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  1. #1
    Join Date
    Aug 2014
    Posts
    4

    Default Hobart Plasma Cutter Problem main arc extinguishes

    Hi All,

    I have an old (late 80's?) Hobart Smooth Cut 60A plasma cutter. I do not have the original torch, but installed a thermal dynamics PCH-51 55A manual torch.

    I have used the maching like this for about 10 years and mostly always on 1/4" and thicker materials at 40 amps. (it has a three postion toggle switch for 25 amps, 40 ams, and 60 amps). It works great for this.

    Recently I have been doing work on 0.050" thick sheet metal and installed an 0.035 orifice nozzle and turned down to 25 amps.

    The problem is that the main arc extinguishes alot, resulting in the pilot arc starting then the main arc starting then extinguish and cycle repeat. I thought the reason was that on thin metal I need to move faster and was simply going slow enough that the arc ran out of metal to cut and go out.

    However I did some testing on 1/4" thick material at 25 amps and at 40 amps last night. At 40 amps, the arc is very stable and system runs fine. at 25 amps even on 1/4" thick, it is very unstable and difficult to get the speed right so it stays lit..

    I put an analogue amp gauge in series with the ground and while cutting stable, It read steady at 25 amps and 40 amps respectively.

    I checked the machine for loose connections, checked the spark gap, blew the dust out, checked the ground, etc... no improvement.

    Anyone have thoughts as to why the lower amp setting would be vey sensitive to cutting speed and main arc unstable?

    Air pressure is right at 70 psi, big air compressor and a motorgaurd water filter.. I did try lower air pressure down to 50psi, that did not help..

    thanks

  2. #2
    Join Date
    Oct 2004
    Location
    Edmonton, Alberta
    Posts
    7,905

    Default

    I would have gone with a SL60 T-dyne torch myself, that 51 is still quite old, . The tips have an irridium core, if that is burned, and holed, you have water in your system, The motorguard canister needs to be changed out constantly. Like the toilet paper filter, they are both perberbial crap. An over sized cheap hurricane filter is much better. merely use the manual version, and crack the drain open slightly.

    Dropping your air pressure definately won't work well, increasing it past spec won't work well either as the air will not ionize fast enough.

  3. #3
    Join Date
    Aug 2014
    Posts
    4

    Default

    Thanks Criuzer, I dont believe I have a water problem. Although I dont have a refrigerated dryer, I have a large compressor tank and drain drop leg set up. I use the compressor for painting and running a flame spray set up and don't have water problems in those systems. Flame spray is sensitive to water..

    Even changing consumables does not help. I was thinking that the torch may not be matched well to the plasma cutter.

    My original thought was that the power supply was unstable at low range. But my testing showed that the current was quite steady (until the arc went out).

    Do I need a smaller orifice in the nozzle for lower amps? Would that help maintain the arc? ( I am currently running an 0.035" dia). When I run at 40 amps, I run an 0.043"

    Maybe the theory is smaller orifice, the lower amps can ionize the air better creating a lower resistance connection to the work?

  4. #4
    Join Date
    Oct 2004
    Location
    Edmonton, Alberta
    Posts
    7,905

    Default

    Is that a sulzer Metco sprayer, Kinda cool, I've done warranty and fixed those as well. As for the tip, maybe, most tend to run the same tip for pretty much everything, Few switch tips. Could even be a diffuser issue. I prefer the SL60 . Nothing much to go wrong, except for the start cartridge, which only tends to stick if there is moisture in the lines.
    I find the guys with the spin on canisters, don't change them out as often as they should. Air pressures are set with the air running through the torch, so while you might have 70psi at the gage, as soon as you start it, the air pressure drops off, especially if the canister is clogged.. Might want to check that.

  5. #5
    Join Date
    Aug 2014
    Posts
    4

    Default

    Yeah, Metco 10E. I spray Aluminum and Zinc for corrosion protection. I do set the air pressure with it flowing.

    What is the starter cartridge in the SL60? What does it actually do?

    thanks

  6. #6
    Join Date
    Oct 2004
    Location
    Edmonton, Alberta
    Posts
    7,905

    Default

    Hit the trigger, air starts flowing, drops the elecrode back from the tip via the start cartidge, makes a spark between the nozzle and tip, pilot comes on, dc comes on. On yours the hf would probably start the pilot. They have kits to fit everything. I really like it

  7. #7
    Join Date
    Aug 2014
    Posts
    4

    Default

    Thanks. So it sounds like air pressure activatives the electrode movement? Is the arc still started with high voltage, high freq? or is it contact between the electrode and nozzle? Does the electrode stay in the start position during cut? Or does it move back for starting and after start, returns to the previous position. Yes mine has no moving parts, uses HV, HF to start the pilot arc, then controls turn off pilot arc once current is detected in the main arc..

    So if I installed the SL60, I would no longer need the HV, HF?

  8. #8
    Join Date
    Oct 2004
    Location
    Edmonton, Alberta
    Posts
    7,905

    Default

    Few if any plasmas use hf any more, though yours not having a low DC to start the arc would still use HF to start it even with a SL-60, would start significantly faster though

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