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29th Oct 2017, 06:43 PM #1Most Valued Member
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Convert my Goodwell AC welder to DC?
Hi all,
I have 3 welders, 2 transformer AC welders and a gasless Mig welder. Each welder has it's pro's and cons and I pick which to use depending on the job but the Mig and the smaller AC arc welders are cheap POS and contantly find myself turning to the old faithful Goodwell AC arc welder. It just keeps going.
The Goodwell does have it's limitations though. I can't for the life of me use low hydrogen electrodes on it. It just will not maintain an arc. I have tried the same electrodes on a smaller DC inverter welder and they work nicely so I assume it's the AC welder. I have read on several sites that people have converted their AC welders to DC using very large (200A - 300A) bridge rectifiers. My question here is, how effective would such a conversion be? I can purchase a 400A rectifier on ebay for $40
https://www.ebay.com.au/itm/New-MDQ-...gAAOSwvfZZ8Zkq
Is it that simple? Will I see and improvement in my welder?
SimonGirl, I don't wanna know about your mild-mannered alter ego or anything like that." I mean, you tell me you're, uh, super-mega-ultra-lightning babe? That's all right with me. I'm good. I'm good.
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29th Oct 2017, 07:42 PM #2Most Valued Member
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G'day Simon,
I don't know how welders usually derive their DC output, so I can't say if bridge rectifying your transformer's output will work for DC welding (i.e. I don't know what shape the DC waveform of a welder looks like). What I can say is that you will need a VERY large heatsink to keep the BR below its rated 150C max temp.
The power dissipated by the BR equals its forward voltage drop multiplied by the current through it. The eBay add says "Forward pressure drop is small" which is completely useless info, but it should be around 1.5V. So welding at 200A will result in the BR dissipating 300W. That's a lot of power. A large, preferably fan cooled heatsink will be required.
Also, I suspect the screws on that BR will be too small to adequately secure the lugs on your welding cables.Chris
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29th Oct 2017, 08:11 PM #3Most Valued Member
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Hi Jack,
Yes, I'm not sure if DC welders are in fact DC or just rectified AC (as in what I am looking at). as for the heat dissipation, I can't see it being too big a problem with the correct heat sinking and fan cooling as with other welders. The rectifier would be housed inside the cabinet and I would use some other form of connection such as copper straps etc. since attempting to attach welding cables to it would be very clumsy.
SimonGirl, I don't wanna know about your mild-mannered alter ego or anything like that." I mean, you tell me you're, uh, super-mega-ultra-lightning babe? That's all right with me. I'm good. I'm good.
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29th Oct 2017, 08:51 PM #4Most Valued Member
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This might give you some ideas. It uses 24 discrete diodes bolted to copper plates with fan cooling:
DC Cheater - HomeChris
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29th Oct 2017, 09:01 PM #5Diamond Member
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If you want to do it out of interest, then go for it - I'd imagine you'd learn a heap along the way.
If you just want to be able to run low hydrogen rods, get a lunchbox DC inverter. They're cheap, they do clever things with current regulation that makes welding easier, they're much more efficient, they weigh bugger-all, you can run them off an extension lead - so many upsides compared to an AC buzz box.
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29th Oct 2017, 09:51 PM #6
Hi Simon. Does your Goodwell have a "special electrode" output? Many of these old welders did. Mine does. Try that for low hydrogen rods.
The issue is the open circuit voltage. You need at least around 60V. Also, LH rods are designed for either DC or AC. Maybe you have the 'wrong' kind....
I have built a rectifier for my transformer welder decades ago. Wasn't very successful. You need both inductance and large capacitors to smooth out the resulting 100Hz half sine waves..... Without that, the arc will still extingish in between pulses. Quite different from a rectified 3 phase transformer welder, where the resulting pulses overlap and the arc won't extingish!
Just my 2cents worth.
Oh, before I forget, I recently tried 2 modern DC welders without success with LH rods. Only my current one has sufficiently high open circuit voltage. So look out for that too....Cheers, Joe
retired - less energy, more time to contemplate projects and more shed time....
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30th Oct 2017, 07:02 PM #7Most Valued Member
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Thanks Rusty, Jack and Joe.
Joe, my big welder has a "special" terminal which is 80V. It's supposed to provide a better start voltage for LH electrodes etc. I tried it and found it did not help at all.
Here's another question for the brains trust. Is welding at 120A (for example) with a DC inverter welder the same as welding with 120A with an AC welder? The reason I ask is that I often find that I need to use a bit more current (with my AC welder) than is often recommended in order to achieve the same weld flow and penetration.
SimonGirl, I don't wanna know about your mild-mannered alter ego or anything like that." I mean, you tell me you're, uh, super-mega-ultra-lightning babe? That's all right with me. I'm good. I'm good.
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30th Oct 2017, 07:28 PM #8Most Valued Member
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Here's an example of a recent weld job. I had reason to but weld two 22mm (7/8") plate tegother using a multipass full thickness weld. Now I'm not a professional welder so time will tell if it does what I want it to but I was happy with the way the end result looked. Others here will no doubt point out my faults which I will be happy to hear. However what I learnt was that I had the welder set to 160A using a 3.2mm rods. The max welding current according to the electrode maker was 140A. They are RD460 E6013 made by Nikko weld. They are approved by Lyods shipping and the US bureau of whatever. I tried 140A and lower but I just did not get the penetration and often resulted in inclusions. I also pre-heated the join to a temp that was hot to the touch and let the plate cool naturally after a pass on each side to a "reasonable" temp.
Any pointers or observations...
20171020_092614.jpg20171020_094414.jpg20171020_110306.jpg20171020_123217.jpg20171020_134420.jpg
Cheers,
SimonGirl, I don't wanna know about your mild-mannered alter ego or anything like that." I mean, you tell me you're, uh, super-mega-ultra-lightning babe? That's all right with me. I'm good. I'm good.
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30th Oct 2017, 08:30 PM #9Philomath in training
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Those don't look too bad Simon. Once upon a time I read on the side of the electrode box that the electrodes should be used between a certain amp range, so decided to try to weld at the low end of the amp range. It did not work, and when I talked to a qualified welder about that, he explained that I should be welding for the end result rather than to a number on the packet. I'm happy to defer to a more experienced welder but I would say that the results you have there are quite good and if that amp range works for you then that is fine.
Michael
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30th Oct 2017, 11:02 PM #10Most Valued Member
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With regards to your welds, they look excellent.
When you ran a root run, did you get full penetration on the underside? As a general rule, I used to use the next size rod down at the same current for the runs, to burn through to get maximum penetration on the underside. Then using a cutting disc grind out the slag if it was done by stick, before welding over it.
When grinding/cleaning up the surface of the welds, run across the welds not along the welds, as it creates minute scratches which under load/stress could start to fracture. Most important on structural work. Plus, it's easy to grind dips and hollows in following along the welds. I always had problems with staff grinding along the welds instead of across.
KrynTo grow old is mandatory, growing up is optional.
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31st Oct 2017, 07:13 AM #11Most Valued Member
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Hi Michael & Kryn,
Thanks so much for the positive feedback. Being a self taught hack welder, I have gone through quite a few years of frustration and learning through experience but I never really knew if my welds were OK or not.
Michael, what you say makes sense to me. I was more concerned that my extra current setting was a consequence of poor welding technique and that I had to inadvertantly increase the current to overcome this.
Hi Kryn,
WRT penetration, I'm not sure if it's obvious but I ground a V on both sides of the plate and when I butted the plates together, I left a very small gap to enable full weld penetration. I have used this technique once before with very average results but I used 2.5mm electrodes and struggled to lay a good root weld in the bottom of the V. I had big issues with slag inclusions and ended up doing the root pass with mig and then finishing with arc. Probably not the correct thing to do but those parts were not going to be stressed anywhere near as much as this part. Changing from 2.5mm to 3.1mm also made a big difference.
WRT grinding back the welds, yes I know exactly what you mean. It's very difficult to grind them back along the welds and not introduce scallop type hollows. I think it should be OK on this occasion as the weld will be in shear stress. This part will be the knife of a log splitter.
Thanks guys,
SimonGirl, I don't wanna know about your mild-mannered alter ego or anything like that." I mean, you tell me you're, uh, super-mega-ultra-lightning babe? That's all right with me. I'm good. I'm good.
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31st Oct 2017, 01:17 PM #12Diamond Member
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I was also going to note that if you were going to lunch on that joint, a couple of millimetres of root gap wouldn't have gone astray, but other than that, it looks pretty damn good.
In the absence of a specific weld procedure, I can't see a problem with running a root pass with MIG - it's a low-hydrogen process, so provided wind isn't a problem, it should make for just as strong a weld as stick, and it should be tougher than a 6013 or other non-LH rod.
As for whether DC runs hotter than AC for the same number of amps, I've heard anecdotal claims that it's true, along with semi-plausible explanations as to why that might be.
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31st Oct 2017, 07:58 PM #13Philomath in training
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One thing to think about Simon is that if you want complete penetration then do your V prep on one side, put down the root run and then grind the prep on the other side so that you grind out any slag inclusions and get to clean metal. Not such a problem with MIG but if using stick I always find that some slag gets to the underside of the weld.
I did manage to bring my lunchbox welder with me in case I wanted to whip up a stand or something. If you want to try it out for a week or two it is light enough to post across.
Michael
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1st Nov 2017, 10:01 PM #14Most Valued Member
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Thanks for the generous offer Michael. Ill keep if in mind. Yet to decide if i will look further into a DC welder.
Cheers.
Simon
Sent from my SM-G900I using TapatalkGirl, I don't wanna know about your mild-mannered alter ego or anything like that." I mean, you tell me you're, uh, super-mega-ultra-lightning babe? That's all right with me. I'm good. I'm good.
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4th Nov 2017, 11:59 AM #15Most Valued Member
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I would support what others have said regarding amperage. Weld for effect, not a number. After all, you don't know how accurate your amperage adjustment is anyway on most welders, then throw in losses due to poor contact/long leads. On many critical jobs, the supervisor will verify that current is within spec using a clamp meter.
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