Author Topic: Microwave owen transformer spot welder?  (Read 16002 times)

Offline PekkaNF

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Microwave owen transformer spot welder?
« on: November 17, 2015, 08:13:49 AM »
Ten years ago I read about MOT spot welders and confiscated one MOT from korean owen that was scraped.

Fast forward 10 years: I found that MOT when I was cleaning up garage and desided to find out if it works.

Has any of you build MOT spot welder? How much power you get out of it and what V/A?

I removed secondary and noticed that there are shunts between primary/secondary. Are these shuts good to have?

I put one round of solid copper wire on secondary and got about 1,6 VAC (per round, no load current was 3A on primary). One, two or three sounds of as thick cable I can cram in?

There are estimates on transformer VA-rating vs. core area. Are those estimates any use here or are they out of whack?

Plan is to check if this transformer is any good for small jobs and lab timer circuitry with it. There is a buzz box type welding machine waiting to be butchered for real attempt.

Pekka

Offline vtsteam

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Re: Microwave owen transformer spot welder?
« Reply #1 on: November 17, 2015, 08:58:39 AM »
Wish I could help Pekka, I started to build a spot welder from a big transformer about ten years ago. Pulled out the secondary (which was a pain) and re-wound with a few turns of heavy gage cable, but never actually tried it out. It's sitting in a box upstairs. Harbor Freight offered a sale (half) priced spot welder in the mean time, and I ended up buying that. I still wonder how that first one would have worked out.  :zap:  :loco:
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Offline John Rudd

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Re: Microwave owen transformer spot welder?
« Reply #2 on: November 17, 2015, 08:59:55 AM »
Pekka,
The mot you generally find are ok for use as a spot welder.....

Have a read here.....http://www.chatzones.co.uk/discus/messages/7/16416.html

You need around 2-3 turns of thickest wire you can fit giving maximum current....
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Offline BillTodd

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Re: Microwave owen transformer spot welder?
« Reply #3 on: November 17, 2015, 09:12:24 AM »
I've been playing around with a MW transformer making a bandsaw blade welder ( The butt weld mechanism has to be surprisingly accurate  - so far I have made two good welds from several feet of blade :LOL: )

Initially, I tried using a spot welder to power the blade welder, but the spot welder is low voltage (~1.7vac - one turn with a 230v primary) .The blade welder needs two or three turns ( I've been experimenting with more but 2-3 seemed to work best)

For a spot welder you'll need to mount the transformer as close to the jaws as possible to keep the resistance down. The current flow is huge: I measured way over 600A from a small hand-held spot welder.

Use the as much copper you can squeeze into the transformer gap (I'd use multiple strips lacquered or taped to insulate them ).
« Last Edit: November 17, 2015, 09:38:53 AM by BillTodd »
Bill

Offline vtsteam

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Re: Microwave owen transformer spot welder?
« Reply #4 on: November 17, 2015, 09:19:37 AM »
If I remember correctly I did use some solid copper sheet in the "construction"  -- I think I  did a search at the time and found that liquid styrene was a good coil dope type insulator, so I made some by dissolving large quantities of polystyrene foam in a small quantity of acetone. Not a recommendation, just an account of my madness in general.  :zap:
 
I love it when a Plan B comes together!
Steve
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Offline PekkaNF

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Re: Microwave owen transformer spot welder?
« Reply #5 on: November 17, 2015, 10:37:26 AM »
Thanks.

Bill, my friend is fixing one old east german bandsaw blade welder and he got it working....problem is annealing cycle after the weld. Old transformer had leaked all smoke out. I gave him a small buzzbox welder and he sawed secondary out put there three (I think???) rounds of 95mm2 fine wire cable coaxed into soft shrink tube (all from local winding shop). It's almost finished.

I'll follow up the links&leads.

I'm still wondering the shunts between windings. Some instructions tell to leave them, but they "rob" part of the flux. i don't get it. Why they are good, unless you want to limit current....or maybe they are there to load the primary a little even when secondary is open circuit????

Pekka

Offline vtsteam

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Re: Microwave owen transformer spot welder?
« Reply #6 on: November 17, 2015, 10:41:12 AM »
I've been playing around with a MW transformer making a bandsaw blade welder ( The butt weld mechanism has to be surprisingly accurate  - so far I have made two good welds from several feet of blade :LOL: )

Initially, I tried using a spot welder to power the blade welder, but the spot welder is low voltage (~1.7vac - one turn with a 230v primary) .The blade welder needs two or three turns ( I've been experimenting with more but 2-3 seemed to work best)

That looks handy, Bill.
I love it when a Plan B comes together!
Steve
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Offline BillTodd

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Re: Microwave owen transformer spot welder?
« Reply #7 on: November 17, 2015, 10:53:45 AM »
Quote
I'm still wondering the shunts between windings. Some instructions tell to leave them, but they "rob" part of the flux. i don't get it. Why they are good, unless you want to limit current....or maybe they are there to load the primary a little even when secondary is open circuit????

I took mine out to give more room for windings. I suspect the shunts are necessary in the original, to add inductance (the MW load is mainly capacitive, so might be require to limit current in secondary) .
Bill

Offline PekkaNF

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Re: Microwave owen transformer spot welder?
« Reply #8 on: November 17, 2015, 02:16:27 PM »
Ineresting reading on shunts. Some nice anecdotes on magnetrons and radars.

Now I have to deside what to test next.

Pekka

Offline Manxmodder

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Re: Microwave owen transformer spot welder?
« Reply #9 on: November 17, 2015, 06:11:46 PM »
I've been playing around with a MW transformer making a bandsaw blade welder ( The butt weld mechanism has to be surprisingly accurate  - so far I have made two good welds from several feet of blade :LOL: )

Initially, I tried using a spot welder to power the blade welder, but the spot welder is low voltage (~1.7vac - one turn with a 230v primary) .The blade welder needs two or three turns ( I've been experimenting with more but 2-3 seemed to work best)

For a spot welder you'll need to mount the transformer as close to the jaws as possible to keep the resistance down. The current flow is huge: I measured way over 600A from a small hand-held spot welder.

Use the as much copper you can squeeze into the transformer gap (I'd use multiple strips lacquered or taped to insulate them ).

Not a specifically a spot welder but I do like Tom's techniques method and apparatus for silver soldering bandsaw blades.

Youtube video link:
https://www.youtube.com/watch?v=mzKSk_Zs0Ew

...OZ.
Helixes aren't always downward spirals,sometimes they're screwed up

Offline PekkaNF

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Re: Microwave owen transformer spot welder?
« Reply #10 on: November 18, 2015, 04:17:34 AM »
Quick measurement shows core cross section area of 22cm2, which quick&dirty formula says would produce 470 VA of power....

On spot welder dirty deeds done dirt cheap....what voltage (vs. amps) I should coax it, before something starts to smell?


Pekka

Offline John Rudd

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Re: Microwave owen transformer spot welder?
« Reply #11 on: November 18, 2015, 04:49:20 AM »
Pekka,
A few links to try....

http://www.giangrandi.ch/electronics/trafo/trafo.shtml

http://sound.westhost.com/articles/xfmr3.htm#s3.0

In the last link, at the bottom of the page are some zip files containing various calcs/spreadsheets/data....
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Offline PekkaNF

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Re: Microwave owen transformer spot welder?
« Reply #12 on: November 18, 2015, 06:20:42 AM »
Looks like they are geared of designing continuous power. I'm trying squese every little red hot electron trough it max. two seconds....but if core saturates a lot of fireworks can happen in two seconds :zap:

Pekka

Offline Noitoen

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Re: Microwave owen transformer spot welder?
« Reply #13 on: November 18, 2015, 01:03:52 PM »
The little piece of metal core between the windings act as a current reducer. Removing them increases the output current.

Offline Fergus OMore

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Re: Microwave owen transformer spot welder?
« Reply #14 on: November 20, 2015, 04:07:01 PM »
I had a spot welder for years to go with my then selection of other welding tackle. It was all sold on my present move but I bought a cheap scrap MIG for the hell of it. In another forum there was guy from Oz who had built much the same  thing but principally the arms etc were wood. Nevertheless, it was a fair idea for intermittent use. His arms went on fire.

I thought that there was a simpler and more reliable tool in a 40 arc welder and coupled with another 10 or so for a copper earthing rod, it was all quite practical  using scrap 1" square arms from an old table set of legs.

I'll let you know when I have time from doing a massive probate job following the death of my wife who had-- would you believe- a turret ( 4 heads)spot welder for orthodontic stainless steel band and wires.

Offline PekkaNF

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Re: Microwave owen transformer spot welder?
« Reply #15 on: November 21, 2015, 04:11:10 AM »
When I had a Morris Marina 1,8 l TC, I was studying how to use MIG. I got the gat cheap because it had an encounter with a deer. I was able to cut out front pilars and part of a roof from othervice crubled similar car. Fair amout of fitting and welding before windshield, bonnet and doors fitted. MIG had a timer and a special nozzel for "spot" welding. Needed a lot of clamps and preparation to imitate spot welding.

I found one small industrial machine 350, but it was big. Well over 35 KVA, water cooled and all that.

I found out that just a stone throwing distance away from my work is shop/factory that stock anything you need for carbon brushes and motor/transformet winding. I bought a metre of 25 and 35 mm2 copper rope (used for carbon brush manufacture), 10 mm silicon sleve and class fibere backed adhesive tape good util 155C.

Old isulation had taken a little hit from secondry removal. Put some tape over the core.

I was able cram 2 2/3 rounds of copper rope on the MOT. That produces 2,4 VAC unloaded on secondary. I measured less that sec partial secodary shot circuit (tried to make it short, but it buzzed and arched), primary pulled 14A and voltage dropped 230->220 VAC. Looks like this will do.

I wonder if copper grounding electrodes could be used as a welding arms or electrodes.

Pekka

Offline Fergus OMore

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Re: Microwave owen transformer spot welder?
« Reply #16 on: November 21, 2015, 05:12:31 AM »
I think that I can go some way to answer your Mig/Mag issues. Basically, all that was needed to create a spot weld was a modified Mig nozzle or shroud which is nothing but a nozzle with two raised lugs and a a the ability to punch a hole in one of the two sheets of body metal. The Mig nozzle was placed touching the intended join and the hole was filled with metal. As the arc was shielded there was no necessity to use a face shield.

It may interest you to know that some 30 years ago Alan Robinson the Lecturer in Motor
Vehicle Restoration at Gateshead College produced a set of notes for student use and this was subsequently made into a book both in English and Swedish.  A revised edited which adds a Mr Livesy to the authorship is still available and is obviously updated. Again, the Welding Institute in Cambridge, UK produced a series of videos for the amateur and I recall the repair of a MGB  in the Mig one being shown to students on the City and Guilds qualification course of which I was a 'manure' student.

Further digressing does suggest a Mig-Welding.co.uk series of information which may help . What must be remembered is that the actual sticking bits of metal together is the easiest learnt whereas the English Wheeling and panel beating are demabnding prior to pulling a vehicle back in correct alignment once major structural damage has been  undertaken

Offline awemawson

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Re: Microwave owen transformer spot welder?
« Reply #17 on: November 21, 2015, 05:43:23 AM »
Strictly speaking, if you use a MIG to 'spot weld' you are actually doing a 'plug weld' which is obviously so when you look at them. (Perfectly sound way of joining stuff though)

As I was reading Pekka's post it went though my mind that ordinary MIG copper tips could perfectly well be used as the tips for a proper 'spot welder' ie one that joins by passing an enormous current though the two metals and joins them without added extra metal by way of filler wire.
Andrew Mawson
East Sussex

Offline Fergus OMore

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Re: Microwave owen transformer spot welder?
« Reply #18 on: November 21, 2015, 06:33:32 AM »
Andrew is perfectly correct in describing the joining process as a 'plug weld'.  It does get a bit pedantic when a spot welder has to melt its way through normally used galvanised body steel to create a joint and replace the anticorrosive properties of the zinc- which should- if you get it right- create an as new joint- with(( clears throat again) the same ability to be torn on impact rather than the incumbents. Its all in Thatcham, the exact number of spot welds to be replaced.

Regarding the spot welder, mine had the existing copper arms but with home turned inserts  but in order to reach more inaccessible spots, I made up a set of longer arms in copper with copper electrode tips. However, after discussion with the gurus at Gateshead College, it was suggested that the arms could have been made out of steel again with copper tips.

My present plans are inherently simpler and will utilise nothing more complicated that cheap and easily obtainable copper earth rods material which comes in 4 foot lengths which could have supplied the local Nissan factory where we obtained donor reject Nissan Primeras. Air hacksawing and stitching up almost pristine body panels  was a joy rather than a chore. I did, however, design and make welding clamps to hold the compound shapes.




Offline awemawson

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Re: Microwave owen transformer spot welder?
« Reply #19 on: November 21, 2015, 07:41:43 AM »
My spot welder, which is a hand held type,  has approximately 1" diam main arms and 1/4" electrodes. I am always amazed how hot the 1" arms get in use. I cannot believe that it is directly the current flowing in the 1" bar that is causing the heat. I suspect it is the resistance of the brass coupling to the body of the machine and the tips themselves whose heat is being conducted by the fat copper bars.

Andrew Mawson
East Sussex

Offline PekkaNF

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Re: Microwave owen transformer spot welder?
« Reply #20 on: November 21, 2015, 12:27:30 PM »
Oh, I never have used those small spot welders, they look a little special and probably something worth imitating.

If you do the math, there must be very little of restance on every joint, therefore my goal is to minimize number of components and connections on secondary. Plan is to terminate other end of the copper rope (secondary winding) on lower (fixed) arm and the other end to pivoting upper arm.

I was toying with the idea of using steel profile/pipe as arms and put the conductor iside of them, but big current here might interfere here and my intuition tells that this might be of complication.

Thanks to Andrew, I got a bag of M6 MIG wire tips, they feel like a good candidate for this small spot welder.

I have one grounding electrode, OD is about 13 mm and it looks like copper.

Which is better, linear movement of the upper arm or pivoting arm? On my doodles looks like a over arm is the most simple solution. Long arm should have very small radial movement. I.E. lateral displacement of the tip should small enough not cause too much trouble.

Pekka

Offline awemawson

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Re: Microwave owen transformer spot welder?
« Reply #21 on: November 21, 2015, 01:40:58 PM »
From a theoretical point of view, a straight line squash is best, but an arc should be fine, as the displacement is microscopic (or should be). I set mine up (with the power off !) to firmly nip the pieces together. Then adjust performance by varying the welding time. It often pays to do a few trials to check what time is best. In my test piece the one on the right is when I was happy with it functionally and cosmetically.

Clean electrodes help enormously  :clap:

The electrodes in mine are fixed using a tapered cotter in like the old bicycle pedals used but without the retaining nut, so they are just tapped in - rather crude but it seems to work.
Andrew Mawson
East Sussex

Offline PekkaNF

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Re: Microwave owen transformer spot welder?
« Reply #22 on: November 21, 2015, 02:45:19 PM »
Thanks. More doodles with arm then...

Another avenue: Which type of solid state relay to choose? I have used triacs with zero crossing switching...apparently not that great here: Highly inductive load.

Pekka

Offline awemawson

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Re: Microwave owen transformer spot welder?
« Reply #23 on: November 21, 2015, 02:59:10 PM »
Assuming that you intend to switch the primary of your transformer (which I'm sure that you do), so long as you use a solid state relay that is very well over specified for the load you should be fine. Some useful gen here:

https://www.omron.com/ecb/products/pdf/precautions_ssr.pdf

Andrew Mawson
East Sussex

Offline PekkaNF

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Re: Microwave owen transformer spot welder?
« Reply #24 on: November 22, 2015, 03:41:57 AM »
Yesh...primary. And that one is chalenging eneough. Some swears on zero (voltage) crossing switching some swears at it. I tried to google answers and I got even more confused that I were before.
Like this, halfway down and not geting anywhere....
http://electronics.stackexchange.com/questions/181874/when-where-to-open-and-close-a-transformers-output


Finally I setled here:
http://www.automation.com/library/articles-white-papers/industrial-io/understanding-solid-state-relays

Transformers
In controlling transformers, the characteristics of the secondary load should be considered, because they reflect the effective load on the SSR. Voltage transients from secondary load circuits, similarly, are frequent in transformers and can be imposed on the SSR. Transformers present a problem in that, depending on the state of the transformer flux at the time of turn-off, the transformer may saturate during the first half cycle of subsequently applied voltage. This saturation can impose a very large current (10 to 100 times rated typical) on the SSR that far exceeds its half-cycle surge rating. SSRs having random turn-on may have a better chance of survival than a zero cross turn-on device, for they commonly require the transformer to support only a portion of the first half cycle of the voltage. On the other hand, a random turn-on device will frequently close at the zero cross point, and then the SSR must sustain the worst-case saturation current. A zero cross turn-on device has the advantage that it turns on in a known mode and will immediately demonstrate the worst-case condition. The use of a current shunt and an oscilloscope is recommended to verify that the half-cycle surge capability is not exceeded.

A typical approach in applying a SSR to a transformer load is to select a SSR having a half-cycle current surge rating greater than the maximum applied line voltage divided by the transformer primary resistance. The primary resistance is usually easily measured and can be relied on as minimum impedance, limiting the first half cycle of inrush current. The presence of some residual flux, plus the saturated reactance of the primary, will then further limit, in the worst case, the half-cycle surge safely within the surge rating of the SSR.

Although. I was about to give it up ans use a contactor and RC-snubber (that's hard eneoug..)

Oh, and I found this, but those rocks looks diminutive.
http://www.avdweb.nl/Article_files/Tech-tips/Spot-welder/Spot-welder-controller-circuit.jpg
http://www.avdweb.nl/arduino/hardware-interfacing/spot-welder-controller.html
Second link confirms of switching SCR:s on near maximum voltage.

Pekka