 volts and 50 to 80 amps. This way it can run off a 115V. It is the same as a 115V stick welder.

Sedit - 26-9-2009 at 16:45

Here is an old paper from my website. It was published in an old science magazine known as The experimentor and it talks about making something just like this only with one major difference that I feel is better then the nichrome resistor mentioned here.

In this article they use a salt water bath for the resistor where the depth of the electrode and the concentration of salt can change the desired current to what ever one desires. Just though I would track this down for ya'll as I read it sometime ago and knew I had to have it uploaded already.

densest - 26-9-2009 at 17:17

The suggestion of an electric welding apparatus for this sort of work is probably the best idea. For example, a used 115V (input) 50A (output) welder is for sale locally for $US 35. I looked on the Internet for about 30 seconds to find that one. Not free, but much safer than any direct connection to the mains!!!! For more money one can get 100s of amperes at 10-50V AC or DC.

The big advantages of the welder are (a) isolated from mains (b) knob to adjust current (and some have a knob for voltage too) and (c) big thick cables and big thick power cord and big thick electrode clamps already attached. The case, wires, and clamps alone are worth paying for if you believe in safety or convenience. I believe that the older units use an adjustable series inductor to limit current; more modern ones use semiconductor inverters with current limiting.

For arcs or resistance heating the cheapest ancient electric welder will work very well and save a great deal of trouble and electric cost.

Besides, if you don't want it any more you can sell it, as opposed to an improvised apparatus.

[Edited on 27-9-2009 by densest]

watson.fawkes - 27-9-2009 at 05:59

As a pointer for anybody who's serious about building a good arc furnace, I can recommend reading up on the power supplies and, particularly, the control circuitry for EDM, electro-discharge machining. An EDM supply is pulsed, and an arc is continuous (usually), but the important similarity is that there's a servo control loop that keeps the EDM electrode at a constant distance to the work. By analogy, the same kind of control can keep the electrodes at constant distance from each other, automatically adjusting for wear and allowing a fairly good measure of temperature regulation.

densest - 27-9-2009 at 14:13

@watson.fawkes: good advice... though perhaps the topic of power supplies has come up often enough to warrant an FAQ for experimentalists not totally immersed in power electronics? I'd be glad to contribute a few paragraphs.

As an addendum to what watson.fawkes said, old electric arc lighting used mostly mechanical ways to gradually feed the carbon rods inward as they burned off. At a first approximation, once the arc is established and the system is stabilized at a particular temperature, feeding inwards an experimentally determined number of millimeters an hour would do a lot to keep an arc running.

EDM arcs are interesting since they're under fluid. I'm only passingly familiar with them; I do know that they operate as a "short arc" meaning that direct ionization from the intense field at the moving electrode keeps the plasma going, and the fluid is in motion to wash away the material removed from the work piece. I'm less sure that a carbon arc, which could be 100 times longer, behaves exactly the same.

An electronic control system for temperature stabilizing an arc furnace could be interesting.... 'fuzzy logic' or proportional-integral-derivative temp controllers with integral thermocouple conversion aren't -too- expensive ($US 100 or less for a cheap one new, less for a used one). Proportional electronic power control at KW levels can get more expensive: a new proportional voltage-controlled "dimmer" rated at 20A costs $US 75 or so.

The interesting part would be extra logic to detect arc failure, running out of carbon rod, and other out-of-the-linear-region conditions. Or watch it all the time and have a big OFF switch. What I don't know is whether or not arc length increases or decreases power output over a wide region of length.

A purist's approach might be to use two control loops: the "primary" one regulates current into the arc sensing temperature while another one attempted to keep the length stable sensing voltage, since voltage across the arc changes with length and current. It is complicated because many arcs have effectively negative resistance: the voltage across them decreases with increasing current, at least over a large region of operation. I believe that voltage across an arc at atmospheric pressure is roughly proportional to length since it must continually ionize new gas molecules to replace ones leaving.

watson.fawkes - 27-9-2009 at 16:27

Arc discharge is full of non-linearities and various kinds of parameter dependencies. In my opinion, it's a problem for purpose-built software. Now I do a lot of software, so this doesn't seem as daunting as it might to others, but when you have a system that's full of, well, behavior like an arc is it's time to step back and think about what's important, which is not killing yourself, and this is mostly about thinking about the edge conditions: start-up, shut-down, faults, and emergencies. I've thought some about building an arc furnace, though it's not on top of my priority list. Then again, I really have to care. I have 277/480 V 3-phase-wye power in my shop on 50 A circuits. When something goes seriously wrong with that, it's 70 kW before the breaker even thinks about tripping. If you don't want to make something this big, you may not need to concern yourself with the things I do.

Top of my list is built-in ground-fault sensing, both a shutdown circuit in hardware for faults and, for smaller faults, a software monitor on the sensors to keep an eye on leakage current, say, from failing insulation or corona discharge. This subsystem includes current sense on the electrode feeds, because a fault there is potentially more dangerous that a fault on the mains.
A crowbar circuit for failure, so that when it shuts off, it stays off. Were I to build a DC arc furnace, I'd be tempted to feed the mains through a mechanical contactor to the arc supply section, wired so that start up requires a mechanical button press and that the crowbar shuts it off.
Current-limiting engineering, including picking a saturable-core transformer, fusing, output current sense (feeding the crowbar), perhaps an output ballast coil, etc.
A high-frequency arc start generator. This one's not as obvious. Striking an arc directly is just chock full of transients, high inrush currents, brand new non-linearities, and generally a whole lot of extra things to go wrong. It's a whole lot more predictable.

After all this packaging, I'd use, internally, generic kinds of switching power supply blocks: a DC power source, a switch matrix with isolated triggers, sensors, passive output network, and a central microprocessor controller. To this add servos and sensors for the carbon rod feeding subsystem. If you're building something like this, it's a project that will be a constant subject of tinkering, as applications change. So it's a good thing to build it with a relatively good amount of reconfigurability.

12AX7 - 28-9-2009 at 07:54

Why does that need a microcontroller? It'll sit around twiddling its thumbs. All those features are already covered by the very hardware you listed. Maybe you can put in some digital panel meters (DPMs), but that hardly deserves a uC when off-the-shelf units are available.

Tim

densest - 28-9-2009 at 11:14

For a system as complex as watson.fawkes suggests (and it sounds like a good idea to me), an industrial process controller should work well. I'm thinking of one of the uC controlled relay logic emulators which can do computation as well. I/O is their business, so they come with lots of inputs and outputs which is what's necessary. Sequencing startup and shutdown, for instance, would be much easier with one. It would be very nice to have a simple "on" button, a "temp" knob, and a big "off" button, and after that a way to set a temperature profile over time. A high frequency ignition scheme could be lifted from a commercial welder.

Don't forget the exhaust fan for a 70KW arc! CO, NOx, O3, metal vapors, evaporated firebrick, .....

Stepping back a little further, what would an arc furnace be able to do that (for instance) an induction furnace couldn't do? That's a lot cleaner, heats more selectively, and by choosing susceptors one can heat in very precisely controlled regions. Obviously, something which needs 10,000C from the arc itself is probably not a candidate. The arc is a lot more dramatic, too.

watson.fawkes - 28-9-2009 at 12:26

Quote: Originally posted by 12AX7

Why does that need a microcontroller? It'll sit around twiddling its thumbs. All those features are already covered by the very hardware you listed.

The safety gear I spent time talking about would, indeed, be outside the microcontroller's reach, and on purpose, since you don't want your fail-safes to fail unsafely because of software defects. I didn't talk nearly so much about what I'd put in a microprocessor for. What I was imagining the microcontroller for was for holding parameters of the arc model, largely to keep away from phase change points, such as arc dropout and sudden conductivity drops. As such it would be controlling the electrode servo system and setting the operational points of the power supply. And, for certain experiments, it provides a ready place for remote-control operation, if you don't want to be standing next to the gear during the run.

watson.fawkes - 28-9-2009 at 12:45

Quote: Originally posted by densest

For a system as complex as watson.fawkes suggests (and it sounds like a good idea to me), an industrial process controller should work well.[...]It would be very nice to have a simple "on" button, a "temp" knob, and a big "off" button, and after that a way to set a temperature profile over time.

Personally, I'll probably use an Arduino, since they're quite handy and the software development environment very clean. If you are more comfortable with a process controller, I'm sure it would work too.

It's true I'm thinking about a complex system. The utility of such a device is exactly that the external controls are simpler, as you point out. In my thinking, there are two kinds of reasonable ways to do this. The first is the kind that you pay attention to full time while it's in operation. These require less internal complexity at the cost of requiring manual control. But you shouldn't walk away from one while it's operating, because you are the safety device. The second is the one as I described, which is capable of taking care of itself to a great extent, and you can walk away from it. Both, of course, should have a big red emergency stop button you can slam down on.

Quote:

Don't forget the exhaust fan for a 70KW arc! CO, NOx, O3, metal vapors, evaporated firebrick, .....

Oh, yes. I was just addressing the power supply.

Quote:

Stepping back a little further, what would an arc furnace be able to do that (for instance) an induction furnace couldn't do? [...] Obviously, something which needs 10,000C from the arc itself is probably not a candidate. The arc is a lot more dramatic, too.

You have it exactly. Some processes need the higher temperature of an arc.

And as long as we're stepping back, I should point out that there's a lot of overlap between the supply as I've outlined it for an arc and that for a induction furnace. In particular, the safety-specific features are almost all the same. Then, if you've got a solid DC stage, you can put different switch topologies on it for different applications. If you're designing for hacking, you can make each individual switch unit (the switch device such as a MOSFET or IGBT, gate trigger isolation, gate bias supply, and optional sensor for switch failure) a separate module, allowing reconfiguration by bolting to new bus bars. Finally, you load in new control software.

12AX7 - 28-9-2009 at 18:31

You still don't need a controller, arcs are not nearly as complex as you imagine them to be. Constant current supply and you're set. Electrode spacing can even be done mechanically, but if you must have an electric servo, match it to arc voltage. Nothing more mysterious than that.

What a microcontroller would be useful for is reducing parts count (if you have one or a few controllers and PWM systems, you can nix a lots of chips and resistors this way) and adding far more complex features, like something above and beyond PID (possibly higher order time constants and nonlinear terms -- assuming, that is, that you've mapped the model quite thoroughly enough to be confident that you can actually write the correct inverse transfer function), temperature profiles, digital I/O, etc.

Microcontrollers are useful, but please don't fool yourself on what they are used for. They are often good for reducing parts count. They aren't good for reducing price, and they have no effect on development time whatsoever.

Note also that microcontrollers are sensitive equipment and not exactly robust around power electronics of ameteur construction.

Tim

IrC - 28-9-2009 at 23:47

It is well known that the impedance of a typical burning arc is around 0.02 to 0.05 ohms. If we say the impedance of the heater coil is around 19 ohms then the bulk of the energy is being wasted in the heater coil. Assuming 15 amps in a series circuit the power in an arc of say 0.035 ohms is just a few watts, meaning you are wasting large amounts of energy.

A neon transformer has a built in shunt which limits short circuit current to 30 or 50 milliamps. Using one constantly will boil the potting tar and burn it out. Far better is the idea of using a transformer from an oil burner furnace, where the transformer is designed to operate in this type of setup.

bquirky - 29-9-2009 at 04:59

I remember reading about alot of high power carbon arc electric lighting before the lightglobe was around

One of the ways that was reguarly used to maintain a good arc was by mechanicly coupling an electro magnet to the carbon electrodes and then running the windings in serise.

The idea was that when the current passing through the arc dropped the magnetic feild weekened and allowed the spring loaded electrode to advance a little more.

There where all kinds of variations on this them incuding electromagnetic rachets that cranked the arc rods acording to current variations.

There was evan one scheem called a Yablochkov candle that had the two electrodes running parralell to each other like this l l the space in between was filled with (i think) plaster. the idea being that the plaster whould crumble away at about the same rate as the carbon rods did.

I have evan made a miniature arc lamp by busting open a relay and gluing a graphite pencil lead to the moving part and wiring the relay in series with the two electrodes.

Carbon arc lamps i believe are still used in some parts of the world in cinema projectors using that kind of system.

But if you want an arc furnace... how about doing something with a ballasted microwave oven transformer. or a cheap arc welder in an argon atmosphere perhaps using cooled tugston electrodes ?

12AX7 - 29-9-2009 at 07:29

Quote: Originally posted by IrC

It is well known that the impedance of a typical burning arc is around 0.02 to 0.05 ohms. If we say the impedance of the heater coil is around 19 ohms then the bulk of the energy is being wasted in the heater coil. Assuming 15 amps in a series circuit the power in an arc of say 0.035 ohms is just a few watts, meaning you are wasting large amounts of energy.

No, that can't be right, and at best you must be confusing incremental with average resistance. A short arc at 15A might drop 20V (a typical welding voltage), which is 20/15 = 1.3 ohms average. It might be 30V at 200A, which suggests 0.15 ohms average and (30 - 20) / (200 - 15) = 0.05 ohms incremental.

Regardless, an arc is nonohmic, so it's useless to rate one in terms of ohms. I can burn a handsome thermal arc from a flyback transformer, that might be dissipating 20W at 20kV and 1mA (apparently 200kohms average resistance). Or I can burn steel at 20V, 100A (2kW and 0.2 ohms). Or I can leave it to the power company to occasionally produce some quite spectacular accidents in the 500kV, 10kA range -- hundreds of megawatts, and still only a few ohms.

Tim

watson.fawkes - 1-10-2009 at 07:36

I should mention two more safety items that are not complicated, and which I should have listed before:

Emergency stop button. You know the big red kind, preferably one that requires some second motion to reset.
A power-on lamp, possibly on a pole. Just so that you know it's on, and more importantly, so that other people do.

For those interested in understanding how fault-sense circuits work, first read up on residual-current devices. Then to understand why the picture in that article applies to alternating current, see Ampere's law and electromagnetic induction. Note that if you're planning on a DC arc, you'll need a different technique, either using sense resistors and op amps, or perhaps the Hall effect.

12AX7 - 1-10-2009 at 14:25

Wow, a brief, informative post by Fawkes. I believe that's a first!

FYI, they're called GFCI receptacles over here.

densest - 2-10-2009 at 17:20

@12AX7, @watson.fawkes: one could build an AC leakage breaker, but they're commercially available for a wide range of power levels and leakage levels and show up on the surplus market for reasonable prices.

It would be easy

enough to make a DC leakage sensor with an old laminated core, a hacksaw, and a few turns of heavy bifilar wire. Hmmm... quick calculation suggests that you'd want the gap as small as possible to admit the thinnest possible (.5mm?) sensor. 100 turns @ 10mA difference @ .5mm = 25 gauss, enough to be sensed with an analog hall effect sensor but it would need amplification. It would be well out of the noise and earth's magnetic field.

I don't believe that commercially available arc welding apparatus (AC or DC) come equipped with such a sensor. The arc circuit is not referenced to earth and the voltage is low enough that leakage is not a safety issue.

I still think that for anything up to a 5 or 10KW arc or so a welding supply with perhaps modified controls and run at a level where it is capable of continuous operation would be by far the most cost effective solution and easiest to get working. Constant current, constant voltage, limits, high frequency arc ignition, all are ready made. The electrode feed would be the major area for implementation, I think.

For the truly insane, take a 20-100HP (15-75KW) VFD (variable frequency motor drive), reverse engineer the I/O on the CPU, and replace the firmware chip.

AC, DC, up to 7 or 15 KHz ignition drive... use a couple of very high current chokes on the output if necessary to give the CPU enough time to recognize shorts, perhaps. Total outlay (on EBay) of about $500. Again, the only hardware construction necessary is the electrode feed. The VFD will have uncommitted digital and analog I/O sufficient for (say) a set of photocell arc length and status monitors, an electrode drive motor, etc. If reverse engineering the whole thing is too much trouble, it's quite likely that with a little study the existing CPU can be cut away and a new unit of one's choice installed instead using the IGBT drives, current sensors, and general purpose I/O already existing.

Anything to avoid creating the -whole thing- from scratch if it's not necessary and/or not fun.

watson.fawkes - 3-10-2009 at 07:50

Quote: Originally posted by densest

one could build an AC leakage breaker, but they're commercially available for a wide range of power levels and leakage levels and show up on the surplus market for reasonable prices.

This is true, if you want a separate module for leakage all by itself. And that's not a bad idea at all, really. On the other hand, these are very easy to make, particularly if you pick up a current transformer on the surplus market. I have several at this point, which is occasionally amusing, since the sizes aren't always apparent from the photos, and I've got one that has capacity way larger than I would ever need (but which I'll use anyway).

I have two reasons for making them myself. The first is that I want two of them in a power supply. One is for the device as a whole, right as the mains enter the cabinet. The other is for the high-power supply separate from the other circuitry in the cabinet. The difference, for me, is that the outer unit passes all four conductors of the three-phase wye circuit through the inductor, and that the inner unit passes only the inner three wires. The inner unit enforces a design criterion on the power supply that it draw balanced phase currents. Thus the outer unit detects earth faults, where net current is leaving the device as a whole. The reaction to this fault should be device shutdown. The inner unit detects a more general class of leakages, including, particularly, neutral currents (that is, imbalanced phase draw) and phase-to-ground faults (as opposed to phase-to-neutral current, which is ordinary). The reaction to these faults need not be immediate shutdown, since they're not posing any immediate life hazard. In such a fault condition, having a parameter logging facility in the controller is useful for forensics.

The second reason is that I want to have an interruption system that has more inputs than just that for the leakage current. This is really for the inner interrupter. One is a deadman switch so that if the control circuit loses voltage, it shuts off the main power supply for the arc. Another is an exceptional stop switch, which shuts off arc power without shutting down control circuits, which is useful if you do parameter logging. (There's also an emergency shutdown switch that cuts all power.) There's also one for leakage current on the inner interrupter, using a second, higher threshold than the one that only yields a fault indicator.

So, in summary, if I find an adequate fault interrupter cheap enough, I'll certainly use it for the outer unit.

watson.fawkes - 3-10-2009 at 08:15

Quote: Originally posted by densest

It would be easy

enough to make a DC leakage sensor with an old laminated core, a hacksaw, and a few turns of heavy bifilar wire.

This would be one way to do it. Another would be to use the Hall effect directly. Take a strip of soft iron sheet metal; this is an adequately low resistivity and high enough permeability to capture most of the field from a conductor. Bind the two DC conductors to either side of this piece of metal, passing the conductors in opposite directions from the feed so that their induced fields should cancel. (You could also use small H-channel for this.) Now pass a bias current across the narrow edge of the strip; perhaps you braze on some copper strips to distribute the current. Then measure the induced EMF between the two ends of the strip. This can be made pretty sensitive since you can mode-lock to a modulated bias current.

Quote:

I still think that for anything up to a 5 or 10KW arc or so a welding supply with perhaps modified controls and run at a level where it is capable of continuous operation would be by far the most cost effective solution and easiest to get working.

I'm in agreement. The old big-iron versions of these are ideal for this kind of hacking, since they're cheap and the modules inside are easy to get to and rewire.

densest - 3-10-2009 at 16:07

Quote: Originally posted by watson.fawkes

Quote: Originally posted by densest

It would be easy

enough to make a DC leakage sensor with an old laminated core, a hacksaw, and a few turns of heavy bifilar wire.

This would be one way to do it. Another would be to use the Hall effect directly. ... Then measure the induced EMF between the two ends of the strip. This can be made pretty sensitive since you can mode-lock to a modulated bias current.

I'd have to disagree and go for something like the Honeywell linear current sensors ($10-$75) with Hall effect (3ms) or giant magnetorestrictive (0.5us !) semiconductor elements. The magnetic properties of iron change with stress and temperature history sufficiently to make getting a good sensor pretty difficult. Getting truly balanced magnetic properties while folding & bending sheet iron is not a project I would like to undertake. I've already lost too much hair doing amateur magnetics!

Besides, the Hall effect in metals is very much smaller (1000x or more) than in semiconductors and the GMR sensors are even better. The Honeywell units come with the magnetics integrated and a fully temperature compensated and linearized output. It's hard to beat that.
Now, if you want to learn all about grain and residual stress and impurity inclusion distribution, go right ahead. Electronic Goldmine is selling sheets of super high permeability magnetic material right now...

Alternatively, one could break open a GFCI of the appropriate size and cut the wires to the trip coil. I had three of various sizes but I threw them away last year

Various size sensors are up on EBay right now for what I consider too much ($100-400). A deal might wander by... The brand name "LineGard" seems to be the most available.

One could also disassemble a single phase GFCI and run more wires through the sense transformer & put a SSR or optocoupler on the trip output - total cost about $30 (one or two tries learning how to open the thing without destroying the parts you want) and a couple of hours messing around.

watson.fawkes - 3-10-2009 at 20:34

Quote: Originally posted by densest

I'd have to disagree and go for something like the Honeywell linear current sensors ($10-$75) with Hall effect (3ms) or giant magnetorestrictive (0.5us !) semiconductor elements. [...] Now, if you want to learn all about grain and residual stress and impurity inclusion distribution, go right ahead. Electronic Goldmine is selling sheets of super high permeability magnetic material right now.

Good information. I hadn't heard about the giant magnetostrictive materials until your post. (I assume these are the same as gaint magnetorestrictive materials, no?)

In any case, there's no need for a very fast reaction times in the sensor and some reason to avoid it. There's always some capacitance in the load device and this leads to current imbalances when the current changes. If you're using a pulsed DC arc, for example, each arc-start and arc-stop operation would definitely show off this capacitance. On the other hand, if you can detect "steady-state" operation, say, by getting a signal from your controller, then you could switch on a fast sensor when it was appropriate. Regardless of this, capacitance limits the sensitivity of any kind of leakage device. If building your own, it's a good idea to put a trimmable lower limit on it, so as to avoid nuisance trips.

Another consideration for a coil-wound DC sensor is the inductance. This wouldn't matter in a continuous setting, but it might in a repetitive pulse one. I haven't done any estimates about how significant this might be.

I bought a sheet of that mu-metal from EG, but not for this. Hmm.

densest - 3-10-2009 at 22:03

My bad. Giant magnetoresistive not magnetostrictive. Both GMR, just entirely different mechanisms... though the side trail through MRAM and other applied quantum physics objects was interesting to look up (4Mbit 35ns static MRAMs for about $30 each - what could one build?)

And what might one make in a high power arc furnace? A high power resistive furnace is used to make CaC2, a route to acetylene and with another, lower temperature, step, to dicyanodiamide, two interesting raw materials.

UnintentionalChaos - 3-10-2009 at 22:20

Quote: Originally posted by densest

My bad. Giant magnetoresistive not magnetostrictive. Both GMR, just entirely different mechanisms... though the side trail through MRAM and other applied quantum physics objects was interesting to look up (4Mbit 35ns static MRAMs for about $30 each - what could one build?)

And what might one make in a high power arc furnace? A high power resistive furnace is used to make CaC2, a route to acetylene and with another, lower temperature, step, to dicyanodiamide, two interesting raw materials.

We have what should be a very nice, clean preparation of calcium cyanamide already, via the pyrolysis of calcium cyanurate:

https://www.sciencemadness.org/talk/viewthread.php?tid=2762

http://www.sciencemadness.org/talk/viewthread.php?tid=8594#p...

Birkeland-Eyde with a pulsed arc

watson.fawkes - 3-10-2009 at 23:35

Quote: Originally posted by densest

And what might one make in a high power arc furnace?

Why funny you should ask. I've been thinking about pulsed power because it's occurred to me that you might be able to improve the efficiency of an arc for burning nitrogen (for the Birkeland-Eyde process) by running a pulsed arc rather than a continuous one. The principle is to increase the surface-area--time product of the arc. Reactions happen at the surface of an arc, not its interior. They don't happen in the arc itself, since that's plasma and is too thermally energetic to form stable bonds. It's at the surface of contact between the arc and its atmosphere that reactivity is high, as ions (free radicals) leak out of the plasma and react with the neutral atoms nearby. So by periodically quenching the arc you liberate the ions inside for reactions.

Now this whole process, in order to work well, seems to require that you put an electric field transverse to the arc, providing a certain amount of separation of the ions in the plasma before they recombine with each other. So now you've got a high voltage bias supply to deal with and all its attendant risks, plus the possibility of arcing to your bias plates. So this is a little challenging.

Practically speaking, you'd really like to have a residual ion channel between the electrodes, which means a high-frequency arc stabilizer (not just arc start) that can keep an ion path open, even if little current flows through it when the main arc is off. This reduces the initial overpotential required to reestablish the main arc at the start of a pulse.

[Edited on 4-10-2009 by watson.fawkes]

watson.fawkes - 3-10-2009 at 23:37

Quote: Originally posted by densest

Giant magnetoresistive not magnetostrictive. Both GMR, just entirely different mechanisms...

Ah. Not both GMR, but GMR and GMM, which was the abbreviation I was always seeing for "giant magnetostrictive materials".

Panache - 6-10-2009 at 15:05

Funnily after reading this thread i stumbled upon this, do you think any publisher would allow such advice in a trade text/manual these days. I especially like how he uses a gray lead pencil, you have a built insulator with the wood.
It comes from R.H. Wights manual on lab glass blowing

Attachment: Pt Welding.tiff (174kB)
This file has been downloaded 1037 times

12AX7 - 6-10-2009 at 15:58

Yup, that'll work fine. If you're good enough at it, you don't even need the smoked glass, just close your eyes, tap and it's done.

I prefer burning the pencil lead first, because it may explode from wax impregnated into it. Use a soft pencil, because the higher graphite content makes them more conductive. The clay and graphite mixture vaporizes somewhat, producing a conductive plasma and reducing atmosphere better than plain graphite.

Tim

metalresearcher - 10-9-2010 at 12:22

Nice try !
But only direct on the mains voltage is DANGEROUS !!

I have done the same but with a $100 hardware-store-around-the-corner welder which yields 40V with much more Amps.

http://www.metallab.net/arcmelt/

I can boil Aluminum, melt steel and melt MgO easily.

Continued from the Phosphorus Thread

careysub - 16-6-2016 at 19:00

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.

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).

wg48 - 16-6-2016 at 23:54

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.

wg48 - 19-6-2016 at 12:07

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.