Spot Welder from Three-Phase Transformer?

Discussion in 'The Projects Forum' started by benha, Feb 5, 2016.

  1. benha

    Thread Starter Active Member

    Jan 4, 2011
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    Hi, folks.

    I'd like to build a spot welder - ideally a bit beefier than the typical Internet DIY versions built from a MOT. Looking for something in the 15-25KVA range. Given most transformer ratings are for duty cycles WAY in excess of what a home-shop spot welder is going to see I figure I can get away with more of a 5-7KVA transformer.

    My plan has been to source a suitably large transformer and replace one side's windings with a few beefy wraps of welding cable. (Or perhaps Litz wire.) However - somewhat serendipitously - a 7.5KVA transformer just dropped in my lap.

    The problem is that it's a 3Ph model rated to take 480V down to 230V.

    Anyone have thoughts on how I would go about building a spot welder from this thing?

    I have 3Ph power in the shop so I can power the line side. I just haven't a clue how to deal with the mods or the load side. Looks like most commercial spot welders run from 3PH are inverter DC models. Not sure if there's a good way to do a traditional "hack the transformer" version of the home shop spot welder from 3PH transformer......

    Thanks!

    -Ben
     
  2. tcmtech

    Well-Known Member

    Nov 4, 2013
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    What do you need to spot weld that would require 15 - 25 KVA of transformer capacity?

    Inless its long runs of continuous seem type sheet metal welding at a high rates of speed regular welding techniques are a way better method of joining two metals.
     
  3. benha

    Thread Starter Active Member

    Jan 4, 2011
    35
    1
    I have a tig machine I use for that kind of stuff. But if you're tacking together 3/16" steel bar stock and don't want to wait too long between welds you need a bigger spot welder.

    Industrial spot welder are often north of 100KVA so this isn't a crazy proposition.
     
  4. MaxHeadRoom

    Expert

    Jul 18, 2013
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    Use one section of the transformer but you will need a very large cross section secondary of 1 to 2 turns, the electrodes will have to be pneumatically operated to bring the electrodes together before firing.
    The secondary current would be upwards of 1kamps.
    Max.
     
  5. shortbus

    AAC Fanatic!

    Sep 30, 2009
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    Why do you need to wait between tacks using the Tig? If your thinking the spotwelder won't give as much heat and warpage as the Tig spots, think your wrong. The heat comes from the amperage put into the weld, the steel doesn't know what type of machine is doing it. I use Mig myself for tacks.

    And like Max said, you need some real force to get the weld nugget to form with that much amperage. Been around welding/fabricating for over 55 years myself.
     
  6. tcmtech

    Well-Known Member

    Nov 4, 2013
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    Yep that doesn't make sense to me either. I have been doing welding for years and a spot welder is a poor choice when tacking much of anything together other than sheet metal that is too thin to weld with more conventional methods.

    As for spot welding capacity I have a old Miller 2.5 KVA portable spot welder that I occasionally use that is rated to do something like 4000 - 5000 amps with the set of tongs and electrodes I have on it and it has no problem tacking anything up to 1/4" combined thickness together but at that thickness it is not anywhere near as fast or easy to use as my mig gun with standard .035" wire.

    To be honest the huge commercial spot welders that can do that sort of work have outputs of 15,000 - 20,000+ amps at 5 - 6 volts AC which takes massive cabling and buss bars to carry that current with any degree of efficiency and are not something a guy can just throw together with any old cable and a salvaged multi KVA transformer.
     
    shortbus likes this.
  7. MaxHeadRoom

    Expert

    Jul 18, 2013
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    The type of resistance welder for continuous welding is a seam welder, this is identical to a spot welder but has copper wheels instead of pointed electrodes, the wheels clamp the seam and rotate in order to traverse the seam through, the current is fired in the same way but pulsed, with the pulses overlapping creating a liquid proof seam/seal.
    Max.
     
  8. MaxHeadRoom

    Expert

    Jul 18, 2013
    10,554
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    Like so.

    upload_2016-2-6_11-31-42.jpeg


    Max.
     
  9. Kermit2

    AAC Fanatic!

    Feb 5, 2010
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    We actually use a spot weld/resistance welder unit to etch serial numbers in hardened and ground gages. Hard copper alloy terminal shaped like an oversize pencil and copper plate to lay the gage on. Etching instruments just scratch the surface. But the welder leaves a very visible "scar" on the hardened metal.
     
  10. MaxHeadRoom

    Expert

    Jul 18, 2013
    10,554
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    One other use for resistance welding is Projection welding, this is a nut or pipe fitting etc that has 3 or 4 nodules around the edge and it is sandwiched between two copper dies of the similar shape and then when under pressure the weld takes place on all in one shot.
    Max.
     
  11. benha

    Thread Starter Active Member

    Jan 4, 2011
    35
    1
    Ok. Interesting thread. Still looking for wiring guidance on the feasibility of using a 3ph transformer to construct a spot welder, ideally without just using one of the phases since that makes for a rather big and heavy unit relative to its output.
     
  12. tcmtech

    Well-Known Member

    Nov 4, 2013
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    The most effective way to use a three phase transformer on single phase is to tie the primaries of the two outermost coil assemblies in parallel and then wind your new secondaries on any one of all three of the core sections.

    The reason being you typically have to leave the center primary winding out of the system is because if its powered up it will usually drive the transformer core into saturation which makes everything overheat even without a load on it although some three phase transformers can run with all three winding sets powered by single phase with the center set running reverse of the other two but it rare that it will work with most unless they are working at reduced input voltages like say running a 277 primary on 240 volts or less.
     
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