Handy Plasmarc 125 repair

Discussion in 'General Electronics Chat' started by HarveyH42, Dec 4, 2007.

  1. HarveyH42

    Thread Starter Active Member

    Jul 22, 2007
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    My brother just dropped of hi plasma cutter for repair (why me?). And unfortunately have only a brief understanding of how they work. Basically, I did a quick study after being asked to take a look on Thanksgiving. Anyway, a little late for me to really get into it much, but took a quick look.

    Plugged it, hooked it up, turned it on. Everything seems to work as expected, except its not cutting (thin galvanized sheet metal). Definate arc in the tip, constant and consistant. The air pump is running, nice bright arc at the gap.

    Took the tip apart, parts look new and shiny. No way of knowing if they are the correct parts though (gotta know my brother).

    Anyway, from my poor understanding, you pull the trigger, touch the work, and the arc ignites the air, and 15,000 degree air is blown onto the metal, melting it. Looks like the air isn't igniting. Figure weak spark (bad connection/wire?) or air flow/pressure. *Suggestions from the manual.

    Will get into it a little more in morning, but would appreciate any words of advice, as this is my first time with this sort of equipment. Having never used one, or even seen one before this, any help would be appreciated.
     
  2. beenthere

    Retired Moderator

    Apr 20, 2004
    15,815
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    Have no idea about the equipment. But, as it's your brother, just give it back and ask "what was your problem? Works fine for me".
     
  3. HarveyH42

    Thread Starter Active Member

    Jul 22, 2007
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    Good one. Actually, this would be great for some aluminum pieces I need to cut for a project I'm working on.

    I plugged it in and gave it another try this morning, with a little different results. Getting the high frequency arc, but it doesn't seem to be of very high current. Not to sure what to expect here, but my 250kV Tesla coil put out stronger arcs. These are thin, maybe 1/4 inch between tip and work piece. A spark plug is much stronger.
    Iv'e got goo airflow. The compressor is small, so it's probably working correctly.

    Noticed the heat shield is damaged and worn around the tip, not sure if that has any influence on the plasma starting.

    Anyway, this morning there was a lot of relay switching going on, and it was only run the compressor/HF for a few seconds per press of the trigger button. Starting to think maybe the ground cable might be damaged. Going to leave the thing unplugged for an hour or so before putting the meter to work. Its fairly packed and tight inside the case, definately don't won't to bump into anything nasty (hate getting shocked).
     
  4. SgtWookie

    Expert

    Jul 17, 2007
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    I did a quick search on Google:
    http://www.google.com/search?hl=en&q=Plasma+cutter+help

    Found this article in Google's cache; the original website appears to be down:
    http://209.85.165.104/search?q=cach...m+Plasma+cutter+help&hl=en&ct=clnk&cd=1&gl=us

    One thing I'd suspect right away is the nozzle; it's a critical part. If it's damaged in any way, forget about getting the arc to light. Being so close to where the work is, it would be very easily damaged.

    And no, I don't have any experience with plasma cutting, or repair of such rigs. A little knowledge is a dangerous thing ;) Caveat Emptor!
     
  5. mrmeval

    Distinguished Member

    Jun 30, 2006
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  6. thingmaker3

    Retired Moderator

    May 16, 2005
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    I presume you've checked for good solid ground connection? Plasma-cutter won't do jack without it.
     
  7. HarveyH42

    Thread Starter Active Member

    Jul 22, 2007
    425
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    That's kind of a question mark. There is a lead that clamps to the work, it's not an earthed ground. Unsure if there also needs to be a seperate ground. The clip-on lead is attatched inside to a good size capacitor. The other side runs out to the torch.

    Should also mention the ground prong on the plug was clipped off, and looks like it's been that way for quite some time.

    From the links in previous posts, I got the impression that the arc would come down through the nozzle, so might explain some of the weak looking arc.

    The heat shield definately needs replacing. Got a hunch it was a contributing factor. It's literally crumbling a part, and it's what holds the electrode and nozzle in place.

    This has been quite educational, and think I'd like get one sometime. This one has a lot of use/abuse issues. Going to find a new heat shield to start with. Will give providing an earthed ground to the work piece a shot, hopefully its just that simple. Really is a cool tool I could use.
     
  8. SgtWookie

    Expert

    Jul 17, 2007
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    Clipping the ground prong from the plug was a very bad idea from a safety standpoint. Such a device should never be operated without a proper ground on the chassis; if there is a fault inside the "box", the outside of the box may be energized with line (or higher) voltage. I strongly recommend replacing the plug.

    Ideally, the ground circuit should never carry ANY current. In the case of 115VAC powered devices, you have a "hot" lead (Black, L1), a "neutral" lead (White, N) and a ground (Green or copper). Normally, the power flows between the L1 and N leads, and there is no current in the ground circuit. If there is an electrical fault, some power might be conducted by the ground circuit. GFCI breakers/outlets (Ground Fault Circuit Interrupter) will sense this current in the ground circuit, and "trip" or interrupt the flow of power to protect the operator of the device.

    In your plasma cutter, the power flows from the tip of the torch through the piece being cut, and then returns to the box via the clamp. Adding an "earth ground" clamp would not help the plasma cutter work better, but it might prevent you from being shocked if the box is "live".

    If the heat shield looks damaged, replace it. As long as you're replacing it, I suggest replacing the nozzle as well; if it's had as much use as it appears to have had, you can be reasonably certain that it's worn, and is not holding a proper vortex pattern.
     
  9. HarveyH42

    Thread Starter Active Member

    Jul 22, 2007
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    Yeah, I pointed out the ground on the plug first time I picked it up to plug it in. I've got a replacement plug around, just need to find it.

    Ordered a heat shield online for around $27 (including shipping). Kind of a spendy little part. The tip, electrode, and vortex baffle all look new, and basically unused. Thinking my brother replaced first when it started to fail. Brought it to me when it didn't fix the problem.

    There is an 'O' ring in the back of the torch head. It looks kind of a brownish orange. Not sure if it's just a regular type (usual black rubber), that's old and baked, or if it's normal and of some higher temperature material. It's a rust color, but everything is brass in the torch head.

    Too many things that need to be right. This is going to be one hell of a challenge.
     
  10. SgtWookie

    Expert

    Jul 17, 2007
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    Black O-rings are generally made of Buna-N or nitrile; they're limited temperature-wise from -35ºF to around 250°F. They also contain carbon, which wouldn't be good at all in a reactive plasma environment.

    I'll bet your O-ring is made from silicone; those are generally good from -85ºF to 400°F or better.

    If it's not brittle or broken, it's likely OK. The silicone O-rings are much spendier than the Buna-N (nitrile) versions. They're much more fragile than the Buna-N/nitrile rings; treat it nicely. :)
     
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