Plasma cutting sequence

Could anyone say what is the proper plasma cutting sequence? How about this:
when trigger is pulled:
- open air valve for a certain amount of time (like 0.25 sec)
- Then start current
* When arc voltage is above certain value, turn on HF, turn off HF if voltage drops below that
When trigger is released:
- stop power - keep air valve open for a period of time, like 5s
does it make sense?
I am programming my microcontroller, which I mostly did for stick and tig (proper relays are closed when they should be), but I have no idea what to put in for plasma.
thanks
i
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Ignoramus27153 wrote:

I've never measured what our machine (Hypertherm) does, but that certainly sounds like what the sequence is.
I think the post cut air purge is 8 seconds on ours... Probably just enough time to cool the torch components. Likely not a critical value.
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wrote:

Thank you.
It's OK, the numbers could be fudged with, I just need to get the sequence right. I already programmed this particular sequence (with delays and voltage thresholds as parameters)

Sounds good, I will change my value from 5 to 10 then.
A ignorant question, do plasma cutters need only air? (no second gas going through another line)?
i
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The purpose of the post cut purge is to cool the torch components significantly. Since in a plasma torch the arc is contained within the nozzle if you shut off the air, you will have a puddle of copper electrode in the tip. I think mine depends on the time the torch has been on, but I have heard it go on for as long as 30 seconds. Makes sense, the hotter the torch, the longer it should run.
Plasma cutters need only a compressed gas to plow the hot metal out of the cut. Obviously the most economical is clean, DRY compressed air but professional plasma cutting often uses dry nitrogen instead (no oxidation when cutting Aluminum or Stainless). I've never heard of anyone plasma cutting with shielding gasses (argon,etc).

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Got it. Will increase the limit. Thanks.

Thanks, that's nice. I do have a compressed air dryer, actually.

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Ignoramus28229 wrote:

Suggest you check out the plasmacutting group on Yahoo groups. It has a couple techs from plasma cutter manufacturers as regulars that could give you a lot of info.
Pete C.
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Thanks. I am also a member of homemade welders yahoo group, but it is pretty quiet. I joined the plasmacutting group and already asked my question.
Thank you!
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Are you using the same welding machine you talk about here for plasma cutting? If so, what did it take to make the change?
Thank you.
Best Regards
Steve Sousa
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Yes, the same machine. Its transformer has six secondaries, two per each leg of three phase. If I connect these secondaries in parallel (two on each leg would be paralleled), delta connected, I get 66 VAC.That's for welding.
If I connect these secondaries in series (that is, two in series on each leg), Wye connected, I get 220VAC on each leg. That's what I would use for plasma cutting.
I do this reconnection using contactors, three of them connect for low voltage, and two for high voltage.
This partis already working (contactors are mounted and doing all that reconnection).
i
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I hope the insulation resistance on those secondaries is high enough for the higher voltage series connection. 220v isn't very high, but the inter-winding insulation might not be designed to take that under continuous duty, since the transformer was designed for 66V operation.
LLoyd
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wrote:

Remember that voltage from one turn of a winding to the next turn is pretty small (and does not change when I reconnect the transformer). These windings seem to be fairly far apart.
See a picture here:
http://igor.chudov.com/projects/Welding/11-New-Rectifier/welding-xfmr.jpg
i
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I wasn't refering to the inter-turn insulation; inter-winding. If the windings are separated from one-another and any other point that might be called "common" by enough insulation, then it'll work. I wasn't nay-saying, just raising an issue.
My first real job was in the test lab of Florida Transitron in DeLeon Spring Florida. My job was to "hi-pot" all the prototypes, and to set up burn-in rigs for temperature-rise and core-loss determinations.
LLoyd
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Lloyd E. Sponenburgh wrote:

Just from the photo, this looks like it could take the few extra volts. The isolation has do deal with full primary/ground voltage and therefore should be fairly well built to get by the safety standards. Id just give it a good cleaning and im rather sure it wont fail.
--
Robert

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On Fri, 02 Jun 2006 04:05:25 GMT, Ignoramus27153

I know you've had pretty good luck so far on your conversions and I commend you on what you've achieved so far. However my experience tells me that what you're about to try is going to be fraught with disappointments.
Plasma cutting is much more complex than most people think. But I'll try to list the basics real quick (I really don't have time to get into details at the moment, we are having to take turns watching my wife at the nursing home at the moment).
To start with you have the air coming on, the high frequency engages and initiates the pilot arc (this is something you've not taken into account). The pilot arc is a low amperage arc from the electrode to the nozzle but it's fed from the main cutting arc (this is where I had trouble fitting the new torch to my old PCM-100 plasma cutter). If the pilot arc is to hot then it'll fry the consumables, to low and it's not long enough to initiate the main arc. The main arc is initiated when the pilot arc crosses to the work piece. This is the main cutting arc.
Things to consider are to much amperage or voltage, to sudden a shut off, to sudden a ramp up, and many other little details play hell with the consumables on a plasma cutter. This is what makes the more modern machines so much better than the old ones. They have better control of the whole process which is what makes the cuts cleaner, the consumables last longer, and the overall process much nicer.
Good luck.
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Thanks Wayne. Yes, I have to consider the low amperage pilot arc indeed. I will try to rewrite my procedure to account for it. In any case, I think, it makes sense to get welding right (stick welding is already working, but for TIG I would use the controller). So I have quite a while before I can try plasma. As for modern plasma cutters, mine is controlled by a microcontroller, so I can program it to do anything I want (knowing what to do is a problem). I wish that I could have a local friend with a plasma cutter to see how they work.
I will post more on this.
i
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wrote:
[snip]

[snip]
My sympathies. Just did that with a son... it's really tough.
...Jim Thompson
--
| James E.Thompson, P.E. | mens |
| Analog Innovations, Inc. | et |
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On Fri, 02 Jun 2006 14:14:19 -0700, Jim Thompson

Thanks.
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Wayne Cook wrote:

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wrote:

Thanks to everyone for the support.
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Oh, and I must say that I am sorry about that.
i
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