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post #1 of (permalink) Old 09-11-2005, 09:39 PM Thread Starter
 
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On Board Plasma Cutter Problems



First off, anyone have any good sites or information on On Board Plasma Cutters?
The information I'm finding is VERY limited, and I'm pretty much doing mine from scratch.
If anyone has any sites, I'd sure like to know about them!
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I've tried a couple of on board plasma cutter designs, and I keep having a problem with blowing through expensive torch tip pieces...

Tried low pressure, medium pressure, high pressure, CO2, Nitrogen and Argon, all to no avail.
It's an old, but still serviceable Thermodyne 100 torch and it works just like new when I put it back on my shop plasma cutter, so I know the problem isn't with the torch.

I'm thinking maybe I'm throwing too much amperage at the arc, but that doesn't seem likely being powered with an automotive alternator just off idle...
With new tip parts it will blow through 3/8" nearly as fast as you can trace your cutting scribe line...
(Probably about 30-50 amps, but I haven't tested it yet.)


Anyone have any ideas?
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post #2 of (permalink) Old 09-11-2005, 11:09 PM
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Re: On Board Plasma Cutter Problems

I'm wondering if it not too much amperage, but spikes in the amperage causing the problems. How's the current regulation?
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post #3 of (permalink) Old 09-12-2005, 12:10 PM Thread Starter
 
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Re: On Board Plasma Cutter Problems

Current regulation is done by...
Voltage supplied to the rotor,
OR,
The engine RPM...

I can track both, but I'm not sure that's helping anything...

I thought about too much amperage, but more air flow through the tip should have carried away excess heat, and at least improving the situation a little, and it didn't...
Unless I'm just so excessive the head can't displace the heat even with 80 psi, and you think that would also do damage to the rest of the torch internals, but it isn't baking or melting anything else...

I'm right on the edge of something here, I can feel it!
I just can't figure the last part out...
This would be a GREAT field unit if it didn't scrap parts every two or three minutes of use...

(The good side is, you only use it 5 or 10 seconds at a time, so two minutes will let you get most of the way through a project...)
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post #4 of (permalink) Old 09-12-2005, 02:17 PM
 
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Re: On Board Plasma Cutter Problems

My understanding of plasma cutters is that they have a fairly high open circuit voltage to get the arc started, then voltage drops down to around 90 volts once the arc is started. Either that,or high frequency is used to start the arc and voltage remains fairly constant. There are several different systems, some with a pilot arc as well. Plasma cutters in the 30-60 amp range will be capable of a rough sever of around 3/4" and a clean cut on say 1/4-3/8". I'm wondering if you are getting overcurrent as well?

There was a guy on the diy welder list (list since MIA) that built a homemade plasma cutter by simply using a diode bridge and a water resistor to limit current through the 220 mains. Struck me as dangerous as all hell. But anyways he used some sort of plasma tip that was a "blow open" or similar to initiate the arc. Other guys have used diode voltage doubler circuits similar to a stun gun to start their arcs. Hope this incoherent rant helps.

I'd aim to design a constant current (10-60 amps) design that holds voltage under arc at around 90 volts.
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post #5 of (permalink) Old 09-12-2005, 03:11 PM
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Re: On Board Plasma Cutter Problems

I wonder - if the current is too high it might be the cause.

You need the fairly high open circuit voltage to get the arc started, but what's controlling the current after it starts?

I was having a similar problem with my Ready Welder. 12 volts it's too hard to start, on 24 it's too high a current - too hot - burns through the thinner stuff.

I took an old carbon pile battery tester, hooked it in series with the batteries. Screwed all the way down it's very low resistance, but as you unscrew the carbon pile resistor it cuts the current down. Open circuit it's still 24 volts so it's easy to start, running it's cooler - can do things like tie rod repairs on the trail.
Mounted the carbon pile itself in a small box.

Old Sun testers are a good source - or Harbor Frieght sells a Carbon pile battery load tester for about $50.

Could be a similar situation?
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post #6 of (permalink) Old 09-12-2005, 03:13 PM
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Re: On Board Plasma Cutter Problems

Get that working and you should make a fortune!
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post #7 of (permalink) Old 09-13-2005, 09:28 PM Thread Starter
 
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Re: On Board Plasma Cutter Problems

For anyone trying to build a plasma cutter! Read this first!
Build in a low air pressure disconnect into your primary circuit!
The air/CO2/Nitrogen/Argon or whatever you are using, if the pressure drops the heat in the torch chamber will do MAJOR BURN DAMAGE, and possibly electrocute you in a matter of a second or two!
(if you have to ask how I know this you are NOT ready to try and build one of these things)

I used a 35 PSI 'Hobbs' normally open switch wired to the rotor feed on the alternator/generator, and moved up to a 50 PSI switch after the first near catastrophe...
Plasma cutters are volume hungry, so have plenty of reserve tank for your air compressor.

A moisture filter is pretty much a requirement if you are using an on board air compressor.
Moisture does weird sh--- Uh... Stuff to the primary arc. Unpredictable stuff...
------------------
------------------

Actually, I remember you TiminMb from 3 years ago, and I appreciate the ideas/help!
You seemed to have a pretty good head on your shoulders, so please speak right up!

I started out with microwave oven parts... BUT...
As for forced step up, I am using a circuit very similar to a 'Hot Shot' or 'Stun Gun' for the pilot, and it's working well.
(I actually used a cheap stun gun in testing, but 9 volt batteries were getting expensive, and when I tried 12 volts, it let out the magic smoke...)
Easy to delay for air flow (555 or 556 timer), cheap to build, and easy to install.
555 or 556 works good for purge/cool down after the burn also! You can build the entire circuit for about $6. and you can't beat that with a stick!

As for over current...
I think you and LEVE may have hit it right on the head...
I was staring at my set up sheet, and I didn't figure my pulley speeds correctly.
I'm looking at 60 to 80 amps as near as I can tell, and this thing will wack off 3/8" plate nearly as fast as you can pull the torch. 1/2" plate takes only slightly longer, but comes out very clean... As clean as 3/16" does on my shop cutter.
I REALLY need to get an amp gauge that will work with this thing!

I just can't figure out why it's not melting plastic parts or burning the swirl ring to ashes...?
I've cooked those parts on my shop cutter, and I just can't figure out why I'm not doing it on this one.
--------------------
--------------------

[ QUOTE ]

TiminMb Wrote:
There was a guy on the diy welder list (list since MIA) that built a homemade plasma cutter by simply using a diode bridge and a water resistor to limit current through the 220 mains.


[/ QUOTE ]

Two questions & a comment...
1. What's "list since MIA"? Don't know the shorthand...
2. Where is the DIY Welder list you are talking about?

I use water resistors when I do carbon arc smelting when I cast metal parts (iron, aluminum), and they can be VERY dangerous!
I've had several near misses... Now I do it out in the yard with long cables!
------------------
------------------

[ QUOTE ]

RRich Wrote:
Get that working and you should make a fortune!

[/ QUOTE ]

This is the alternator that I tried to recommend here for an on board welder and everybody gave me crap...
The self anointed 'Electrical Engineer' (you know, 'trail ride' or whatever, the guy that didn't know how an alternator worked, and didn't know how a diode worked...) declared my ideas 'CRAP'...
... AND YOU CAN'T TEACH A PIG TO SING...
Trying only wastes your time, and annoys the pig...

So far, with the same unit I can do,
MIG (with a CO2 bottle) straight or reverse,
TIG (with a CO2 bottle) straight & reverse or AC,
Stick, both 3 phase AC and DC straight & reverse polarity,
Plasma cutter, (not quite got that perfected yet...) You will need CO2 or air compressor...
115, 220, 220 3 phase AC & DC power,
And a 160 amp continuous load, adjustable regulator alternator.
--------------------

Several of the farmers around here are using ones I made for them, and lots of the guys at the race tracks use ones I've made...
The OBW's I made in high school are still working in the fields of our family farms.

But the guys here all said my idea wouldn't work, and if they are sure it won't work, there is no point in beating my head [img]/ubbthreads/images/graemlins/wall.gif[/img] against a wall trying to educate them...[img]/ubbthreads/images/graemlins/wall.gif[/img]

I suggested two alternators, neither of which was the GM SI series they were used to seeing.

You know how those self appointed 'On Board Welding Gods' are...
Just like the self appointed 'On Board Air Compressor Gods' were when I suggested an electric compressor, or smaller compressor switches...
Or the "GM HEI in I-6 Gods" were when I suggested using the larger cap and rotor instead of scrapping the entire ignition system...
---------------------
--------------------

If anyone has any CONSTRUCTIVE ideas, comments, links or rants...

I'd SURE like to hear them!
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post #8 of (permalink) Old 09-13-2005, 10:31 PM
 
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Re: On Board Plasma Cutter Problems

There was a mail list on the internet that was called "diy welder". There was some guy named Stockley or Shockley that was involved. I guess his server was running it. My typing is often sloppy, but what I meant to say was that he and his list has gone "MIA" or as you know, missing in action. I don't know where the mail list contents reside anymore, or what happened to him and the wonderful discussions that went on, homemade welders, tigs, tesla's, etc. There was a young buck on there who was trying to perfect his transformerless plasma cutter. It used 220 volt mains, a diode bridge and a water resister (copper sulfate solution I believe). He claimed to be having great success and was trying to go on to market his plans over ebay. I felt that the lack of a transformer and isolation posed a significant risk to the user. But that's another story. He described what he believed was the best kind of plasma torch to use, and it did not require a high frequency or high voltage start. He said it had a blow open arc start, or something similar, but I have no idea what that is.

I tried finding Stockley or Schockleys name on the internet, but to no avail. I hope someone saved it somewhere. Great place to learn about things that make sparks.

In any event, I don't have a plasma cutter myself, but I understand that you select current, not voltage. That suggests to me a constant current design. Either a transformer with a lot of slope, or a regulated circuit. There should be a simple way to pass the output of the alternator through a shunt, measure the voltage across the shunt feed it into the regulator to control the field. That way when no current is flowing, the voltage across the shunt is zero, the regulator feeds the field full voltage and output voltage is high to start the arc. As soon as current flows and reaches the desired level, sufficient voltage across the shunt tells the regulator to back off input to the field. Constant current, variable voltage should result.

A 0.5 ohm shunt would result in 15 volts across it when 30 amps runs through it. Feed that 15 volts into a 12 volt regulator and you would have a starting point for regulating 30 amps into a dead short. Just an idea for using off the shelf parts.
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post #9 of (permalink) Old 09-13-2005, 10:34 PM
 
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Re: On Board Plasma Cutter Problems

Look here: I think I found it and it looks like someone hacked into his website and messed it all up. Homemade plasma projects are alive and well on this site. Search alternator plasma cutter and blow back torch.

http://forums.diywelder.com/forum/index.php
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post #10 of (permalink) Old 09-13-2005, 11:01 PM Thread Starter
 
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Re: On Board Plasma Cutter Problems

Thanks man!
I'll poke around there tomorow, bed time for the fat guy now, but I do appricate it!
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