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MOT welder

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LAN_403

Platinum

Sat Nov 10 2012, 12:26AM

Registered Member #3926 Joined: Fri Jun 03 2011, 08:32PM
Location: UK.
Posts: 525

I want to make a welder so I can have as they are very useful. I have a MOT, rated at 1.6KW roughly how many turns would I need on the secondary? And will one MOT be enough power?

Harry

Sat Nov 10 2012, 12:47AM

Registered Member #4081 Joined: Wed Aug 31 2011, 06:40PM
Location: UK
Posts: 139

What kind of welder do you want? A spot or an arc?
A spot welder will need a few turns of very heavy gauge wire for maximum current and an arc welder will need somewhere around 30 - 60 turns depending on your transformer.
I think you may struggle with 1.6kW, but good luck!

GrantX

Sat Nov 10 2012, 06:38AM

Registered Member #4074 Joined: Mon Aug 29 2011, 06:58AM
Location: Australia
Posts: 335

Yes, as Harry said it all depends on what type of welder you wish to make, and also what sort of work you want to achieve with it.

1.6kW is a little low for an effective welder (my MIG uses 3.6kVA and it still struggles when using flux-cored wire on thick mild steel), but you should be able to safely draw more than that if you drop the duty cycle considerably (say, 30 seconds to 1 minute out of every 5 mins).

If you want a spot welder, then you will want an output of about of 1 - 3 Volts, and about 1 kA, for efficient and quick spot welding of thin sheet metal. That means about one to three turns of supremely thick welding cable or busbar material.

If you want an arc welder, then things will be a little more difficult. I've seen several MOT-based arc welders on Instuctables, and they all suck. A high quality arc welding transformer will output roughly 50 - 80 Vac and should be capable of sourcing at least 200A. For mild steel welding the best results will be achieved with a rectified output. You can use the straight AC output if you're brave enough to attempt welding aluminium.

At uni our large three phase arc welders are rated for 100% duty cycle at 12kVA, and they rip through 6mm mild steel plate! (we also have a pair of 15kVA spot welders, which can effectively weld two 3mm plates together in about 2 seconds :D)

Ash Small

Sat Nov 10 2012, 10:02AM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

With 1.6kVA you should easily weld 2mm steel plate with no rectification. You may even weld thicker stainless steel, but rectification would be advisable for stainless.

1.6 kVA is easily enough for an air cooled TIG welding torch, there are several designs for simple TIG welders out there, my design is one of the simplest, using an SG TC circuit, driven by a flyback and 2N3055

macona

Sat Nov 10 2012, 01:14PM

Registered Member #3272 Joined: Mon Oct 04 2010, 11:40PM
Location: Beaverton, OR
Posts: 101

I will start this by saying I was a Miller certified tech (Expired April)

A MOT wont be good for squat when it come to a welder short of a small spot welder.

I suppose you could make a micro tig machine but TIG is generally a constant current process, DC for steel and stainless. So you wont be able to do aluminum with out a HF source. You will need a way of controlling current though.

You need about 80v OCV or you will have a tough time establishing an arc.

Ash Small

Sat Nov 10 2012, 04:00PM

Registered Member #3414 Joined: Sun Nov 14 2010, 05:05PM
Location: UK
Posts: 4245

macona wrote ...

A MOT wont be good for squat when it come to a welder short of a small spot welder.

I disagree.

1.6kVA is plenty for an air cooled TIG torch. Most air cooled torches are good for around 75A. 1.6kVA should give close to that. Open circuit voltage will be higher than the voltage when the arc is struck. you don't need much voltage at all then (25V is plenty)

You may need to ballast the MOT in order to limit the current. (Welding transformers use other methods to limit current, etc.)

Platinum

Sat Nov 10 2012, 07:43PM

Registered Member #3926 Joined: Fri Jun 03 2011, 08:32PM
Location: UK.
Posts: 525

Well I've been working (apprenticeship) for a pretty big welding/fabrication company, I want to make an arc welder for practice, and because of the usefulness of an arc welder. I'm pretty good at MIG, and the welders they have there are very big indeed.

I'll have to find another MOT for more power. I'm sure arc welders have a movable winding or lamination, I'm unsure.

Harry

Sat Nov 10 2012, 09:08PM

Registered Member #4081 Joined: Wed Aug 31 2011, 06:40PM
Location: UK
Posts: 139

Sounds like fun!
I think the best way to go is to get an old arc welder, they're not too expensive (~Â£50) and then you don't have to worry about whether it's not going to work or it not being adjustable.
I saw one arc welder that used a large inductor with variable taps for the current control. I think it was on Photon's channel. Clickety click, here it is:

GrantX

Sun Nov 11 2012, 12:44AM

Registered Member #4074 Joined: Mon Aug 29 2011, 06:58AM
Location: Australia
Posts: 335

Photons arc welder transformer certainly is an example of a beefy welder. The multi-tap inductor is a good idea for variable power, and the arc welders at uni have an enormous shunt in the transformer which can be moved in and out of the core by winding a knob.

I guess if you had 2 MOTs and wound each one for 25-30V at maybe 50A, then you could use both of them in parallel for a reasonable arc welder. You could improve on the design by using a variac/variable ballast of some kind to control the output (thus you could wind it down for welding very thin sheet metal or something). Perhaps even add a diode bridge to the output to get a bit more efficiency when welding steel.

Although, at this stage it might be cheaper just to buy an arc welder, as Harry stated. I've seen some massive second hand ones around for roughly $100 (to put that in perspective I just spent $230 on 4 600V 150A diodes, which would be a perfect bridge for a homemade arc welder). A few local hardware stores sell some cheap iron core arc welders, the one I was looking at buying for use as a DC power supply could put out 180A and was only $80 brand new and shiny.

macona

Sun Nov 11 2012, 03:44AM

Registered Member #3272 Joined: Mon Oct 04 2010, 11:40PM
Location: Beaverton, OR
Posts: 101

Air cooled torches are available up to 300amps, but that is not the issue. You will get about 20-25 amps of useful current out of a MOT with it's secondary wound correctly. That is good for a .020 tungsten, maybe a fine tipped .040 tungsten.

35 to 30v is OK for a mig machine, not an arc welder though. Too low of OCV. 50 amps might run .023 wire. Might...

Watch the duty cycle, probably about 10% max with a MOT.

Most people that have built them end up needing about 4 transformers. There was a site years ago where a guy built one with eight. He used some large SCRs set up as triacs on the primary for current control.

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