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A DIY Inverters - what's there to these things?

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  • A DIY Inverters - what's there to these things?

    I picked myself up off my ass after checking out the price of TIG inverters and started to wonder, what the heck is there to these things that makes them so outrageously expensive.....and since we make just about everything, why not one of these?

    Basically could you go ----> transformer, rectifier, couple of big ass caps then silicon control relay(s) or banks of heat sunk power transistors controlled by a pic? you could even get cute and program in some of the pulsing and what not....I'm enough of a pic beginner that i wouldn't call anything easy, but it sure is cheap.

    So what am i missing that makes these things worth 5k without the bottle and that's not even with a water cooled torch "double eek". ....Of course I'd buy torch kit, regulator etc...I'm more thinking the controller & power supply should be fairly easy...or not?

    come on, 5k for this?

    http://www.homedepot.ca/webapp/wcs/s...k=P_PartNumber


    someone call the Sheriff!
    Last edited by Mcgyver; 04-19-2011, 06:49 PM.
    .

  • #2
    I think you pay $3000 for the sticker and maybe $50 extra for better components.
    However you could buy a Everlast 250 tig/stick/plasma for about $1500 and you'd be hard pressed to make it yourself for that money. Even assuming you're doing it first time right and without the hundreds of dollars in burned components figuring out what works best......

    Igor
    Last edited by ikdor; 04-19-2011, 07:07 PM.

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    • #3
      There is a guy that hangs out on the rec.crafts.metalworking usenet group that did just that a few years back. It worked out quite well, as I recall, but was not a trivial project. He is not an EE or anything like that, but is a very bright, clever, and persistent guy. Not sure if he would say that it saved him an bunch of money but he sure seemed to learn lots.

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      • #4
        There is a lot of design time into building an inverter welder, which in reality is a switching power supply. Its kind of a black magic getting these things to work right. Early inverter welders were known as smoke stacks.

        Good machines like the Millers use IGBTs to do the switching. Cheap machines like the chinese ones use pass banks of small mosfets. To keep the size of the machines down they use input power modules to do the rectification, inrush limiting, and main switching all on one module.

        And adding AC output is another hurdle. You need to invert the DC into AC. This requires another set of IGBTs. These things are not cheap. On ones and twos these modules are around $300 to $400 bucks a pop.

        The home depot price for that machine is way high. Especially with the strong canadian dollar. A Miller Dynasty 200DX is about $3k US. And this thing will run on anything from 120v single phase to 480v 3 phase. Try doing that with a cheap chinese unit.

        Edit:

        So how do one of these things work. AC comes in, is rectified and filtered. At this point, depending on the machine, the bus voltage is modified through sense circuity. The smaller Dynastys use a buck/boost converter to raise the low line voltage to the operating voltage in the machine, about 600v. The bigger machines reconfigure the capacitors through contactors to create a voltage doubler when running on 240v and then pass straight through when on 480v. This filter bus voltage is then passed through a H-bridge of IGBTs (Isolated Gate BiPolar Transistors) where the DC is inverter to high frequency AC in the 40khz range. This allows you to use a much smaller transformer to do all the work. The higher the frequency the more efficient the transformer is and the smaller it can be. (This is why 400Hz AC is used in airplanes, to make the radios smaller and lighter). The voltage is stepped down and is then rectified through a bridge rectifier. The resulting DC is fed through a hall effect current sensor for current feedback. Thats the end of the line on a DC only machine. On an AC output inverter the output from the final bridge rectifier is passed through another H-Bridge of IGBTs and inverted to create the AC output. The ones in a Dynasty 300 are 400 AMP, 1200v modules. Also a high voltage arc starter and coupling coil is added for touchless ignition.
        Last edited by macona; 04-19-2011, 08:35 PM.

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        • #5
          I see the Everlast unit mentioned is TIG stick, and a plasma cutter combined. When ever I have enquired about this sort of combination, I've been told they aren't a good idea. Miller and Lincoln multui process welders only offer a plasma cutter as an add on feature, which really just means you have to buy a plasma cutter as well. So why wouldn't a combination that includes plasma be a good idea.

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          • #6
            Originally posted by Robin R
            I see the Everlast unit mentioned is TIG stick, and a plasma cutter combined. When ever I have enquired about this sort of combination, I've been told they aren't a good idea. Miller and Lincoln multui process welders only offer a plasma cutter as an add on feature, which really just means you have to buy a plasma cutter as well. So why wouldn't a combination that includes plasma be a good idea.
            All your eggs in one basket....

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            • #7
              The impresion I got was that it was a problem with the function, rather than a reliability issue.

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              • #8
                Originally posted by Robin R
                I see the Everlast unit mentioned is TIG stick, and a plasma cutter combined. When ever I have enquired about this sort of combination, I've been told they aren't a good idea. Miller and Lincoln multui process welders only offer a plasma cutter as an add on feature, which really just means you have to buy a plasma cutter as well. So why wouldn't a combination that includes plasma be a good idea.

                Powcon made a tig/stick/mig/plasma combo. I think it was call a starcut. Very rare. Mostly because they had a tendency to blow out their front end electronics.

                Powcon also made a unit that connected to you welder (I think 300 amp minimum) and gave you plasma cutter functionality. I have only seen a couple of them. They weighed about 80 lbs.

                Part of the problem is the voltage. A typical stick/tig machine has an open circuit voltage around 70-80v and the arc voltage under load drops to somewhere around 10-20v. A plasma cutter uses an OCV of about 200v and 100v under load. Huge difference. The powcon unit inverted the input DC to high freq AC (Possibly pulsed DC) and ran it through a transformer and re-rectified it. It also had all the logic to control the torch and HF for starting.

                Kind of like the all-in-one machines. It does everything poorly.

                So, buy yourself a Miller welder and a Hypertherm Plasma and you will be happy.

                For those who dont know I used to be a service tech for airgas working on welding equipment, plasma cutters, and cnc shape cutters of all makes and models from 20 amp plasmas up to 1000 amp welders. I am still a certified Miller tech, at least till next year.

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                • #9
                  That makes sense, it's basically a plasma cutter shoe-horned in with the welder.

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                  • #10
                    Jerry, I'm surprised the voltages are so high? At the arc its more like 30 volts isnt' it.....so why transform it up only to have to lower it again? couldn't one use solid state relays, basically FETS aren't they (?) instead of IGBT's, to switch?

                    i probably won't make one, more just trying understand why they seem to be priced so high.

                    I like the idea of getting a plasma cutter as well, but there's lots of mixed reviews on the Everlast. I've also read that inverters are less robust than more old school boxes. and here they're like $1925 and a boatload of canuckstan taxes f(or the powerpro256) ....not chump change, or at least it makes me a bit nervous shelling out that much for something iffy. Iffy in the sense of mixed reviews. just missed out on a used 200a Lincoln inverter...a use Miller or Lincoln might be the way to go.
                    .

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                    • #11
                      The bare minimum:
                      AC line -> rectifier -> H-bridge -> transformer -> clamps

                      The H-bridge runs north of several kilohertz to reduce both the copper and iron needed in the transformer. A feedback circuit controlling the H-bridge determines output current.

                      Many machines have a PFC circuit in between the rectifier and H-bridge. It can help with ripple under heavy load and if done right reduce the size of any capacitors needed to reduce ripple feeding the H-bridge.

                      To get DC output 2 diodes are used with a center-tapped transformer. The CT is ground and the diodes run the other two legs. They have to keep up with the frequency of the H-bridge, handle a few hundred amps, and survive any transient fun that occurs.

                      To build an AC/DC box, a 0-200 (or 500) Hz high current H-bridge goes on the rectifier after the transformer.

                      Arc-start is basically another transformer, oscillator, and some brains on the output. Capacitive start is another way to do it without being as brutal to other nearby electronics.

                      Machines with a plasma cutter us different taps on the first transformer to put out about 100V at whatever amperage is available.

                      Toss in a solenoid to control gas flow, a simple display, and a half-dozen switches and knobs and you've got a machine that can do anything.

                      Just to recap for AC/DC tig/stick:
                      AC line -> rectifier/PFC -> H-bridge -> transformer -> rectifier -> H-bridge -> arc start -> torch/clamp

                      And plasma:
                      AC line -> rectifier/PFC -> H-bridge -> transformer -> rectifier -> arc start -> torch

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                      • #12
                        Originally posted by Mcgyver
                        Jerry, I'm surprised the voltages are so high? At the arc its more like 30 volts isnt' it.....so why transform it up only to have to lower it again? couldn't one use solid state relays, basically FETS aren't they (?) instead of IGBT's, to switch?
                        A welding power supply is a controlled current source and the voltage has to float to keep current constant as resistance changes during welding. The IGBTs have a little bit time with the transient spikes.

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                        • #13
                          Originally posted by macona
                          Part of the problem is the voltage. A typical stick/tig machine has an open circuit voltage around 70-80v and the arc voltage under load drops to somewhere around 10-20v. A plasma cutter uses an OCV of about 200v and 100v under load. Huge difference. The powcon unit inverted the input DC to high freq AC (Possibly pulsed DC) and ran it through a transformer and re-rectified it. It also had all the logic to control the torch and HF for starting.
                          .
                          Yup,the little Miller spectrum plasma at work lists the OCV as 380 vdc
                          I just need one more tool,just one!

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                          • #14
                            Has anyone seen this guy's DIY tig welder?

                            http://www3.telus.net/public/a5a26316/TIG_Welder.html

                            Uses a cheap stick-welder as the power source.

                            It looks pretty good and a quick glance over the plans tells me the guy knows electronics although i've not studied them in detail...

                            Cheers
                            Batt

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                            • #15
                              This is another one i found some time ago.

                              http://myweb.cableone.net/rschell/TIG.htm

                              Again i've not looked at the plans in detail...

                              Cheers
                              Batt

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