Welder made with microwave oven transformer

[debe]

One of the problems with Microwave oven transformers, is there is an air gap in the center of the E core. This acts as a current limiter, this is desirable as a magnetron powersuply but may not be helpfull in an arc welder aplication.

 

[AlainB]

I said I salvaged 9 microwave ovens. It is true. I found them in garbage, last july, in about one week, not even really looking for them.

https://www.cbc.ca/news/canada/ottawa/story/2010/07/12/scrap-metal-on-moving-day.html

 

[gary350]

One of the problems with Microwave oven transformers, is there is an air gap in the center of the E core. This acts as a current limiter, this is desirable as a magnetron powersuply but may not be helpfull in an arc welder aplication.

That is not true. There is NO air gap. It is a shunt transformer. The shunts limit the current of the transformer to 14 amps on 120 VAC. The primary winding has only 100 turns this causes the transformer too pull too much current so it runs 12 amps of idle current. Microwave oven transformers are great for all kinds of home projects, stick welders, spot welders, choke transformers, and others. I have about 30 MOTs I have not found one with an air core yet.

 

[dr peppers]

I can back up there isnt an air gap, I've pulled a few apart and not seen an air gap.

The idle current is a concern there are safety issues as mentioned with the welder burning up if left idle, the way round this is to fit thermal trips on the transformers so they cut out when hot.

I reccomend 6013 rods, known in welding circles as the farmers rod, I think a 7014 is a modern version of a 7018 which is a professionals rod, the 7014 being suitable for ac and less sensitive to moisture, check that up first its been a while since I delved into all that, the 7014 will give a better weld, the 6013 a little more forgiving.

Commercial 2kw microwaves make good donors, I had one transformer that would burn 3.2mm 6011 rods which are very aggressive.

Incidently the rod numbers, the first 2 are the strength I think in psi, the 3rd is the welding position 1=all positions and the last refers to the type of weld current ac/dc and a few other things, do a google.

 

[MrAl]

Hi,


Very interesting. I'll have to see if i can dig up the ones i took out of ovens and examine them more closely. Im sure they are smaller units though coming out of 700 watt mic ovens.

 

[DerStrom8]

You are aware he took the HV windings out and replaced them with 20 turns of 10 ga which now only put out 41 volts open circuit right?

I think as long as he doesn't stick his tongue on it he may be okay!

I'm officially eating my own foot right now

My apologies, everyone. I didn't read it as carefully as I should have

 

[AlainB]

View attachment 63467

 

[DerStrom8]

View attachment 63467

Thanks for that

 

[gary350]

If it were me I would put a spring loaded normally off foot pedal on/off switch on the welder. Step on the switch to weld then let off. That is the way a lot of the new welder work it keeps you from wasting electricity when not using the welder. It is also a nice safety you can lay your welding rod holder down and it won't short out on something if the welder is off.

I have a box of 6013 rods, 7014 rods and 7018 rods. I have an old Lincoln stick welder and one of those cheap 90 amp Harbor Freight stick welders. The 6013 rods are a pain in the ass to use. They are well known to farmers because they weld through rust, dirt, mud, paint but they make a very ugly weld. It takes some effert to get them to weld. The 7014 rods almost weld themself but they will not weld over paint, mud and dirt. The low hydrogen 7018 rods are hard to get started and they make an extremely strong weld but they do not like to weld up vertical or over head. Go to your local welding store ask for 1 sample rod of each kind then go test them on your welder.

 

[DerStrom8]

If it were me I would put a spring loaded normally off foot pedal on/off switch on the welder. Step on the switch to weld then let off. That is the way a lot of the new welder work it keeps you from wasting electricity when not using the welder. It is also a nice safety you can lay your welding rod holder down and it won't short out on something if the welder is off.

I agree. A dead-man's-switch would be a good idea, for safety and for better operation.

 

[dr peppers]

I dont really argue with that gaz, however the 1 in 7018 means any position including overhead.

I must be from the farmer clan, it usually is rusty old junk I weld up anyway.
6011's weld through rust paint and junk, they are aggressive, with a fast freeze time so the weld doesnt look as pretty, pipe welders like these for welding round pipes, well they did things might have changed.
Its too much hassle but if you prewarm 7018's they start much better, you can even get heated quivers fo them.

Professional tig welders have a foot pedal (or thumb control), more for controlling the weld current, but no foot - no power, good if something bad happens - no or much less power.
I've seen a circuit that switches on a welder power tranny when the electrode is shorted, cant remember how it worked, possibly a resistor in series with the primary and a circuit to close a relay when current flowed in the secondary.

 

[AlainB]

Here is a newer MOT compare to an older one. Even with the shunts in place there is (I think) more room in the older one than in the newer one with the shunts removed.

 

[dr pepper]

Are those both from the same wattage oven, if so theres a big diffo in size.

 

[AlainB]

Unfortunately, I can't tell. I did not check that. Next time i'll get an oven as a donor I will check the wattage but also I will salvage more parts. There is so many nice things inside a microwave oven.

 

[Sceadwian]

Alain can you measure the width and height of the gaps and the distance between the gaps on both transformers, in metric for me? I'm curious to work out the math my minds eye says that the newer one even with the shunts on it has substantially more winding room.

The wire gauge of what you're going to wind the secondary with is required as well, if you don't mind. If it's anything other than enameled wire you're wrapping it with measure the insulation thickness as well.\

 

[AlainB]

For the small one, without the shunt the free opening is 15 mm by 32. The shunt is 5 mm and the distance between the 2 openings is 32 mm.

For the big one, the opening is 20 by 40, the shunt is 8 mm and the distance between gaps is 40 mm.

 

[AlainB]

Even if I did not used the welder a lot since I built it I can say that I am happy with the way it is working right now, using the rods that I have.

But if there was a possibility to improve the machine, I would be all in.

This guy https://aaawelder.com/indextoo.html say about is project:

"The transformers are installed so that their magnetic fields don't interact with each other and cause adverse effects. I accomplished this by turning the plane of the transformer/coil center lines 90 degrees to one another. "

he also said:

"The addition of capacitors to the primary circuit is probably the most significant improvement in the mega buzz box. Properly installed, the capacitors will correct the power factor and minimize tripped breakers."

He installed some 30uF 370 volts capacitors, I beleive they are starter capacitors.

I am willing to make these changes but I would like to know what you think about that. Will it improve the welding and also what range of the capacitor values should I look for if I don't find those values. I see some from time to time at the Flea Makket. The capacitors from the microwave oven are 0.86 uF 2100 VAC. Could they be used?

 

[canadaelk]

The transformers should be at 90 deg to each other and as far as is possible apart. I will not comment on the use of the caps othe than that they would be motor star caps. E

 

[MrAl]

Hi,

You could probably measure the current phase of the input and that would tell you what size caps to use. Of course they need to have an AC rating but im sure you know that. The current phase will tell you the inductive part and that would lead to calculating a capacitor value that will 'correct' the power factor. I would bet that there is a lot on the web about power factor correction, but finding the value of the equivalent inductance isnt hard at all. With the welder running full steam, measure the current with a scope and get the amplitude and phase angle relative to the line voltage, and of course measure the input voltage amplitude too.

The reason this could help with the welding itself is because if there is a 'bad' power factor then that means the line voltage is supplying current for both the resistive part of the load and also for the reactive part of the load. IF the power factor is corrected, then the line voltage has to supply current for only the real part and so that reduces the load on the line. This in turn allows you to plug into a 20 amp circuit rather than say a 30 amp circuit, or if the power is low like 10 amps then adding more transformers could get you up to 20 amps. Without correction however, adding more transformers might easily take you over the 20 amp limit.

We could do a few circuit simulations to see the difference, and if we had the data from the measurements above it would help a lot to determine what, if anything, we could do to improve either the power of the welder (add transformers) or reduce the load current demand on the line from what it is now.

 

[AlainB]

Hi, thanks!

Rotating one transformer is easy to do and I will do it. It should not lower the performances of the welder. For the capacitors, I am lost with the calculations needed. Something like a rule of thumb advice or just an approximate value would be more what I need as long as it is relatively sure that it will improve the welding. Maybe this 30uF value is the way to go?