DIY plasma cutter

[Hobweld]

A gentleman on the yahoo welding group has produced a DIY plasma cutter and has posted the schematic for others to use. Could someone please look over the schematic and let me know how practical this is. His cost was about $277.00 or so. Thanks

[Mike W]

Hmmmmn.......directly rectifying 220 without an isolation transformer is living on the edge. I don't know the specs on a plasma cutter to compare this to.

[fla jim]

Needs a little further design work. No fuse or circuit breaker!!!
No ground.
Kind of "Rube Goldberg"

[Hobweld]

here is a regular one

[Sberry]

I think even without looking at it carefully he has dam near 300$ in it and for twice that or 1000 you are going to get something that works well. With the labor its not a bargain to build. Tuning a plasma to work well is going to be complex and engineers with all the resources are going to way better job on something so sophisticated. If he could build it an afternoon for 75$ it might be worth a shot,,, or if one like that cost 2000$ it would be a different matter. That cant be any better than a Chicago Electric junker at 600$.

[Sberry]

We have seen some DIY stuff here and the home made tig might have been passable. The one that did actually impress me was the engine drive, which would have been ok for occasional use like a lot of guys might need. The reason I thought that one was good was that a couple of the major parts could have been salvage items such as engine and alt and it would have been quite cheap. Also with a slightly better alternator and couple hp larger motor it was upgradeable.

[Mike W]

Wait until you see my 28 volt 100 amp alternator on a 10 hp engine for a mig welder. :D

[MrElectric]

Actually, this is very possible. My whole electronics career has been spent designing AC phase control circuitry. I've found some plasma cutter power supply designs that were much more simple than most can imagine. Typically those $2500.00 machines that cut 1 inch steel use less than $600.00 in parts, and some use less than $400.00. If the system uses a transformer, that's when it gets expensive and heavy. Typical mains isolation transformer will run $321.00 for a 12KW toroidal, and weighs 130 pounds.

I do have a power supply design which is not isolated from the mains, but is completely amperage limited and adjustable. No high frequency start but should give at least 40 ampere @ 300VDC cutting current and cost less than $200.00 to build. Uses capacitive reactance to limit current, so no dangerous water resistor with chemicals etc. Total package should weigh less than 40 pounds. You should just buy the torch off ebay etc for $100.00 and then build the power supply.

I do have an itch to try it just to see it work. So if I do I will tell you how it goes and any pitfalls that I find.

Tad

[vdospec]

How did the the DIY Plasma Cutter Turn out?? Can I have the design?

[BeePig]

hey dudes... I have successfully built a plasma torch plus the power supply. The torch parts are constructed from PVC pipe, ball valve, and copper. Too much details to list, but sure will give detail information if anyone of you would reply... The power supply is portable, similar to a on-board welder (engine driven alternator)... .. if interested, give me a shout, happy to help anyone who are interested in designing power supply for welding, cutting ...etc.


BeePig

[vipermanz]

hey dudes... I have successfully built a plasma torch plus the power supply. The torch parts are constructed from PVC pipe, ball valve, and copper. Too much details to list, but sure will give detail information if anyone of you would reply... The power supply is portable, similar to a on-board welder (engine driven alternator)... .. if interested, give me a shout, happy to help anyone who are interested in designing power supply for welding, cutting ...etc.


BeePig

Feel free to post what you know, it sounds interesting to me

[BeePig]

I should start with the power supply...the power supply is engine driven alternator. The engine must at least 5 hp, it doesn't't matter whether is horizontal or vertical shaft.
The alternator must have an output of at least 50 to 60, the higher the better and must be modify.( I will explain why).
The modification is very simply, you have to remove the voltage regulator and the ****ty rectifier (diodes) inside the alternator, sometimes you may find the voltage regulator located on the outside of the alternator on some older model.

As soon as you have finished the above process, the alternator must have three poles on the stationary coil. The power are come from the stationary coil not the rotor coil.
Don't be surprise, an alternator is very different then a generator, but serves the same function, except different design.

I will give more detail information once you have fully understand the above ...

Beepig

[gimpyrobb]

I understand the above, so lets hear more! I am very "thrifty" and always looking for more info. :waving:

[BeePig]

Ok, A plasma cutting machine is use DC not AC, because the plasma arc has to be in transferred mode.. the ions tend to move from the Neg to the positive elctrode.

An Alternator has three pole, and is three phase, if rectified it will give almost clean DC .. however, if a not clean DC is use in a plasma cutter, the electrodo and/ or the nozzel will erode very quickly, so after the rectifier, you will need to filter out the ripples ^^^^^^^^ to -----------..... so the arc will transfer smooter and the cut will preform better.

I used two types of filter, one is Capacitor rated at 1500v @ 400uf. and an inductor. an inductor is almost like an transformer, but not quite.. it coils around an iron ring or rod. (notes. too much of a loop will decrease the voltage)

My torch head is made out of solid copper rod....... machined to the desire size, the starting process is 1) start engine (warm up), 2) supply field current to the alternator rotor 3) increse engine's rpm 4) torch is already hooked up to compressor 5) created a shot circut to the torch (electrode and nozzle is touch at this point) 6) turn on air valve, then the pressurized air will escape through the nozzle by pushing the nozzle away from the electrode, then a spark will created.... 7) Boom, there you go, the pressurize clean dry air will then swire around the electrode and mix with the arc and shoot out to the work picece in which you already have ground.

[gimpyrobb]

When do I get to see pics of [(mainly) the torch] and the rest of the setup. I have a good picture in my mind of the rest, but the torch I'm havin trouble with. I have lots of guys tellin me I'm going to blow something up :laugh: .

[BeePig]

I cannot send you any pics because my computer is currently under virus attack, I am using the Library's CPU to send you this message.

I would like to send you some pics of the plasma torch, but i really want to know what are you trying to do. What are you going to blow up?

Tell me your progress

[gimpyrobb]

I don't intend on blowing anything up. I am a do it yourselfer and while some projects have failed, they've never blown up. Alot of the guys I hang with are not inclined to do anything so they keep saying "your going to blow yourself up one of these days". I have too much money invested into my tool collection to be able to afford a store bought plasma cutter (getting divorced doesn't help either) so I would like to build my own. My math isn't strong enough to finish electronic engineering and build one by myself, so I am following along your path. I have no need to make one other than to help myself finish other projects and to prove the naysayers are lost.

[digital_life]

hey hobweld, could you let me know how to get your shcematic in a better resolution?
thanks, please keep on posting DIY PLASMA information!!!!!!!

[dcwelder]

Hobweld --if your looking for a big charge out of your life then go ahead an try this cocomame thing so called plasme cutter . i have a real one that is a esap 1250 plasma cutter and the amount of power it takes on start up is large , it says that you need a 100 amp designated circuit for start up an runs on 80 amp when cutting and that it also needs 280 cfh of air supply while cutting . it is a sweet unit never let me down an haven't found a thing you can't cut with it . doug

[beemerb]

hey dudes... I have successfully built a plasma torch plus the power supply. The torch parts are constructed from PVC pipe, ball valve, and copper. Too much details to list, but sure will give detail information if anyone of you would reply... The power supply is portable, similar to a on-board welder (engine driven alternator)... .. if interested, give me a shout, happy to help anyone who are interested in designing power supply for welding, cutting ...etc.


BeePig
I would like to have the details if possible.Will pay any costs incurred.
beemerb

[elvergon]

I would also love to make a plasma torch as the one you did Beepig!

Mind to show what you did when you can?

[Brewman_ca]

Would like to see picture as well

[comp]

same here

[awright]

As MikeW said, the original circuit sketched at the beginning of this thread would be a potential killer due to lack of isolation from the power line.

You know how careful product designers are to be sure that the user or a passer-by cannot make contact with electrically energised parts of their product? And yet, we read about workmen being electrocuted by touching the body of a defective tool.

Well, the circuit as sketched would connect your entire welding table and work piece to the line, creating a situation in which it would only be a matter of time before someone would be injured or killed. The diode bridge does nothing to eliminate this lethal problem.

awright

[comp]

wow,,,,i'd buy new unit :eek:

[halbritt]

As MikeW said, the original circuit sketched at the beginning of this thread would be a potential killer due to lack of isolation from the power line.

You know how careful product designers are to be sure that the user or a passer-by cannot make contact with electrically energised parts of their product? And yet, we read about workmen being electrocuted by touching the body of a defective tool.

Well, the circuit as sketched would connect your entire welding table and work piece to the line, creating a situation in which it would only be a matter of time before someone would be injured or killed. The diode bridge does nothing to eliminate this lethal problem.

awright

How is this any different than a commercial plasma cutter that would connect one's entire welding table and run at 100+VDC?

[awright]

halbritt, the difference is that with the commercial plasma cutter, you have your welding table connected to the GROUND lead of the plasma cutter and you are manipulating a plasma torch with 100 volts or more ONLY on the 3/8" tip of the torch that is barely exposed to personnel contact and is only electrically activated when the trigger is pulled. This results in an extremely low probability that a person will come into contact with the electrically hot tip. This is especially true because when the trigger is pulled, applying voltage to the tip, you also get a noisy gas jet and a hidden, noisy pilot arc inside the torch giving even the unwary lots of warning not to touch the tip. You can stand in water and hug the cutting table with no risk whatsoever.

Conversely, with the homemade plasma cutter shown in the original post, you are, in effect, connecting the entire cutting table to one hot lead of the incoming power line through the forward-biased power rectifier of the diode bridge. This is the same as taking a power cord plugged into the 240 VAC wall receptacle, stripping one of the hot leads (remember that both leads are "hot" with 240 VAC power), and connecting it to your cutting table. Your table is now energised at 120 VAC. (Each line of a 240 VAC supply is carrying 120 VAC with respect to ground.) If the breaker doesn't pop right away (which is quite possible if the floor is not very conductive or the table feet are insulated from the floor by slag grit under the feet), YOU become the conductor from the hot table to ground if you grab or lean on the table, or even brush the back of your hand against the table. And the likely path of the current passes through your heart. The heart can be sent into fibrillation (rapid, random, shallow beating without pumping blood) by a few tens of milliamps. You may get away with this configuration for some time before some combination of sweaty feet, wet floor, or holding onto a power tool with a grounded, metal body with one hand and simultaneously brushing against the electrically hot cutting table leads to shock with a potentially lethal outcome.

The difference between the commercial plasma cutter and the homemade cutter shown in the original post is enormous. The commercial unit is very safe, but an extremely persistent and determined person could kill himself by sitting on the grounded table, touching the torch tip with one wet finger, and pulling the torch trigger with the other hand. The homemade rig is a lethal accident waiting to happen to any person who casually touches any part of the cutting table while holding a grounded power tool or standing on a moist concrete floor.

Stay safe.

awright

[halbritt]

Not quite the same as the output would be mostly DC. However, you are absolutely correct about the circuit being unsafe. If I understand the schematic correctly, you'll end up with about 200 volts of rectified DC on the other side of the bridge with each lead being biased relative to ground. Without the resistor, the negative lead would be around -100VDC and the positive lead would be +100VDC. Either represents enough differential to give a nasty and potentially deadly shock.

[awright]

halbritt, I'm relieved that you agree with my conclusion about the potentially lethality of the "homemade plasma cutter," but I have a somewhat different analysis of the details of the threat.

I don't think the presence of C1, L1, or R1 in the original homemade plasma circuit would make any difference whatsoever on the lethality of the setup or the voltage waveform felt by the victim. I will concede that the diode bridge will pass only the positive half-cycles of the incoming waveform on to the table (and to anybody touching the table), subjecting the victim to a positive, full-wave rectified waveform with a large DC component. But it doesn't take a balanced, sinusoidal AC waveform to kill. The victim (I'm making an assumption here that someone will eventually come along and touch the table while wearing sweaty socks or while holding a grounded power tool in the other hand, thereby becoming a "victim") will still "feel" the same peak voltage (170 volts, peak, relative to ground) and the same peak current through the heart with the same repetition rate as someone grasping one line of a 240 VAC circuit while grounded. It's just that all the peaks are positive instead of alternating positive and negative. The fact that the peaks are all positive makes no difference in lethality, and the victim has no way of sensing that there is a capacitor charged to 340 volts hanging on the lead that is killing him since the PEAK-TO-PEAK AC or DC voltage is never felt unless the victim grasps the torch tip with one sweaty hand while touching the table with the other.

The alternating, forward-biased diodes of the bridge are essentially hard conductors from the hot lines to the table with negligible resistance. They are capable of supplying thousands of times the current necessary to kill. The nominal 12 ohm water resistor is a negligible impediment to the relatively low but lethal current required to kill.

I also disagree with the voltages you predict on the ground clip and the torch. Neglecting voltage drops through the diodes, resistors, and inductors, (all very small at low, killing currents), the peak positive and negative voltages would be sq. root (2) x 120V or + and - 170 V. The nominal voltage across the energy storage capacitor would be 340 Volts which is moot because the victim will (almost) never touch the negative bus. All the rest of the circuitry hanging on the incoming line will also have no influence on the voltage felt by the victim touching the table while grounded.

I have no idea if the homemade plasma circuit would make a good cutter, but I do know that it will make a good killer. God knows I would love to see a good, workable homemade plasma cutter circuit, but this isn't it. The fundamental defect of the circuit is the absence of isolation from the line. I can also visualize someone with limited knowledge of electricity putting a galvanized bucket on the floor for his water resistor and connecting the positive output from the bridge to it, creating an altogether new electrocution hazard.

Be safe.

awright

[coachgeo]

I God knows I would love to see a good, workable homemade plasma cutter circuit, Be safe. awright

What about the other DIY cutter discussed in here using the Alternator? What are its drawbacks when it comes to safety.

[tech-ad]

Have you thought of using a rewound microwave transformer as the isolation device? With the magnetic shunts left in, you could probably eliminate the need for the current limiting ballast resistor. Most microwave transformers I have stripped have 99 to 110 turns on the primary. for a secondary, it should not be too difficult to wind a 200 to 220 turn coil for a final voltage of about 220 volts. It would be much safer than a direct line connection. If you want to run off of 220 volts, put 2 similar 1:1 transformers in series. To test volts per turn, strip out the high voltage and insert 10 turns of any size wire. Put a voltmeter across your 10 turns and read the voltage. Divide your desired output voltage by your 10 turn voltage reading and multiply that by 10 to figure the number of turns for your secondary coil. round up to the next whole number.

I have attached three images. two showing removal of secondary wires, the last ready for rewinding. Either chop off the wires or grind open the welds of the I laminations. The welds are extremely shallow so only a touch with an angle grinder is needed to remove one side. Tap the loosened side with a chisel and the other side should open like a hinge and snap off.

Note: keep the magnetic shunts in place otherwise the initial inrush current when powered will blow your electric circuit breaker.

[comp]

quick question,, when a microwave goes out what is the "normal" problem ???

[Clunker]

"quick question,, when a microwave goes out what is the "normal" problem ???"

It used to be the Magnetron would just burn out or sometimes the Triad (the rectifier that controlled the Magnetron cycle would go bad. But things have changed greatly since I last looked inside one and I would imagine with the advanced control circuits today there is a plethora of potential for fault.

[comp]

can you tell a "slo" guy how to check ????

[awright]

I don't like to come on as only a Pollyanna safety nut, but here I go. again. For some unknown reason I clicked on this old thread and saw several hanging questions that had never been responded to.

Actually, I guess my history of repeatedly giving myself really unpleasant, shocks as a kid with a toy train transformer with exposed terminals under my workbench driving an auto ignition coil eventually made me aware of the danger of electricity. I never did learn not to reach blindly under the desk to adjust the voltage and end up touching some exposed transformer contact. Probably alive today only because the ignition coil couldn't supply enough current to kill.

But a microwave oven is a completely different kind of beast. It can easily kill the careless or unwary. That may be the (legitimate) reason that nobody responded to the request to advise a "slo" guy how to check his microwave. "Slo" guys should not be messing around inside a microwave oven because you really have to have your wits about you when you do.

I'd advise anyone thinking about checking his microwave's guts to buy one of the several good, inexpensive books on the subject and study it. Especially the safety stuff that they all emphasize. If you are accident prone or know you are careless, don't do it. Checking inside a microwave is just not something that can be properly communicated by e-mail.

That said, one of the causes of failure, and something that can be RELATIVELY safely tested (relative to trying to measure the magnetron circuit) is the safety interlock system that detects whether the covers are on and the door is fully closed or whether the latch has started to be opened. Sometimes the tang or lever on the latch that closes a microswitch to tell the controls that it is OK to turn on the high voltage is broken or bent. This can be checked with an ohmmeter without power to the oven and with minor disassembly. You do, however, have to remember that the high voltage capacitor can retain a lethal charge for a long time after use of the oven, even after disconnection from the power line. Use a screwdriver with an insulated handle and a clip lead to ground to discharge the capacitor before going inside. Doesn't hurt to use a low value resistor (say, 100 to 1000 ohms) in series with the clip lead to limit the surge current upon first contact with the capacitor. After draining the charge through the resistor for several seconds, repeat with a hard grounded screwdriver to finish the discharge.

On another matter, what happened to beepig? He enticed us on with the hint that he had built a plasma cutter from stuff from the local ACE hardware, but then disappeared when half a dozen guys enthusiastically asked for more info. I didn't chime in because I thought there were enough requests, but I'd also like to see more details about it. How about more info, beepig? Do we each individually have to go behind the curtain to get more details?

Additionally, Hobweld posted an interesting schematic that is almost impossible to read and was asked how to access a higher resolution image. Any suggestions how to see a better image, Hobweld?

Still interested:

awright

[comp]

i'm lurgin' cause i hope a DIY is possible,,,not that i can build

[Roy Hodges]

Wow! I've been shocked (accidentally,when sweaty) with as much as 1000 amps, 50-100 volts d c - when i was AIR-arcing with a 1/2" carbon,and flashed at the same time -no fun ; but this plasma thing is scary . I wouldn't have the nerve to try a homemade plasma like this . -Roy

[awright]

Roy, I'd hate to see people conclude from this thread that homemade plasma (if it is, indeed, feasible) is inherently dangerous. It is not the plasma technology that is dangerous, it is inappropriate application of the technology, as expressed in the original "DIY Plasma" post. Yes, plasma cutters use higher voltages than the normal welding technologies that are limited to about 80 volts, and that adds to the hazard. But the hazards can be addressed with careful design and communication among the membership. If you read the earlier posts about the REASONS that I regarded the DIY Plasma proposal dangerous, you will see that it was the inappropriate application that was my problem, not the fundamentals of plasma cutting.

I hope that somewhere among the Welding Web community there resides the knowledge and initiative to come up with a DIY Plasma cutter that is safe, cheap, and effective.

Probably a pipe dream, but not impossible.

By the way, comp, for us plodders, what is "lurgin' "?

awright

[yellow73bb]

I'm a new guy to the forum, and I have successfully built and am using a home made plasma cutter, similar to, but with some significant improvements over, the initial post in this thread. First of all, I agree with the safety issues that have been raised. In my cutter, I use a contactor that disconnects both leads so that the danger is reduced to only the time that the button on the plasma cutter is pressed. The biggest danger on mine is that the capacitors hold a potentially lethal charge for a long time (days?) after power is removed, so they must be covered. I have a much better way of doing the arc start (the stun gun is pretty innefective). The water resistor is not a very good idea either, and I have a nice clean solution to that. I did make a simple printed circuit board for the control functions. I built mine last summer, and have been using it for doing a 1957 chevy truck restoration in the US, and am planning on building a CNC plasma cutter now that I am living in Shanghai for a while and don't have access to the truck.

In the time I have been using it, I have not yet even felt a "tingle" when touching the work piece. I don't usually touch the work piece while cutting, and have always worked while on dry concrete.

By the way, it only takes a few milliamps (doesn't matter if it's AC or DC) of current to cook your innards. Remember the old hot dog cookers that simply ran 110V through the hot dog to cook it? The resistance of a person's DRY skin is high enough that it USUALLY takes higher voltages to kill (12V won't usually kill of course, 110 V can kill but often doesn't, 220V or higher is very likely lethal). If your skin is wet, you can be cooked by much lower voltages. Electric shocks, especially AC, can also do funny things to your body's own electrical system (stop the heart, etc.).

I'd like to build an IGBT based cutter next, but have not gotten very far on that idea. Such a cutter would allow isolation of the return lead so that it could be grounded.

[awright]

GREAT! Please post a schematic of your cutter. I'm sure that many others would be as interested in duplicating your equipment as I am. Please don't wait until you have the perfect solution or a fully developed circuit board layout. We're anxious to see how a working device is put together.

I can't promise that I won't be critical of any device that does not provide isolation from the line, but I am encouraged that you apparently have at least been attentive to the safety issues of non-isolated cutters. And it's intrigueing to hear that your design is actually a successful cutter.

I do worry about any design that relies on the user remembering to not touch the workpiece while the trigger is pulled or always working on dry concrete as the main safety measures, since sooner or later fatigue, inattention, or environmental conditions may combine to create a dangerous situation. I do know that on more than one occasion when I knew I was doing something potentially dangerous, but that I would be very careful, I got some very embarassing surprises. Fortunately, just embarassing, obviously not fatal.

When you say, in the last paragraph of your post, that you'd like to build an IGBT based cutter, do you mean one using IGBTs in an inverter configuration with an isolating transformer as part of the inverter? Now THAT would be a great contribution.

You mention the danger of the capacitors holding a charge long after power is removed. The common practice of installing a permanently connected capacitor discharging resistor should be able to "defuse" that hazard with minimal penalty in power dissipation, expecially since you are apparently taking the power directly from the line. Even a fairly large resistance could discharge the capacitor bank in a reasonable time with little constant power drain. An alternative is a discharging resistor that is placed across the capacitor bank by a NC (Normally Closed) relay contact that is held open while power is applied.

Please follow up your intrigueing post.

awright

[yellow73bb]

Yes, I mean an inverter that is isolated from the line.

One of the tradeoffs I made with this system is that I have a bridge rectifier and a large (100K) resistor hooked from before the power contactor to charge the capacitors. This is to protect the main power contactor. Since I have rather large capacitors to charge, if the contactor is closed at anything other than the zero crossing of the voltage, the inrush current can be quite large. I destroyed one contactor already. So, I put a trickle charge circuit to keep the caps charged. I need to put in a safety discharge mechanism so that I can discharge the caps when I need to work on the system, but I haven't done that yet.

What's the best way to post to this forum?

[comp]

do you mean you print's for the cutter ????

[yellow73bb]

Sorry, I guess that wasn't very clear. What is the best way to post pictures, documents, PDFs, etc. to the forum.

It looks like I can just attach to my post before submitting?

[LuzRD]

that should work. "go advanced" then click the paperclip icon and it will open a box for you to choose the file on your harddrive to upload

[yellow73bb]

It looks like my file is too big to upload, so i'll have to put it on my website: plamsa cutter construction (http://www.jblankenagel.net/plasma%20cutter%20construction.pdf).

The only thing I don't like about this cutter is that I seem to get more slag on the back side of the metal than I want. It may just be that I don't know how to use a plasma cutter though (air pressure, cutting speed, etc.), so maybe I can make good cuts with it if I keep practicing.

[tech-ad]

Yes, I mean an inverter that is isolated from the line.

One of the tradeoffs I made with this system is that I have a bridge rectifier and a large (100K) resistor hooked from before the power contactor to charge the capacitors. This is to protect the main power contactor. Since I have rather large capacitors to charge, if the contactor is closed at anything other than the zero crossing of the voltage, the inrush current can be quite large. I destroyed one contactor already. So, I put a trickle charge circuit to keep the caps charged. I need to put in a safety discharge mechanism so that I can discharge the caps when I need to work on the system, but I haven't done that yet.


Yellow
Have you thought about using zero volt crossover optoisolator and a power semiconductor to switch your precharge circuit at zero volts, or in place of your contactor? They are often used in power circuits to eliminate radio interference when switching high currents. If you use an inductor in series with the output lead, you could probably use a much smaller capacitor.

I would probably use one or more large 1:1 isolation transformers for the front end for safety. They are easily made up from microwave transformers (see post 31).

In your schematic you have a 7 to 9 ohm resistor with the comment "see text" What does the text say?

Your schematic looks elegant in its simplicity. Well done. Thanks for posting it.

[wizard]

Very interesting, will have to go over this later. In any event I'm of the same opinion as tech-ad, transformer isolation would be nice.

Another way to deal with the capcitor issue would be to use a semiconductor to turn on a timed precharge. That is a circuit that limits charge current for a specfic time at start up and then is bypassed electrically for normal run conditions. Right now I see this as the weakest link in the system safety wise, the shut off really needs to shut off everything when flipped. This is a big concern on high voltage devices like this. By the way this may be doable with a minimal of parts if you move the contactor to the other side of the CAP.

In other words at power on the unit is inhibited from turning on the output until the precharge is done. Another set of contacts on the contactor could be used to short out the precharge whenever the gun is turned on. So at best you would need a small time delay circuit that is active at power up and an extra set of contacts on the contactor.

As far as discharge of the CAPs, most systems simply use a bleed resistor mounted across the terminals. It should be sized large enough that it doesn't impact normal operation of the circuit yet produces reasonable bleed down times. I know that there is a bit of powered stored in the CAPs so you won't be doing this with a simple 1/2 watt resistor, a bit of engineering is required.

Thanks
Dave

[tech-ad]

Re: Please disregard my question on the 7 to 9 ohm resistor. I scrolled down on your file and saw the details about it. Thanks again

[awright]

Nice effort, yellow73bb. I'm sure we all appreciate the thought and effort that went into your cutter.

But here I am, again, with my pollyanna safety comments.

I simply cannot agree that it is EVER safe to connect the work piece to a hot line, even for a limited time during actual cutting. Your schematic shows that the "Return Clamp" is essentially connected directly to the "hot," 115 volt line whenever the contactor downstream of the rectifiers is closed. This is a vast improvement over the original post that started this thread, since the risk if instantaneous electrocution is limited to the times during which the gun trigger is pulled. An improvement, but still far too risky to be considered safe.

Isolation, isolation, isolation. That's what can make a the cutter acceptably safe. Not perfectly safe, acceptably safe. The present circuit is a trial lawyer's dream come true.

That said, I think you have contributed significantly to the goal of a functional, homemade plasma cutter. And I'm happy that you are still here to talk about it. But I strongly discourage anyone from implementing your design in it's present form, without isolation.

One approach would be to scrounge a high-power line isolation transformer, say 5 or 10 KW or so. They are not that rare in salvage around here (San Francisco Bay Area), but would be very expensive if purchased new. And, they are heavy. That's why plain transformer welding power supplies are heavy in the higher power ranges.

Ground one end of the secondary, ground the table/workpiece, and pass the hot through the rectifiers to the torch.

The other way is to charge the capacitor bank directly from the line, as you are now doing, chop the voltage on the capacitor bank into a high-frequency square wave AC waveform, pass that through a much smaller transformer (this is where the isolation from the line is provided), rectify the transformer output, and proceed safely. This is exactly what the commercial plasma cutters do, but perhaps with more sophisticated controls.

Because transformer size and weight is inversely proportional to frequency, you get a dramatic size/weight reduction by switching at a very high frequency. The other side of the coin, however, is that it takes more and more sophisticated and expensive semiconductor devices to switch at the desirable high frequencies with acceptably low losses (i.e., destructive heat generation). In general, IGBT modules or a bank of high power MOSFETS are required, and these are expensive. So you end up trading off transformer cost and weight for semiconductor cost - still an advantage, as indicated by the popularity of inverter type machines.

I hope some enterprising member knowledgeable about inverter design comes along and contributes his knowledge to the project.

awright

[awright]

And yet another safety criticism:

Your water-cooled resistor is innovative. Kept in a plastic bucket, I guess it is relatively safe unless the curious dips his hand into the water to test how hot it is.

However, putting the bare element in a tube connected to the house plumbing is definitely illegal and potentially unsafe, depending upon where you live. In many areas using ground water the water can be conductive enough to be hazardous. Remember that it only takes a few milliamps to kill under the right circumstances. We've all heard about people being electrocuted when a radio or hair dryer fell into the bathtub.

Your photos show brass fittings carrying water to your water cooling tube. The brass could be electrically "hot" enough to cause a shock to a person touching the fitting and a ground at the same time. Not a good situation.

Another potentially hazardous condition could arise when the plumbing system is energized through the water path and the system has not been properly "bonded" with a hard wire link bypassing the water heater, or perhaps an insulated union or coupler was installed to prevent electrolytic corrosion between old galvanized pipe and new copper pipe. In that cas the plumbing could be "hot," waiting for someone to come along and complete the path to ground.

However, there is a good, cheap alternative if you want a water cooled resistor. Electric water heater elements have the right order of magnitude of resistance, are electrically isolated from the water, are cheap and readily available, and are easy to mount. I'm looking at one marked, "1650Watt, 120 volt," that measures 8.5 ohms, cold. Grainger's catalog shows them at a cost of about US$12.

awright

[yellow73bb]

I have considered all of the safety issues mentioned, and most of the fixes for those issues. On the zero crossing detector, that would work fine here in Shanghai as they use single phase 220V. In the US, though, the two legs of the 220 are 120 degrees out of phase. So, you can't close the contactor at zero volts of both at the same time.

I tried different inductors on the output, they didn't seem to make much difference, and were heavy, so I left them out. Large capacitors are cheap on the surplus market. I'm currently reading through patents of plasma cutters to see how they do the switcher, and am starting schematics soon. Also, here in Shanghai, it is very easy to get IGBTs, FETs, transformer cores, etc, and they are pretty cheap, so I should be able to build it pretty soon. Also, it's very inexpensive to get boards made here :p .

I am using about 20 feet of 1/4 inch plastic tubing between the resistor and the water supply line to increase resistance. The other solution is to ground one of the brass fittings. That would certainly reduce risk, and the water is not so conductive that it would be a problem. It would be better to ground it at the plumbing end or course, but since the water supply line coming into the house is already grounded, I'm not sure it would help much since I am taking water quite close to where it comes into the house. Other's mileage may vary, of course.

I'll look into the semiconductor drain on power off for the capacitors, that is a good idea, and should be quite simple.

I have seen several places here which sell the water heater elements, and I'm thinking of trying some of them as well. Maybe I can also use it to heat water for my house :rolleyes: . I would have to put a few in parallel, which makes it harder to switch resistance, but i should be able to figure that out.

Thanks for the feedback, and any new feedback that you are thinking about and haven't given me yet :cool:

[wizard]

Hi Yellow73bb

Sorry about running out right after my last post, pressing business!

In any event as I was going out the door I realized that what I had written about the inrush current to the CAPs is a bit confused at best. The goal is pretty simple, that is to manage the in rush current to the CAPs at start up. That can be accomplished via a simple RC circuit that gets by passed by the gun relay when the output is turned on.

Ther are two simply ways to accomplish this in my mind. One is a time delay at power on. The other is a voltage comparator. Either one would enable the rest of the control circuitry when the precharge is complete. This only requires one more high current contact on your gun relay. This should be cheap to do as you can often get industrial relays with auxilirary contacts for the control circuit portion of the unit.

As to the isolation issue I'm in agreement, that something should be done. I have no idea how this is handled on production plasma units but I'm still of the mind that Isolation transformers can be found cheap as surplus. I'm also wondering about the feasability to use a step ups transformer to up the DC voltage. The thought is that a higher voltage might make it easier to start and maintain a good quality plasma.

As to your primary relay, that should be wired up to provide low voltage drop out. In other words if power is lost to the machine, reenergizing the input should not result in the machine turning on agian. This is easy to accomplish with a NC and a NO pushbutton. Frankly this is something that should be on all welders and like equipment.

The water resistor is a problem as awright points out. Safety is a big issue of course. The other issue is the leakage of good power away from where you need it, atleast for anything connected to city water. Unfortunately I don't have any suggestions here that are simple. There is a possibility that putting an inductor on the AC side of the Bridges might help with currentl limiting. Maybe there are electrical engineers here that can comment on that idea.

I'm wondering what you are using for a gun? Is it home made or did you retrofit somebody elses? It would sem to me that this would be the most difficult part of the whole design, that is keeping the gun safe and operator friendly.

Time to go back to yor web site and look things over. Best of luck and play safe.

Thanks
Dave

[yellow73bb]

It's easy for me to check with electrical engineers as I am one, and I manage about 10 of them at my job ;) . Line isolation transformers are heavy, and not used in smaller commercial units because instead they use an inverter that switches at high frequency and then use a much smaller isolation transformer. There are points inside a commercial unit that are every bit as dangerous as my caps are, they are just protected by sheetmetal and screws. The reason for isolation is simply so that the work piece can be grounded. Then, the torch is running at a negative voltage (100-300V) with respect to the work piece, but the torch electrode is protected by an air gap to the outer cap.

I am using a regular commercial handset that I bought on ebay.

I don't actually plan to do much more for safety on the current unit, as it works well and I know how to use it. I'm going to build a non-conductive welding table (ceramic tile top) to make it safer, but I want to devote more of my time to designing and building a IGBT inverter design.

[Rockski]

does anybody have the scematic in great detail? the one on page one didnt look bad but i need an enlarged format, me and my electritian buddy are gonna try one out this summer while im in school, i figure3 that a welder and an sparky might be a good combo for this, hopfully itll turn out good and we can crank her up to go throug 1 inch plate easily

[yellow73bb]

You'll need a lot of current to go through 1". I have the board schematic in ORCAD form, and the system schematic in visio if you want them.

[Rockski]

yea sure, if you'll shoot me a private message ill reply with my email addy.... i figure a plasma cutter would be a cool adn usefull project

[yellow73bb]

There have been a couple of requests for schematics. I can get boards made pretty cheaply here in Shanghai. If there is adequate interest, I'll have some boards made and send them to the US. I know that many on the list are from countries other than the US, so I could send packages to various countries too if people are willing to pay the shipping. If you are interested in a board, please send me a PM with your desired quantity and the country that you live in.

[CarterKraft]

how much cash are we talking for the proposed boards?

[tech-ad]

Yellow

Canada is on the north american electrical grid and domestic supplies are derrived from a single phase of a massive 3 phase system just like Shanghai. The domestic lines are switched occasionally to mailtain a balanced 3 phase grid. One neighbourhood would be out of phase by 120 degrees with another neighbourhood but any individual house would all be a single phase. Therefore, zero volt IC's work well here.

On low voltage circuits I have used a voltage doubler circuit to operate 24 volt contactors and relays off of a single 12 volt transformer. Lower voltages were obtained by the use of 3 pin voltage regulators again taken off of the 12 volt supply. this reduces weight of the overall circuit.

You had mentioned that you get quite a bit of slag on the backside of the cut. Do you also wear out your consumables quickly? If so you may need to check that your air drier is functioning well.

Thanks again for the circuit. It is from people with your initiave that we are able to progress as a community.

[coachgeo]

I see a lot of 12v and 24v being thrown around. How about running it off auto batteries. My welder already does so this would be good for me.

[tech-ad]

When I look over the schematics, I see what appears as a TIG power supply with an HF circuit connected to a plasma torch. Am I missing something here?

Has anyone tried to run a plasma torch off of a TIG power supply and dry compressed air? With what results?

[awright]

Well, tech-ad, the main difference is that the voltage is way higher than a TIG power supply and the only current control is a fixed resistor. Both of those characteristics would make the cutter power supply shown inappropriate for TIG.

And, Coachgeo, the high voltage also demonstrates why it is, if not impossible, at least impractical to run a plasma cutter off batteries. You would need an inverter to step the voltage up to the level required to sustain a plasma arc, which is a lot higher than a welding arc. While that is possible, it would be a pretty major design undertaking and would drain your batteries pretty quickly.

My medium-sized plasma cutter requires a 50 amp, 240 volt supply, or about 12 KW. That would be equivalent to drawing 1000 amps from your 12 volt battery, or 500 amps from a 24 volt battery. You are going to destroy automotive, or even golf-cart or marine batteries at that draw for more than a few seconds. Navy submarine batteries, on the other hand, might work fine.

awright

[B.v.d.Holst]

hi BeePig.
will you help my to send the schematic off your plasma cutter thad you make rond jan 2005, I'verry interested in the cutter.I like iff I gad a answer form you.and sorry for my bad english.
best regards
Bob
Nederland

[pef]

I'm also interested.... ( bad English too, from Belgium )

[badone07]

but had to ask.... who is from Ballwin, Missouri in this thread? Since BeePig is from Canada... wondering who it is profitting from his plans, claiming as there own? http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&item=7622649929

[coachgeo]

but had to ask.... who is from Ballwin, Missouri in this thread? Since BeePig is from Canada... wondering who it is profitting from his plans, claiming as there own? http://cgi.ebay.com/ws/eBayISAPI.dll?ViewItem&item=7622649929This e-bay add is for a home made plasma "Head". is this a different animal than the circuit our friend posted here?

[badone07]

This e-bay add is for a home made plasma "Head". is this a different animal than the circuit our friend posted here?

Actually "Hobweld" posted schematics. Beepig's 1st post is http://www.weldingweb.com/showpost.php?p=20341&postcount=10
More than just a coincidence.

[coachgeo]

Actually "Hobweld" posted schematics. Beepig's 1st post is http://www.weldingweb.com/showpost.php?p=20341&postcount=10....Looks like the two Plasma cutters are completely sepperate do it yourself designs. The ebay unit apparently attaches to a DC welder as it's power source. Must be why he calls it a "Head" in his ad. The one in this thread has it's own power suply right?

I am confused though... what does someone steeling Bigpigs design (if that happend) have to do with anything in this thread.

[badone07]

Stealing has plenty to do with this thread since this is the only place BeePig posted it. Won't find anything posted like it anyplace else or outside this forum. Except for 1 person who did so on a free site recently. The initial power source is irrelevent since, DC is the final product, regardless. Hopefully more examples of stuff like this doesn't happen, so maybe others won't think twice about sharing any info or ideas. Have seen it on other forums. Personally, I'll compare both plans soon enough to see if they are indeed not one in the same.

[coachgeo]

Stealing has plenty to do with this thread...Were spliting hairs in different places. I agree the topic is important!!! It should be in it's OWN thread to garner the importance it deserves. Putting it in here has taken this thread in a whole new direction that confuses the reader till they realize the topic switched. This new direction has sorta killed discussing the original topic of how to make one.

Still though thanx for the info. Maybe the moderator can edit/cut out the theft stuff in this thread and put it in a new thread for us.

Anyone built one of these from his plans yet? Would love to hear about the success.

[nemesistheking]

Here is the user VdoSpec, a.k.a Michael "the lawnyer" Walker

http://www.geocities.com/nemesistheking/mickey_walker_topgun_thief.jpg

don't listen to Mickey, don't support this thief.

Check your pockets when you are near this face.

[Prune]

I'm still hoping someone would post a switching supply schematic. The commercial unit schematics I've come across do not show the drive circuitry for the IGBTs or MOSFETs, though I suppose there are plenty of easy to use inverter driver chips on Digikey. The big problem for me is finding or winding the HF transformer.
I'm hoping to have a supply running from the 110 not 220, at about 5 kW.

BTW, the compressor I found is 4.5 CFM @ 40 PSI and 3 CFM @ 90 PSI, and I'm wondering if that's sufficient for a small cutter.

[awright]

Why in the world would one choose to supply a high voltage, high power (5 KW) device from 110 VAC? That's almost 50 amps from a 110 VAC line, which is a pretty rare, no, a never seen arrangement. And just what is the advantage? Drawing 50 amps from a 110 VAC line will not make it more generally useable in home environments because you will never find a 110 VAC branch circuit with a 50 amp breaker and adequate wiring and receptacles for a 50 amp load. You might say that you will use half of a 230 VAC branch circuit, but if that is the case, what is the advantage of using "110" volts instead of the full 230 VAC at about 25 amps?

awright

[nuts and bolts]

Its been awhile since ive been on here so busy with things.
But im answering a question on here, im from Ballwin and I sell plasma head and power supply plans on ebay with arc start circuits.
I dont copy nobody, never have never will.
If there are others that resemble my plans well thats ok with me.
Atleast there using there Brains not there big mouths to make one.

[pityocamptes]

I stumbled onto this thread and found it very interesting. Would using the hot wire and power supply off of an ARC welder be sufficient to run the DIY plasma cutter? If so would a 110V welder work and how deep would it cut? How about a 220V ARC welder. Thoughts on this? Thanks!!!

[nuts and bolts]

Hey Guys and Gals, if you want to build your own plasma cutter then here is a tested and tried system for you.
just go to (theplasmacutterman.com) and check it out.)

[comp]

I stumbled onto this thread and found it very interesting. Would using the hot wire and power supply off of an ARC welder be sufficient to run the DIY plasma cutter? If so would a 110V welder work and how deep would it cut? How about a 220V ARC welder. Thoughts on this? Thanks!!!

hmmmmmmmm no replys till above

[coilgunman222]

hmm, as to pityocamptes' question, as i understand, plasma cutters run about 140 volts no load, dropping to about 90 while cutting, so an arc welder would probably be too low voltage. really, the main problem with the homebrew cutters i've found so far is the risk of electrocution from the workpiece. i'm sorta thinking of combining four microwave oven transformers to make a 240v isolation transformer and then going from there, but i don't know how practical that is.

[jojo]

very interested in your device love to see plans and pics if possible maybe even parts list

[jojo]

made onboard welder out of large case ford alt and onboard compressor out of york pump would love to have CHEAP mobile plasma cutter

[comp]

very interested in your device love to see plans and pics if possible maybe even parts list

same here

[jakuvsummastruvnone]

what's the status of the DIY plasma cutter, and have all the safety issues been resolved? i'd like to build one.

[keithfoor]

Wow! I've been shocked (accidentally,when sweaty) with as much as 1000 amps, 50-100 volts d c - when i was AIR-arcing with a 1/2" carbon,and flashed at the same time -no fun ; but this plasma thing is scary . I wouldn't have the nerve to try a homemade plasma like this . -Roy

OK, I feel I have to correct you on this.
The current potential of a power source doesn't predicate the amount of current that does flow, only the available amount of current that can flow.

The minimum amount of current that can kill is 20 milliamps, but that is the current directly across the heart.
At 200 ma, death is certain, if that level of current is across the heart, and 2 amps will cause
significant burning of flesh. At 1000 amps of current flow, the heating of the flesh would cause it to vaporize. Current flows if significantly less than 1000 amps have resulted in peoples limbs being burnt off their bodies.

For a better understanding of this, we go to ohm's law. Voltage divided by resistance equals current. A typical humans OUTTER skin resistance when dry is thousnads of ohms per inch. This is on dry skin, it drops when you are sweaty, and if you go below the skin into the wet flesh, it drops to a few hundred ohms per inch.
This is why some electricans, although not advisable, will check 110 lines by toughing them with a dry finger, you feel it, but it doesn't hurt them. If they are sweating, or they have a piece wire stuck in their finger, they get knocked on the floor doing this.

Now, back to the 1000 amp shock. For a human to have 1000 amps pass through them, the voltage level would need to be in the hundreds of thousands of volts. Near the voltage of a lightning strike, and even then, passing 1000 amps through the body is a difficult thing to have happen. When you are sweaty the outer skin resistance is lowered, but there is still the resistance between the outer layer and inner layer of flesh. This is why some people when shocked get significant skin burns but are not killed from the shock. The current passes over their body on the outer layer and does not go through the inner part of their body. The burns are caused by the heating effects of the current passing over the body and heating the sweat and outer layer of skin.

[keithfoor]

Now, as far as the rest of this, and this circuit design. IT CAN KILL YOU.
Any welder, even the inverter models, via the internal circuits have whats' referred to as isolation. Basically no direct connections exists between the line power and the output of the unit.
With the old transformer welders it was done in the transformer. The line voltage connected through the switch or whatever to the primary side of the transformer and the leads connected to the secondary side. The isolation was accomplished via the transformers magnetic field. With an inverter it's basically the same but the transformer is smaller. The AC comes in and is rectified and filtered. It's then chopped back up and run into a smaller transformer where it's stepped up or down depending on the unit (welders is down, 110 plasma cutters is up). It's then chopped up again and fed to a capacitor bank and the current is controlled via the on and off dwell time of the the chopping. I realize that this all sounds complicated, and believe me that this is a really basic explanation and the full one can be rather complex, but the basic idea is that you have to have isolation between the line voltage and the output. This water resistor design doesn't have that. So if you touch the work while the circuit is powered up you get shocked. If you lay a grounded metal cased tool on the work or the table, the fire flies. This is all due to the lack of isolation.

[keithfoor]

I stumbled onto this thread and found it very interesting. Would using the hot wire and power supply off of an ARC welder be sufficient to run the DIY plasma cutter? If so would a 110V welder work and how deep would it cut? How about a 220V ARC welder. Thoughts on this? Thanks!!!

This will not work, here's why.
A stick / TIG welder is a constant current power supply. The open circuit voltage is around 80 volts, and the welding voltage is down around 20 volts with the current flow at teh level set by the operator. A MIG, before anyone asks, is a constant VOLTAGE supply, that ranges from 17 volts on a cheap Sears MIG for doing short circuit MIG operations to 50 volts on a commercial unit capable of doing spray arc. Plasma cutters operate at 200 plus volts between the gun tip and the work. So the voltages are not high enough for any welder, to plasma cut with.

[keithfoor]

What about the other DIY cutter discussed in here using the Alternator? What are its drawbacks when it comes to safety.

This is actually not a bad idea, but it has some limitations.
First limitation is that the alternator would need to be spun VERY fast to get 220 out of it.
The standard setup on a car turns an alternator at about 3 to 4 times the RPM of the engine.
So at 3000 RPM on the motor, the alt is spinning between 9000 and 12000 RPM. Don't believe me, it's simple gearing. the crank pulley is 6 inches in diameter, the alternator pulley is 2 to 2.5 inches in diameter. Figure it out.

Now, I have run an alternator at full field and used it to generate 110 volts DC to run drills and grinders. That takes spinning the engine at about 2100 to 2500 RPM. That means the alternator is spinning at 8400 to 10000 RPM. So to get 220 volts out of it, you would need to spin it at 16800 to 20000 RPM. In the hot rod car and NASCAR scene they put larger pulleys on the alternator to slow it down because of the engine speed or the alternator will throw the windings off it and fail.

If that could be overcome, the diodes in an alternator are not designed for 220 volts.
They are typically only rectifying 15 volts max but are typically 150 PIV diodes.
So a set of diodes with a high enough PIV to take the 220 volts would be in order.

Now, you will still need to generate the high voltage high frequency, so you will still need AC power to run a step up transformer for the spark gap. By the time you build it, obtaining all the needed parts, you could buy a used plasma cutter that wouldn't be be some huge contraption. Also, the required electric motor to spin the alternator would be in the order of 10 to 15 HP. Pricey. If you go with a gas motor, you are needing fuel and 110 to run the unit. Again, it would work but WHY????

[keithfoor]

Have you thought of using a rewound microwave transformer as the isolation device?


You all are probably getting sick of my posts. If anyone ever tells you it sucks being smart, believe them. I see questions and feel compelled to give them answers that are accurate and safe. This will be it for now, so if you are tired of reading my stuff, take heart, this is it for now.

As far as the specific topic of a microwave transformer. It's probably NOT going to work. But you my friend are on the right track with rewinding a transformer, you just picked the wrong one. Here's why, and what you can do that will work.
Transformers are rated in VA or Volt Amps. It's basically the same as watts but in AC there are some differences that need the other rating. I am going to use watts, even though, because it's simpler to understand without going into AC power theory that no one wants to hear.

So, a microwave produces RF energy in Watts via a device called a magnetron. For you older guys, it's a vacuum tube more or less. Now microwaves are rated in watts, the bigger consumer units are 1000 watts. Now keep in mind that they, like anything else, are not 100% efficient, so the transformer is actually a bit bigger about 1500 watt rating.
Now the rating has to do with a couple things. Part of it is simple, things like wire gauge and the current capability of that gauge of wire. but some more in depth things have to do with the magnetic properties of the core material and its size. Without getting complicated, the larger the core the more current capability of the transformer, and the higher the wattage rating of it, for the same input and output voltage. This is why a 300 amp TIG that's old school transformer design has a huge transformer that weighs over 100 pounds. Remember that you are wanting to step up the voltage, but your current needs are still around 40 amps. So for a 40 amp 200 volt secondary you are going to need a 10 or even an 8 gauge wire for the secondary winding. So the mechanically it's not going to work because you will need almost as many windings on the secondary as the primary. Phyically it will just not fit. Beyond that, the core saturation (has to do with the magnetic properties of the core) is not going to support the current levels you are needing for it to work.

I have seen the plans for the microwave transformer based TIG welder using the 6 transformers in series parallel to get the current pumped up for welding. Remember that you are stepping the voltage down a great deal and stepping up the current. With a plasma cutter, the step down isn't as great so the number of transformers would better than double for it to be a workable solution. Again, it could be done but it's not reasonable.

A better option.

EVERYONE LISTEN UP , HERE IS THE ANSWER TO THIS.

Transformer choice is EVERYTHING here. You need a big enough core that will support the needed voltage AND current.
Your best option is to start with a 3 phase step down transformer that goes from 480 to 240 3 phase. These are available in the KVA or 1000's of WATTS and if you know an electrician, might be cheap. What these do in commercial buildings that are fed by 480 3 phase power is to step that down to 208 / 240 3 phase, creating 3 legs of 110 volt power from the 3 legs of 277 (277 is 480 leg to neutral). These are available big enough to be rewound and used. Now the down side (there is always one) you will need 3 phase power (you weld anyway so build a rotary phase converter because a static converter will NOT work with a transformer). The phase converter will come in handy to run all that cheap 3 phase machining equipment that is all over at business auctions.

Trick here is that you don't remove the secondary, you actually remove the primary, leaving the secondary to become the primary. You then wind your own secondary on the core and add in the extra set of diodes. A bridge rectifier for 3 phase needs 6 diodes, not just 4 like single phase.

Some research into 3 phase power will get you on the right track, if you are smart enough to understand what I have said, you will have no problems with doing it. If this all sounds really complicated and you are totally lost. Well, keep in mind I say this to save your life, just go buy a plasma cutter and leave screwing with things that will kill you to guys that know how to handle it safely. I don't mean to sound like a butt, but electricity is a wonderful thing, it's also VERY dangerous if not handled correctly. If you don't have a firm grasp of it, don't screw with it. The outcome can be very bad.

[bonny]

Hello...

I read that thread as I'm interested for Plasma torch.
As I read I think here some post's or image is missing. BeePig's homemade torch? Some other schematics (I see only two on beggining of thread)

I'm interested on small plasma to cut sheet metal only 1 to 2mm thick. ...and have transformer lying around with output 60V@50A. probably nedded to be revound to higher voltage but what is minimal curren't nedded for plasma? (thin material)

[jdchmiel]

soda can? 1 amp. thick / thin is relative.

In case it is not clear yet, Do not build any of these circuits, they are very dangerous, and actually illegal.
The second circuit could be made but eliminate the 7-9 ohm water cooled power resistor and use an isolation transformer on the input. That design is running 240 VAC input, rectified, so you are working with 340v DC output. The commercial ones seem to operate at a safer lower voltage. Lower voltage is less likely to arc through insulating materials. Most wire used for 120 VAC is 600v rated. operating at 340v, adding a hf/hv component to that .. bad things can happen. IF I was going to try to do a diy plasma, I would do two alternators in series with a 20 hp motor for something in the 100-140 volt range at 50 amps. Your 60v 5a transformer would be good if you had two in series.. rewound you can get 120v @25amps which would be a good smaller unit. I would be more inclined to use some high frequency filter from the high voltage arc start to keep the rest of the circuit cleaner and more reliable.. See the diy Tig welder threads with the HF units. Be aware a few of the posted schematics are not correct and are corrected in 1-2 threads.

[BeetleJuice]

i guess a stick welder can not be modified because of the low voltage output?
the same for a mig welder?

i have a simgle phase 220volt power line
if i need 120 volts and 50 amps means 5kw ,can a power line from a house give 25amps @220volt?

moreover i would like to ask how much will cost a transformer that takes 220volt and gives 120 volts @ 50amps?
maybe a toroidal one would be great
as i guess there is no electrical common use device that uses such a transformer
i think that there are some "garden" transformers here in greece .they give i think 40volts and are rated 1,5kw
2 of them in series plus another set i parallel , but i think this is no decent diy project
i hate projects that look awfull

i guess the whole project has problems only because we are trying to avoid the big transformer . why? too much money?
the GM unit could be substituted with an easy HV module , there are many like them in the internet or as kits also to build them on your own
maybe an old pc supply can give you the 12 and 24 volts switching dc power and avoid the extra 2 circuits (transformer-bridge-cap-maybe the relay also)
the point is to make our lives easier and to do decent projects

some people in greece had american refrigerators and used a 220 to 110 transformer
i remember they were heavy
i guess they would be 1-1,5 kw? they could do the job too

as for the protection i should add this
dc can be lethal too over 50 volts
my car battery gives peak current lets say 200amps? 200x12=2,4kw
i am not dead , why? cause the potention difference is not enough to make these 200 max amps through my body
if i use a battery with 1 amp and 70volts it would kick me for sure
there is no straight answer what kills , the volts or the amps , its a wrong question
if you have the volts but the source cant provide the amps then is not lethal
if you have the amps but the volts are low , there is no amps at all!

what i cant understand : my mig gives 30volts lets say
ok , she doesnt kill me .ok she is isolated from mains throught he transformer , so i dont touch any 220 volts ever
lets go to plasma : why the heck is possible not to get hit by 120 volts is the tortch works with 120 volts? what is the meaning of the isolation transformer ?
to bring the amps lower? so lets say it will not me 50amps it will be 30? 25?
it will kill eitherway

i see many of you say about isolation transformers to the plasma
as i can see the plasma kills with or without isolation transformer , the voltage is high enough to allow current through me and the max current is much

if you mean "to use an isolation transformer" in order not to get hit by the table when switch is off , then ok ,you are correct.
but then the DIY plasma with the "isolation relay" is not wrong .some say is is lethal too. i think the new version is not bad.

still the resistor in the tank is ugly . smart use but very ugly .

anyway this is me
hope i didnt say much!
thank you so much , i learned many things today!

[awright]

BeetleJuice, have you read the entirety of this thread? The purpose of isolation from the line by means of either a large line isolation transformer or a much smaller high fequency transformer and the relationship to safety have been discussed extensively. As one example, see my post #27, but others have addressed the same subject. I see no purpose in rehashing the subject of line isolation here.

Considering the amount of confusion and the number of potentially lethal circuit suggestions that have been presented, at this point I would STRONGLY urge people who do not thoroughly understand the safety issues to NOT play with these circuits. Is your life worth the price of a commercial plasma cutter? I am truly amazed at the number of frankly idiotically dangerous circuits that have been presented, some by people who should know better. I am a very strong advocate of doing-it-yourself, but not of electrocution of ones self or innocent bystanders.

I am sorry for sounding officious, but there is simply too much dangerous ignorance on display on this topic.

awright

While I am alienating pwople, let me complete the job by pointing out that the rules of punctuation and grammar have evolved over thousands of years to facilitate written communications, not to oppress people. The complete absence of capital letters at the beginning of your sentances is perhaps cute and trendy and even tweety, but displays a lack of respect for your readers and seriously distracts from clear communication of your questions. If you are too lazy to punctuate your questions clearly, why should I spend time trying to follow and respond? I am not criticizing your use of English, clearly as a second language. I cannot ask any questions in a second language. But I suspect even the Greek language uses capitals at the beginning of sentances.

I guess I'm in a testy, impatient mood due to the cold I'm suffering from

[brslk]

BeetleJuice, have you read the entirety of this thread? The purpose of isolation from the line by means of either a large line isolation transformer or a much smaller high fequency transformer and the relationship to safety have been discussed extensively. As one example, see my post #27, but others have addressed the same subject. I see no purpose in rehashing the subject of line isolation here.

Considering the amount of confusion and the number of potentially lethal circuit suggestions that have been presented, at this point I would STRONGLY urge people who do not thoroughly understand the safety issues to NOT play with these circuits. Is your life worth the price of a commercial plasma cutter? I am truly amazed at the number of frankly idiotically dangerous circuits that have been presented, some by people who should know better. I am a very strong advocate of doing-it-yourself, but not of electrocution of ones self or innocent bystanders.

I am sorry for sounding officious, but there is simply too much dangerous ignorance on display on this topic.

awright

While I am alienating pwople, let me complete the job by pointing out that the rules of punctuation and grammar have evolved over thousands of years to facilitate written communications, not to oppress people. The complete absence of capital letters at the beginning of your sentances is perhaps cute and trendy and even tweety, but displays a lack of respect for your readers and seriously distracts from clear communication of your questions. If you are too lazy to punctuate your questions clearly, why should I spend time trying to follow and respond? I am not criticizing your use of English, clearly as a second language. I cannot ask any questions in a second language. But I suspect even the Greek language uses capitals at the beginning of sentances.

I guess I'm in a testy, impatient mood due to the cold I'm suffering from

Be careful of Muphrys law.

[denrep]

I haven't read much of what's been posted in this thread but the LISTEN UP answer shouting caught my eye.



. . .A better option.

EVERYONE LISTEN UP , HERE IS THE ANSWER TO THIS.

. . .Your best option is to start with a 3 phase step down transformer that goes from 480 to 240 3 phase. These are available in the KVA or 1000's of WATTS and if you know an electrician, might be cheap. What these do in commercial buildings that are fed by 480 3 phase power is to step that down to 208 / 240 3 phase, creating 3 legs of 110 volt power from the 3 legs of 277 (277 is 480 leg to neutral). These are available big enough to be rewound and used. Now the down side (there is always one) you will need 3 phase power (you weld anyway so build a rotary phase converter because a static converter will NOT work with a transformer). The phase converter will come in handy to run all that cheap 3 phase machining equipment that is all over at business auctions.

Trick here is that you don't remove the secondary, you actually remove the primary, leaving the secondary to become the primary. You then wind your own secondary on the core and add in the extra set of diodes. A bridge rectifier for 3 phase needs 6 diodes, not just 4 like single phase.

Some research into 3 phase power will get you on the right track, if you are smart enough to understand what I have said, you will have no problems with doing it. If this all sounds really complicated and you are totally lost.. . ..

Keithfoor - I don't where to start or what to say. :confused:
I will say that I feel "THE ANSWER" seems to describe impossibilities.

How ‘bout posting a simple quick sketch of the "trick" circuit?
Maybe even just the windings?

Good Luck

[GBM]

Did Beerpig die using this circuit ? He seemed to have vanished early in the thread....Maybe R and D was more exciting than he had expected ?

[vwguy3]

As I was reading this old thread the old Debby Boone song "you light up my life" kept running through my head.....but then what do I know about electrics but the 3 union rules,
1)water flows down hill
2)pay day is Friday
and
3)the boss is a prick.
Oh wait that's the plumbers union:jester:.

[Warpspeed]

My first post here, and I found this Forum (and thread), while researching home brew plasma cutting.
This is an old thread, but the most informative I have found so far, anywhere on the internet.

Anyhow, as a retired electronics engineer, I thoroughly endorse the safety concerns raised in previous posts, and consider a mains isolation transformer an absolute necessity.

I have been mulling over how best to build a suitable high voltage constant current plasma power supply, and following are some of my ideas.
SAFETY, low cost, simplicity, and high power are the main objectives.

From what I have read, quality of cut depends on having a clean dry air supply, and a high quality dc supply of constant current, sufficient voltage, and low ac ripple current.
Some of the more crude home made supplies fall well short of most of these objectives IMHO.

Designing and building an IGBT cutter would not be totally beyond me, as I am a retired power electronics engineer. But I could never do it at a competitive price compared to an imported Chinese unit. The huge amount of testing and development time involved, and cost of parts make it just not worth doing.

However I have come up with a better, cheaper, and much simpler idea.......

I have an ancient old buzz box stick welder that I have not used in many years, but it has given me a few ideas. This one has a 240 volt primary, a 75 volt fully isolated secondary, and a shunt air gap with a plunger to set the current. It is advertised as an 80 volt 130 amp welder, and I have made some measurements that indicate that the short circuit current can be adjusted from about 20 amps to 130 amps.

The basis of my idea is to buy two more identical welders off e-bay, they are cheap and readily available these days, nobody wants them, as a modern MIG will do a much better job.

If I run each welder from a separate phase from three phase power, and fit a 150 amp bridge rectifier to each of the three secondary windings, then connect the three bridges in series, I should get a very clean low ripple current dc supply.

Connecting three bridge rectifiers in series this way is rather unusual, but it is the better method for a constant current source. The ripple voltages and currents largely cancel giving a very clean output that can be very readily adjusted from 20 amps upwards.

The wire gauges in this welder will allow full continuous output (100 % duty cycle) of 40 amps dc with no cooling fan. And for short bursts over 100 amps, maybe 20 Kw+

I doubt if lack of power will be a problem.
It may benefit from a bit of extra series inductance to reduce ripple current even further.

Because there will be no large storage capacitors required, there will be no potentially lethal stored energy. When the main power contactor opens, power immediately shuts off, and that should add to safety.

The control system should not be too difficult, the circuits shown earlier in this thread should work fine for controlling air and high voltage.

With a bit of patience accumulating parts, I believe I can build a really low cost SAFE fully isolated high power plasma cutting supply with very good dc characteristics.

What do you guys think ???

[jdchmiel]

what voltage are you targeting? You will still end up with rectified 80vac right? ~115 volts dc

Instead of killing that many welders that can supply way more current than you need, how about microwave oven transformers as a cheaper option?

Another idea.. maybe an alternator can put out enough voltage to work depending on how the fields are energized. stick / tig welders build from an alternator usually just use a 0-12v input to the field coils and get 60+ volts at 60-200 amps depending on the size of the alt. Maybe using a current limited 24v supply will increase the voltage and keep similar current to get you 100+volts are 40ish amps?

[Warpspeed]

what voltage are you targeting? You will still end up with rectified 80vac right? ~115 volts dc

Instead of killing that many welders that can supply way more current than you need, how about microwave oven transformers as a cheaper option?

Another idea.. maybe an alternator can put out enough voltage to work depending on how the fields are energized. stick / tig welders build from an alternator usually just use a 0-12v input to the field coils and get 60+ volts at 60-200 amps depending on the size of the alt. Maybe using a current limited 24v supply will increase the voltage and keep similar current to get you 100+volts are 40ish amps?

Open circuit voltage should be in the region of just over 200 volts dc.
There will be three 75 volt welders each modified to produce dc, and connected in series.

Microwave oven transformers offer no adjustment of output current, and have a very short intermittent power rating. They are made to run for only a minute or so with long rest periods.

Sixty volts will not be enough for plasma cutting, commercial units start at 180 volts minimum open circuit voltage and some work up to 300 volts.
An automotive alternator would certainty reach the required voltage and current, but you will need very high drive horsepower.

Automotive alternators are extremely inefficient, they are designed with minimum copper and iron to be small.
Nobody cares if a 100 amp alternator takes 3Hp or 8Hp to drive it when you have a V8 engine.
But a large electric motor might really struggle turning that alternator fast enough with enough drive torque.

I don't want a troublesome Mickey Mouse setup that is unreliable.
I want a plasma cutter with real balls, that will work trouble free for years and tackle absolutely anything.

[WeldingMachine]

I don't want a troublesome Mickey Mouse setup that is unreliable.
I want a plasma cutter with real balls, that will work trouble free for years and tackle absolutely anything.

Then you want to copy a Hypertherm. I am more that happy with mine, it still makes me smile when I use it. Great stuff!

[Warpspeed]

Then you want to copy a Hypertherm. I am more that happy with mine, it still makes me smile when I use it. Great stuff!

Are you going to scan and post the circuit diagram ?

[jimcolt]

What are you going to do for a torch?

Jim

[Warpspeed]

Haven't got that far yet.
First comes a viable power supply.
I will probably look for a commercial torch that has plenty of readily available spares later on.

[jimcolt]

The primary reason that plasma system manufacturers stopped producing "drooper" style transformer based power supplies was to control the notoriously short consumable parts life (torch electrode and nozzle) that was caused by the current inrush to the torch. When the gas in a plasma torch becomes ionized....the resistance between the electrode (negative) and the nozzle (positive) becomes very low. The current inrush would create a brief overshoot that would damage the nozzle orifice and pit the electrode emitter....causing poorer cut quality with each start.

Chopper power supplied (first used on Hypertherm plasma's in 1985) had a closed loop current control that allowed for a controlled current ramp up....and no current overshoot at the beginning of each cut. The newer technology inverters (from the major manufacturers) use current ramps that vary depending on amperage output wettings as well as consumable designs....for best cut quality and consumable life. Some also have controlled gas flow and current ramp downs at the end of each cut cycle to resolidify the hafnium electrode emitter......further extending consumable life.

Good luck with your design.....and I understand that these projects are always fun.....but you probably could find an older technology drooper....or even a chopper plasma system on the used market for as low as a couple hundred dollars.

Jim Colt

[WeldingMachine]

Are you going to scan and post the circuit diagram ?
Sorry, I have zero electronic knowledge, I'm just a welder.

[Warpspeed]

Yes Jim, you are quite right.
Actually plasma has not just a low resistance, but a negative resistance.
An arc will grow to infinite size if it has enough current to feed from.
As the arc gets fatter, it draws more current, and grows even more.

The trick is to have a very high source impedance to stabilize the arc size. That can be done with a feedback control loop, or by building a constant current power source such as a constant current transformer.

Many loads such as neon gas discharge tubes, and microwave magnetrons behave very similarly to a plasma arc, and they work just fine with a shunt gap constant current transformer.
A stick welding transformer should work pretty well as it is, after all, generating a stable ac arc is what it was originally designed to do.

The three problems are, converting ac to dc (easy).
Getting sufficient open circuit voltage, three standard welding transformers in series should be sufficient to get me there.
And the biggie, getting a very smooth dc output current with minimal ac ripple.
By combining three full wave rectifiers in series running 120 degrees apart, the ripple will be minimal amplitude at six times mains frequency.

There should be no monstrous inrush current spike you will get with a capacitor bank and series resistor once an arc is struck.
The current will ramp up from zero when the mains contactor closes, and the high voltage pilot arc breaks down the air path. That is a fundamental characteristic of inductors.

Although I have never built a plasma cutter power supply before, I have spent quite some time designing very high powered pulsed industrial laser systems, so know a bit about plasma arcs in laser pumping flash tubes, and the physics will not be all that different.

While my knowledge of practical plasma cutting is about zero at this stage, I am not starting completely from scratch.
As a power electronics engineer I have a fair bit of relevant experience in related fields, and I believe this should not only work, but work well, and be a path others might like to follow.

[jdchmiel]

http://www.millerwelds.com/pdf/spec_sheets/PC9-8.pdf 140vdc 60 amps
http://www.millerwelds.com/pdf/spec_sheets/PC9-1.pdf 92vdc 27 amps

I think 3 phases of 75vac added together ends up being quite a bit over 200 volts but that math is really far in my past.. Do you know offhand the math to sum the waveforms? I think 3 phase normal rectification with outputs in parallel is sqrt 3 * vac. This would get you to ~130 volts and let you use off the shelf surplus 3 phase rectifiers.. like this: http://www.ebay.com/itm/100-NEW-THREE-PHASE-RECTIFIER-BRIDGE-MODULE-300AMP-/260863327475?pt=Motors_Car_Truck_Parts_Accessories&vxp=mtr&hash=item3cbcaa7cf3#ht_1905wt_907

Another thread on here that might interest you is the welding with car batteries thread where the experimenter built his own "reactor coil" , ie a big choke / inductor to limit the surge current when the arc was struck. A similar choke type solution might work for limiting the current inrush from destroying consumables.

And final thought this go around - how critical is it to have the 3 transformers have their shunts be in the same position? Building some sort of jig to keep them all in line?

[Warpspeed]

I think 3 phases of 75vac added together ends up being quite a bit over 200 volts but that math is really far in my past..

And final thought this go around - how critical is it to have the 3 transformers have their shunts be in the same position? Building some sort of jig to keep them all in line?
This had me stumped for a while too, it is a very unusual way to build a three phase rectifier. I searched the internet, and all my electrical engineering reference books and could find nothing anything like this anywhere.
It is very hard to visualize how the actual numbers work out.

So I just connected three small identical 16.5 volt transformers to three bridge rectifiers, and hooked it up to the three phase supply and measured the results.
I obtained 47.5 volts dc with about 5% rms ripple current into a resistive load.
Surprisingly very close to three times the rated rms secondary voltages.

On that basis I should probably expect about 216 volts dc at nominal mains voltage with 75 volt secondary windings.

An interesting feature of this rectifier configuration is that the dc current through each bridge rectifier is a constant square wave at the full dc output current, and each winding contributes to the output, as the waveform outputs from the other two windings are falling. This will source current over the whole of each cycle from each transformer, which is very different to how most three phase rectifiers operate.

If the transformer outputs are unequal, it only means that the output ripple will become a bit lumpy. Even if one transformer is removed entirely, it will still be better than having a single phase rectifier.

As I plan to use three identical welding transformers, setting the shunt gaps for the required output current should not be that critical in practice.
I could always get really carried away, and use three motor positioned magnetic shunts, and a microprocessor to do all this automatically with current feedback from each phase !

But more seriously, at this initial stage, just three bog standard welding transformers with each set manually should work well enough to start with.

What I like about this is there is no large electrolytic capacitor bank, and no stored energy. It should provide a very nice constant smooth dc current without massive inrush.

The only way to get low dc ripple with single phase, is with capacitors, to source the energy as the mains voltage swings throgh zero.
With three phases available I don't need to do that.

I have just bought three of these rectifier bridges off e-bay.
http://www.ebay.com.au/itm/150489786251?ssPageName=STRK:MEWNX:IT&_trksid=p3984.m1497.l2649
I only plan to connect four of the six diodes in each package, but the diodes themselves are rated at 150 amps with a 25 Celsius heat sink, and 85 amps with a 90 Celsius heatsink. More than sufficient to survive a brief dead short at full max transformer current setting.

The actual cutting voltage is supposed to end up around 70 to 100 volts, but a much higher open circuit voltage is required to get a good constant current characteristic over that range.
A good rule of thumb is twice the maximum operating voltage for most constant current applications.
Going significantly higher than that, increases safety concerns, so around 200 volts open circuit voltage is probably not a bad compromise to begin with.

[Rod buster]

Well at this time, I'm disassembling a "pole pig" transformer that I picked up from a ham friend of mine. It was rated at 7.5 Kv at two amps, but somewhere down the line, it shorted out. I plan on rewinding it with 240 volts on the primary, and 200 volts on the secondary, starting with the primary at the center of the core.. I will use a full wave air cooled bridge on it. I plan on using the water heater elements, submerged in oil. The high voltage start will come from a car coil. Any other ideas?:rolleyes:

[jdchmiel]

pole pig is prob worth more than you realize.

[Warpspeed]

pole pig is prob worth more than you realize.
If it is burned out, it is only worth scrap value for the copper and iron it contains.
Sounds like a good plan.....

[Warpspeed]

I have just scored a second 40 year old buzz box welder, absolutely identical to the one I already have here, for $40.00
Another 6Kw of fully isolated and very readily adjustable constant current power.
Two down, one more to get.

Anyhow, Hypotherm seem to be the industry leaders in plasma cutting technology, and I stumbled across one of their patent applications which makes very interesting reading.
http://www.google.com/patents/US4225769
If you download the PDF it is a goldmine of information, but very difficult reading.

The main thing Hypotherm have is their unique patented pilot arc transfer system which is very clever, and one of the outstanding features of their product.
There is also mention of some requirement for the high frequency high voltage pilot arc.

Most home made (and Chinese) systems look rather crude, commonly based around using a car ignition coil.
Hypotherm seem to prefer a pulsed radio frequency source 1Mhz to 3 Mhz at a voltage of 6Kv to 10Kv according to their patent.
The power requirement for this is not stated.

So while I patiently wait for my third ultra low cost bargain stick welder to present itself on e-bay, I will see what I can do to generate some bursts of very high voltage RF power suitable for a pilot arc.

I can well imagine that a long burst of very high voltage RF will create a much bigger cloud of angry ions than a single skinny wimpy snap discharge from a car ignition coil.

I have no idea how Hypotherm go about doing this, and I am definitely not copying anything of theirs directly.
Whatever I put together will be a one off Frankenstein prototype, based around junk I already have sitting around here, or whatever really cheap stuff I need to scrounge.

[jimcolt]

The Hypertherm (not Hypotherm) patent you are referring to is one of over 80 patents that Hypertherm holds.....and is for the starting method on a couple of our older technology industrial plasma systems that are no longer in production.

This patent is certainly not "the main thing" that Hypertherm has, rather we are the largest manufacturer of plasma cutting equipment worldwide, and are on the leading edge of the industries technology with continuous improvements in consumable parts life, cut quality, speed and productivity as well as with power supply and control reliability and innovation.

I'd say the best things that Hypertherm has are its customers, its owner-employees, as well as its drive to continuously improve the performance of its products.

Jim Colt Hypertherm


I have just scored a second 40 year old buzz box welder, absolutely identical to the one I already have here, for $40.00
Another 6Kw of fully isolated and very readily adjustable constant current power.
Two down, one more to get.

Anyhow, Hypotherm seem to be the industry leaders in plasma cutting technology, and I stumbled across one of their patent applications which makes very interesting reading.
http://www.google.com/patents/US4225769
If you download the PDF it is a goldmine of information, but very difficult reading.

The main thing Hypotherm have is their unique patented pilot arc transfer system which is very clever, and one of the outstanding features of their product.
There is also mention of some requirement for the high frequency high voltage pilot arc.

Most home made (and Chinese) systems look rather crude, commonly based around using a car ignition coil.
Hypotherm seem to prefer a pulsed radio frequency source 1Mhz to 3 Mhz at a voltage of 6Kv to 10Kv according to their patent.
The power requirement for this is not stated.

So while I patiently wait for my third ultra low cost bargain stick welder to present itself on e-bay, I will see what I can do to generate some bursts of very high voltage RF power suitable for a pilot arc.

I can well imagine that a long burst of very high voltage RF will create a much bigger cloud of angry ions than a single skinny wimpy snap discharge from a car ignition coil.

I have no idea how Hypotherm go about doing this, and I am definitely not copying anything of theirs directly.
Whatever I put together will be a one off Frankenstein prototype, based around junk I already have sitting around here, or whatever really cheap stuff I need to scrounge.

[Bluewelders]

If all that you want is isolation,and 1 or 2 hundred volts,why not hook the low voltage AC outputs together.
Plug one in and use the other input as an output.

[Warpspeed]

If all that you want is isolation,and 1 or 2 hundred volts,why not hook the low voltage AC outputs together.
Plug one in and use the other input as an output.

Several reasons. If you connect the three secondary phases together in series, they will add up to zero volts.
If I connected all three welders up, to only one phase, the mains current would then equal the cutting current, and I just don't have that much mains power available on each individual phase.

Rectifying and then combining all three phases, I can get a very smooth dc output current without requiring a rather dangerous high voltage capacitor bank.

Without capacitors, there will be minimal inrush current when the mains contactor closes, and zero stored energy when the mains contactor opens.
This will make a very robust and reliable power source capable of over 20Kw, also very simple to make and low cost.
And above all, it is fully mains isolated and about as safe as it is possible to make it.
http://i144.photobucket.com/albums/r166/Warpspeed_photos/plasmadcsupply.jpg

[waferhead]

Ignoring the folks who seem intent on electrocuting everyone,...

Backing up a little, the high frequency 3 phase output of a ~200A alternator run through an appropriate transformer>rectifier should be in the right ballpark... Perhaps one of the off the shelf inverter transformer setups would work if run at the right RPM?

(heck, it might be sufficient by itself based on some of the spec'd output voltages and currents posted)

Inherent isolation, keeps the size down etc, and reasonably easy to control the current with ~low power circuitry

Hey, I have an 18HP Briggs and a 215A Ford E350 alternator, and need a new project . :D

Short: What output voltage, current, and regulation quality is required to make a reasonable power supply to effectively cut various metal thicknesses?

[jimcolt]

To cut 26 gauge steel with the best cut quality you need 30 amps at 80 load volts.To cut 1/2" steel with the best quality you need 60 amps at 146 load volts. To cut 1" steel with the best quality you need 130 amps at 130 load volts. To cut 6-1/4" stainless steel you need 800 amps at about 185 load volts. These are specifications out of Hypertherm plasma cutting operators manuals.

There are a lot of variations to these specs based on different torch and consumable designs...and different gases used in the torches. Simple plasma torches use air as the plasma gas. Most industrial plasma systems use oxygen as plasma and air as shield for cutting steel, use mixes of argon and hydron as plasma and nitrogen as shield for cutting stainless and aluminum...these gases have different resistance when ionized, and affect the voltage current relationship differently.

Current regulation is normally held to within 2 or 3 amps at all poawer ranges for cut quality consistency. Voltage is controlled by torch to work distance....and height control (voltage feedback) is used in mechanized plasma and a drag consumable part is often used to maintain the correct standoff/voltage in hand cutting applications.

It is important to have very pure DC at the torch....so often large output inductors are used. Most power supplies today use either chopper (Pulse width modulated) output current control or inverter power supply designs (mostly for size and weight savings).

Industrial torches and older hand held plasma's use a high frequency (15 kv @ 2 megaherts) high voltage circuit to ionize the gas at the beginning of each cut cycle. Newer technology air plasma systems use a moving (air pressuredriven) electrode to create a short circuit spark that ionizes the air.

Hope this helps.


Jim Colt Hypertherm






Ignoring the folks who seem intent on electrocuting everyone,...

Backing up a little, the high frequency 3 phase output of a ~200A alternator run through an appropriate transformer>rectifier should be in the right ballpark... Perhaps one of the off the shelf inverter transformer setups would work if run at the right RPM?

(heck, it might be sufficient by itself based on some of the spec'd output voltages and currents posted)

Inherent isolation, keeps the size down etc, and reasonably easy to control the current with ~low power circuitry

Hey, I have an 18HP Briggs and a 215A Ford E350 alternator, and need a new project . :D

Short: What output voltage, current, and regulation quality is required to make a reasonable power supply to effectively cut various metal thicknesses?

[waferhead]

Thank you.

So with some quick calcs based on those numbers, you would need >5KW to even efficiently do 3/8" plate, easily double the power output of any single alternator, even allowing for significant underrating.

1/8" is well within the output range and 1/4" plate might be doable, barely with the straight output and good regulation, assuming some overdrive and a simple HF starter.

I think a simple welding engine drive for a spoolgun or stick is the only reasonable match though for the weldernator-ish setups, I'll probably keep an eye out on Ebay etc for a real plasma unit.

It looks like clean/dry air is half the battle or more... Maybe steam plasma using a proper power source would be a benefit here, I'll have to look into it.
Unfortunately the only steam plasma unit available on the market is hideously overpriced and very underpowered, but the basic idea looks like a winner...

Ideally one could use it to do plasma spray coating (powdered media) as well as cutting.

[Warpspeed]

Jim,
Thanks for the technical information, it is extremely helpful, and hugely appreciated.

Waferhead,
The alternator idea is probably doable, but the closed loop control of current will be relatively slow acting and potentially may not have the very high stability that appears to be necessary for a really good quality cut. There may also be some interesting problems with initially establishing the desired constant current at startup.
Inverter welders have exactly the same problem, but closed loop control will be much easier and faster with a PWM inverter, mainly because there will be minimal stored energy in the magnetics of a smaller high frequency ferrite transformer, compared to controlling the dc magnetic field of a much larger iron alternator.
The problem of closed loop current control does not arise with a constant current shunt gap mains transformer, or a current limiting resistor. They are inherently self current regulating without requiring any external feedback control.
Jim mentions the very fast buildup of current leading to short life of consumables. The provision of a series choke would control current ramp up rate, as well as establishing a much smoother final constant current. I have a 1.8 millihenry 75 amp iron cored choke here, that originally came from a junked fork lift battery charger that I intend to try out.
The control of current level will not be as tight, as with an inverter but should be pretty well controlled.
Quite a few possible ways to do this, and each has its own advantages and disadvantages.
Inverter is undoubtedly technically the best, but not practical or economic for home construction.

Getting onto startup methods.
The cheap import cutters seem to use an adapted auto ignition system, and that simple system obviously works.
The high voltage radio frequency method sounds much better to me, but will be more complex to build. It does have the advantage of requiring a simple fixed non moving cutting head. So consumables should in theory cost less.
The air powered electromechanical scratch start will certainly be the simplest electrically, if you buy a torch head that has that feature.
Again there are advantages and disadvantages of each approach that need to be considered.

I am going to have a go at building a high voltage radio frequency generator.
Mainly because I am a masochist, and a Ham radio operator with a very convenient stash of radio transmitter parts that I have had stored here for years. And the problem interests me.
I will use parts I already have, which will mean that it costs me almost nothing to try, but would be difficult to duplicate exactly by others but not impossible.

Jim, how much power do these high voltage RF generators usually produce?
Just curious, because what I am putting together can be run anywhere between a very few watts, up to several hundred watts, which I realize would be totally excessive.
Design target is 6 to 7Kv output at 2 Mhz from the power amplifier.
And the possibility of much higher output voltages, by driving the output tank coil from a tapping.
It will basically be like a mini Tesla coil directly driven from a high voltage RF power amplifier instead of a spark gap.
At least those are my intentions at this stage, which may change.

[waferhead]

Flyback transformer/oscillator and a tank circuit, Miller spark gaps.
More ugly HF power than you know what to do with.
Tesla would approve.

Tube-based TVs are ~free these days.
( a small B&W unit will be plenty, you can almost weld with the big color flybacks... hmmmm)

You'll have more $ in the spark gaps than everything else, but they are probably required for long term reliability.

OTOH an ignition coil/oscillator combo is dead simple and could be built as easily and even more cheaply.

I still haven't totally given up on the idea for thin material. (1/4" and less) but the steam plasma idea in lieu of compressed air or N2 has me really curious...
Esp with a possible tiny argon purge to preserve the consumables.

Not much detailed info out there so far, unfortunately.

As a wise Russian Admiral once said, "...the best is the enemy of the good ", we'll see where it ends up.

[jimcolt]

The cheap import cutters use an RF spark gap oscillator.....which is high frequency....not like an auto ignition. The power of the high frequency circuits are generally 100 watts or less.

Inrush current......when the plasma gas becomes ionized...is pretty high. generally a surge injection circuit with some large capacitors is used to mainatin power for the plasma arc while the power source build energy.

Good cut quality and good consumable life depends on a pretty much ripple free DC.....that maintains within a few amps of the torch nozzle designs maximum amperage rating. Current overshoot at the beginning of a cut cycle will destroy the nozzle in a short period of time....so current ramping at the beginning of each cut cycle is critical. Molten electrode hafnium will be ejected at the end of the cut cycle if a coordinated gas pressure/current rampdown is not done at the end of each cut cycle....resulting in short electrode life.

There are alot of tricks to make the process work correctly....based on the physics and thermal transfers within the torch as well as with power supply technology. It likely is easiest o simply purchase a broken plasma system of a respected design....and repair it....than to re-invent the wheel!


Jim Colt



Jim,
Thanks for the technical information, it is extremely helpful, and hugely appreciated.

Waferhead,
The alternator idea is probably doable, but the closed loop control of current will be relatively slow acting and potentially may not have the very high stability that appears to be necessary for a really good quality cut. There may also be some interesting problems with initially establishing the desired constant current at startup.
Inverter welders have exactly the same problem, but closed loop control will be much easier and faster with a PWM inverter, mainly because there will be minimal stored energy in the magnetics of a smaller high frequency ferrite transformer, compared to controlling the dc magnetic field of a much larger iron alternator.
The problem of closed loop current control does not arise with a constant current shunt gap mains transformer, or a current limiting resistor. They are inherently self current regulating without requiring any external feedback control.
Jim mentions the very fast buildup of current leading to short life of consumables. The provision of a series choke would control current ramp up rate, as well as establishing a much smoother final constant current. I have a 1.8 millihenry 75 amp iron cored choke here, that originally came from a junked fork lift battery charger that I intend to try out.
The control of current level will not be as tight, as with an inverter but should be pretty well controlled.
Quite a few possible ways to do this, and each has its own advantages and disadvantages.
Inverter is undoubtedly technically the best, but not practical or economic for home construction.

Getting onto startup methods.
The cheap import cutters seem to use an adapted auto ignition system, and that simple system obviously works.
The high voltage radio frequency method sounds much better to me, but will be more complex to build. It does have the advantage of requiring a simple fixed non moving cutting head. So consumables should in theory cost less.
The air powered electromechanical scratch start will certainly be the simplest electrically, if you buy a torch head that has that feature.
Again there are advantages and disadvantages of each approach that need to be considered.

I am going to have a go at building a high voltage radio frequency generator.
Mainly because I am a masochist, and a Ham radio operator with a very convenient stash of radio transmitter parts that I have had stored here for years. And the problem interests me.
I will use parts I already have, which will mean that it costs me almost nothing to try, but would be difficult to duplicate exactly by others but not impossible.

Jim, how much power do these high voltage RF generators usually produce?
Just curious, because what I am putting together can be run anywhere between a very few watts, up to several hundred watts, which I realize would be totally excessive.
Design target is 6 to 7Kv output at 2 Mhz from the power amplifier.
And the possibility of much higher output voltages, by driving the output tank coil from a tapping.
It will basically be like a mini Tesla coil directly driven from a high voltage RF power amplifier instead of a spark gap.
At least those are my intentions at this stage, which may change.

[Warpspeed]

The cheap import cutters use an RF spark gap oscillator.....which is high frequency....not like an auto ignition. The power of the high frequency circuits are generally 100 watts or less.

Inrush current......when the plasma gas becomes ionized...is pretty high. generally a surge injection circuit with some large capacitors is used to mainatin power for the plasma arc while the power source build energy.

Jim Colt

Once again thanks Jim.
I should very easily be able to raise a hundred watts at 2 Mhz the way I plan to go about this.

I noticed the charge injection circuit in the Hypertherm patent, and assumed it had two purposes.
First to allow a controlled current build up during arc transfer, and second to allow the inverter feedback time to catch up to the suddenly applied load.
I will not need that as such, but I will have a very similar time constant for current ramp up with my series dc choke.

You are quite right about buying a deceased plasma cutter and bringing it back to life as being the most practical path.
But I think I can probably build something far larger, more powerful, and much more ugly for vastly less.

And also, doing it all myself is a bit of a challenge and an adventure.
And I am learning interesting things and occupying much free time in my retirement. And there is also the satisfaction of achievement.

I can probably do a complete 100 amp three phase power supply, high frequency ignition, and compressed air controller for around $200.00 plus all the various parts I already have.
The only moderately expensive item to buy will be the hand torch.

[waferhead]

It likely is easiest o simply purchase a broken plasma system of a respected design....and repair it....than to re-invent the wheel!


Jim Colt

Probably wise advice, I'm pretty decent at fixing vs. designing.

(I'm trying to formulate the question but it seems to sound the same regardless...)

I assume it's like buying a welder, just buy a used Synchro and a MM200-250 and don't look back.
What is the Synchrowave 250 of the plasma cutter world?
(...but hopefully a bit more portable)


build something far larger, more powerful, and much more ugly for vastly less.
;-)

[Warpspeed]

Flyback transformer/oscillator and a tank circuit, Miller spark gaps.
More ugly HF power than you know what to do with.
Tesla would approve.

Tube-based TVs are ~free these days.
( a small B&W unit will be plenty, you can almost weld with the big color flybacks... hmmmm)

You'll have more $ in the spark gaps than everything else, but they are probably required for long term reliability.

OTOH an ignition coil/oscillator combo is dead simple and could be built as easily and even more cheaply.
I have gone through a very similar train of thinking myself.
But my final inspiration is as follows:

Starting with a free microwave oven rescued from the local rubbish dump, use the original magnetron power supply but ditch the original magnetron.
Replace the magnetron with a large transmitting tube with it's own 2 Mhz crystal controlled semiconductor grid driver circuit powered off the filament supply.
So instead of 2,450 Mhz output frequency (or whatever it is), it ends up working at 2 Mhz, with the anode of the transmitting tube more safely at dc ground potential.
By replacing the 1uF voltage doubling capacitor with a smaller value, maybe 100nF (?) it should reduce the maximum output power from say 800 watts to maybe 80 watts.

This has quite a few things going for it, including low cost and safety, a very clean RF output, and potentially a vast amount of what should be very reliable power. And filament warm up time provides the required ignition startup delay without any additional timer !!
The main safety feature will be running the tube anode at ground potential and using a low value very high voltage coupling capacitor, so nothing but RF can ever come out should any of the components fail.

What comes out of this fiendish monster, will be a 2 Mhz carrier 100% modulated with a clean 50 Hz sine wave, peak voltage should reach 6.5 Kv, (13 Kv peak/peak) which is the peak voltage produced by the original magnetron supply.
That then drives a ferrite slug tuned tank coil that resonates with stray cable capacitance at 2 Mhz.

At least that is the grand plan....