MOT arc welder - Page 1
Forum section: electronics / projects

author: nucleopolymere
author_rank: Contributor
timestamp: December 24, 2015, 06:50:35 pm
content: Hi, i wanted to wish to all of you a merry christmasI had a project in my head for quite a lot of time now which is to build an arc welder. Since I am relatively new to electronics there is several things that I must know to make this come true.First of all, is this design safe, would it try to kil me?Also I am scavenging different microwave ovens and all the transformers are not all rated for the same power output, how do I adapt the wiring so that all the transformers give their respective power output ?For the controlling section would a triac be sufficient, I dont know how to choose the variable resistor nor the capacitor of the triac to make it work.Should I add an isolation transformer?I live in canada should I plug this welder on the 120v or the 240 v ?Is 3500W is enough for an Arc welder ?I've been inspired of other schematic found on the internet unfortunately it needs some modification so it can fits my design limitations (number of mots, power of the mots)Also I though it would be a good idea to add the cooling fans from the microwave I've scavenged sould I build a control circuits for them with thermal shut-off in case the mots get too hot or simply running them to full power all the time ?One last thing I don't know how much current it will draw on the output so how do I calculate the gage for the wiring. I though it would be nice if I could weld 1/2 inch steel.I know I have a lot of questionThanks in advance !!!My plan is on the linkhttp://postimg.org/image/u0cumpbpl/

author: Arjan Emm
author_rank: Regular Contributor
timestamp: December 24, 2015, 07:18:38 pm
content: I didn't read your post properly. But Mot's are very dangerous and very ineffecient transformers. They are built down for a price. They are designed on the edge of overload.They draw a lot magnitising current and they are not ready for some extra overloading. output voltage and current is letal.

author: tautech
author_rank: Super Contributor
timestamp: December 24, 2015, 07:41:16 pm
content: Designing an operative welder is not exactly simple, there are several parameters to get correct, safety being the most important of course.This thread should assist and open your eyes:https://www.eevblog.com/forum/beginners/arc-welder-triac-controler/

author: nucleopolymere
author_rank: Contributor
timestamp: December 24, 2015, 10:03:24 pm
content: Posted by: Arjan Emm« on: Today at 06:18:38 AM » Insert QuoteI didn't read your post properly. But Mot's are very dangerous and very ineffecient transformers. They are built down for a price. They are designed on the edge of overload.They draw a lot magnitising current and they are not ready for some extra overloading. output voltage and current is letal.Please, can you explain why are they inefficient?I do not intent to weld on 4kV since I must rewire the secondary of the transformer but i do not know how many turn should I put on the secondary?If I use them to their respective power, I will not overload them ?? I don't understandSince I rewire the secondary why would the output voltage would be lethal?I've read the entire topic but I understand very little I don't know what to do actually.It seems that the schematic is not safe (i dont know actually this is what I understand for the post of others) Is triac is good for controlling the power output of the welder . Or it would be better to control by a magnetic shunt, but since the mots are very small would it work ?The second way I've though of controlling the power is to add a variac in series with the primary of the MOTS it would be simplier I think.

author: Macbeth
author_rank: Super Contributor
timestamp: December 24, 2015, 10:11:28 pm
content: I've been wanting to make an isolation transformer from MOT's. From what I can tell you need to use at least 3 MOTs to make 1 isolation transformer. Not quite sure why I can't just gut the HV stuff and transplant the primary windings from 1 to the other...But they tend to come for free when you go to the tip or find them on the street and I hate waste so if I can make safe use of them then that would be great.

author: johansen
author_rank: Super Contributor
timestamp: December 24, 2015, 11:21:21 pm
content: Find at least 8 microwave oven transformers, wire 6 turns of 8 awg wire through each one, run the secondaries all in series.you don't need to cut the core open to hacksaw the high voltage secondary off and pull it out. this can be surprisingly difficult but its better than cutting the core, which leads to more magnetizing current due to large air gap.you can wire the transformer coils in series like this: 120vac-primary coil-electrode-secondary coil-neut/gnd-secondary coils-electrode-primary-120vacin this way, the primary coils have about 100 volts across them which will pull the transformer out of saturation, reducing the no load idle current from 4 to 8 amps per transformer to more like 1 or two amps. this reduces the waste heat produced during welding.. at the cost of lower output voltage.no the output won't be isolated but it will be center tapped to ground.

author: gadget73
author_rank: Regular Contributor
timestamp: December 25, 2015, 01:16:55 am
content: How are you planning to control current with this? The cheap and cheerful method used on many old welders is mechanically variable transformer coupling. Essentially it has a screw down through the middle of the windings that moves the core in and out to adjust coupling in order to control the max current available. The transformer also gives you line isolation. Typical stick welder output voltage is only about 30 volts. You usually need more than 100 amps of current to weld together metal of any sort of real thickness. If you want to get fancier with it, you can wire in a selector switch and a rectifier stack to get +DC and -DC modes as well.Honestly, this may be something where you're better off just picking up a used one. a quick search on the local Craigslist turned up several for under $200 with the leads. Buying a sizable length of heavy gauge copper, plus the clamps isn't exactly cheap. Personally if I were looking to do something like that, I'd start with a basic old AC buzz box and fit a rectifier setup to make it more flexible. That would get you a suitable power supply to start with and a set of welding leads.

author: nucleopolymere
author_rank: Contributor
timestamp: December 25, 2015, 05:35:56 am
content: "in this way, the primary coils have about 100 volts across them which will pull the transformer out of saturation, reducing the no load idle current from 4 to 8 amps per transformer to more like 1 or two amps. this reduces the waste heat produced during welding.. at the cost of lower output voltage."If i understand you (and wikipedia) if the transformer goes into saturation the magnetic field could not grow further, thus decreasing the inductive reactance of the coils and then increasing the current in the primary resulting in power losses because it would be a resistive load? So I need a lot cores to avoid the saturation? Correct me if i am wrong please."How are you planning to control current with this? The cheap and cheerful method used on many old welders is mechanically variable transformer coupling. Essentially it has a screw down through the middle of the windings that moves the core in and out to adjust coupling in order to control the max current available. The transformer also gives you line isolation. Typical stick welder output voltage is only about 30 volts. You usually need more than 100 amps of current to weld together metal of any sort of real thickness. If you want to get fancier with it, you can wire in a selector switch and a rectifier stack to get +DC and -DC modes as well.Honestly, this may be something where you're better off just picking up a used one. a quick search on the local Craigslist turned up several for under $200 with the leads. Buying a sizable length of heavy gauge copper, plus the clamps isn't exactly cheap. Personally if I were looking to do something like that, I'd start with a basic old AC buzz box and fit a rectifier setup to make it more flexible. That would get you a suitable power supply to start with and a set of welding leads."I know the easiest solution would be to buy a used one but I really want to build the welder, to learn something. and if it can do some decent welds i would be happy. This welder is not something that I would use abusively but it is a nice tool to have.

author: oldway
author_rank: Super Contributor
timestamp: December 25, 2015, 10:57:24 am
content: http://postimg.org/image/u0cumpbpl/Mains short-circuited by triac: a new "spectacular failure of a triac"https://www.eevblog.com/forum/beginners/spectacular-failure-of-a-triac/

author: calexanian
author_rank: Super Contributor
timestamp: December 25, 2015, 05:32:54 pm
content: Arc welders have specially designed variable core transformers. Just buy one. Otherwise flux core wire welders are so cheap. Harbor freight here in the U.S. Sells them for $99 and you don't have to worry about heating and drying sticks and everything. Otherwise used mammoth stick welders are usually pretty cheap and would be way way better than anything you could come up with and safer.

author: nucleopolymere
author_rank: Contributor
timestamp: December 25, 2015, 05:49:39 pm
content: XD oops, it seems that I effectively should have put the triac in series with the primary.Well,Are you guys gonna actually help me or you just going to tell to buy one because if so, what is the purpose of this siteI did not post this to be told to buy one..... I know it would be simplier and maybe cheaper .I made my reasearch about it but I couldn't find enough information to properly design one +I have no experience in designing circuits thus requiring help.I've seen video like the king of random but i do not know if his design is safe or whatever.Thanks for your answers

author: nctnico
author_rank: Super Contributor
timestamp: December 25, 2015, 06:17:16 pm
content: Quote from: nucleopolymere on December 25, 2015, 05:49:39 pmYou don't seem to know it is also much safer to buy one! I've lost a friend because he was tinkering with electricity. Don't do that to your loved ones!

author: tautech
author_rank: Super Contributor
timestamp: December 25, 2015, 07:32:00 pm
content: Quote from: nucleopolymere on December 25, 2015, 05:49:39 pmWith little or no experience designing circuits you have been well advised to bypass thoughts of using a welder for a first step into electronics.The purpose of this forum is to share its collective wisdom for your and others benefit.We don't have to justify our experience, just know collectively we have much.Grab one of the little Caddy welders or a second hand unit and stay safe.

author: Macbeth
author_rank: Super Contributor
timestamp: December 25, 2015, 07:52:01 pm
content: I have to wonder why anyone would want to make an arc welder when you can pick them up for less than £49.99 at the Aldi. They are awful compared to MIG and TIG for welding anyway.Making any of these welders "on the cheap" is not going to happen. Not if you actually want it to make a nice weld.Now a decent home made welder would be something like a battery tab spot welder. That can be done with a bank of capacitors and bus bars.

author: sjg0
author_rank: Newbie
timestamp: December 26, 2015, 08:12:49 am
content: It makes me sad how (when browsing forums) there is always a group of pessimists discouraging a project. "Just buy XXX from the store." crowd being the absolute WORST!Last I checked, this was the ElectronicsEngineeringVideo blog forum. Engineers don't buy shit! They BUILD it!If you disagree with a project, present your argument and then take the extra 4.5 seconds to put yourself in his/her position and outline some potential advantages. Seeing an argument from both sides will show that you are a mature professional who can let someone make a decision for themselves.The fact that this person has come to the community seeking help before he begins kinda speaks for itself.THAT BEING SAID, this is a terrible idea practically speaking. I encourage you to pursue it as a great learning experience.The best way would preferably be using the aforementioned method of chopping off the high-voltage secondaries and replacing it with just a few turns of large wire. Don't hacksaw the core please. In my experience the magnetic shunt will only vary the current to the load by about 20%, so its not really worth it. The triac will certainly need to be in series with the primaries (it's just a short to gnd in your original schematic). But most importantly use a fuse! I'm not sure what the power situation is up in Canada-ville but be aware that 3500W is a lot and your circuit may not be able to provide that.Honestly it's ******* obvious, but as it has been said this is very dangerous. Don't let it discourage you (shit in America ~35,000 people die a year in automobiles. Is that going to stop you from driving?). Just be aware of the level of danger and take appropriate precautions. Take your time with everything. Double check every connection. Work in a clean space (don't be a redneck on the garage floor). Don't ever let wires dangle. Etc, etc...From my experience MOT welding, you *will* melt metal, however you will *NOT* be stacking dimes:https://www.google.com/search?q=stacking+dimes&safe=off&tbm=isch&tbo=u&source=univ&sa=X&ved=0ahUKEwjfyc739vjJAhVPymMKHflDAScQsAQIHA&biw=1920&bih=971. So please don't consider this a genuine shop tool even for light-duty. Also keep a thermometer on the transformer cores. As mentioned, they are super inefficient and can get pretty hot.Lastly, I recommend getting one of these goofy kill-switches and keeping it handy when working on high-energy stuff like this:http://www.amazon.com/uxcell-Mushroom-Emergency-Button-Switch/dp/B00548585A/ref=pd_sim_328_2?ie=UTF8&dpID=41a69f-X4AL&dpSrc=sims&preST=_AC_UL160_SR160%2C160_&refRID=149GFA6A5K1TMKH0357K(do your research. I found this link just as an example.)

author: cs.dk
author_rank: Unknown
timestamp: December 26, 2015, 09:11:13 am
content: Are you sure a MOT is big enough to handle the load. They seem pretty small to me, but I don't know for sure.Just for scaling;Attached pics is two out of a 3-phase welding machine (third is just chopped of) - It's not assembled yet, just all parts thrown in the box, so i can find themIt should make me a 12-15V DC PSU capable of at least 100A. The transformer is quite heavy, 15-20 Kg would be a guess.

author: SeanB
author_rank: Super Contributor
timestamp: December 26, 2015, 09:37:52 am
content: MOT can be used to make a small low current welder. Sub 1mm welding rod, 1 minute on, 10 minutes off duty cycle with fan cooling as operating regime, but it is not really going to do much work, and the usage is limited.Good though to learn a little with, and you can turn them into a pretty good spot welder core, as there it will be pulsed at a low duty cycle and will have plenty of usable core area for the windings.

author: cs.dk
author_rank: Unknown
timestamp: December 26, 2015, 09:46:29 am
content: Seems like it will be more valuable to make a spotwelder then.Sub 1 mm rods, and 10% DC isn't really worth much. It's almost only for tagging stuff, to come back with a welder later on.It's fine to build something to learn, but make something useful.

author: tautech
author_rank: Super Contributor
timestamp: December 26, 2015, 10:24:01 am
content: Quote from: cs.dk on December 26, 2015, 09:46:29 amExactly.Quote from: sjg0 on December 26, 2015, 08:12:49 amAND YOU HAVE THE EXPERIENCE TO GUIDE THE OP THROUGH A PROJECT THAT HAS THE POTENTIAL TO KILL, BE MY GUEST AND BE IT ON YOUR CONSCIENCE.

author: cs.dk
author_rank: Unknown
timestamp: December 26, 2015, 10:53:24 am
content: I just found this;http://www.users.on.net/~endsodds/psrewind.htmIt seems that a MOT will only deliver ~20A max. 30A maybe.. That won't weld a lot. I would say at least 100A is acceptable for a small hobby welder.Ie. ESAB 53.05 or 53.16 2,5 mm rods typical runs from 50-100A @ 24-27V.Seems like a dead project/waste of time from the beginning IMO. If you really need to weld, two SLA's can do it for small things.EDIT: Damn typos.

author: sjg0
author_rank: Newbie
timestamp: December 26, 2015, 10:56:04 am
content: Quote from: tautech on December 26, 2015, 10:24:01 amStairs have the potential to kill, therefore nobody should ever use stairs or have them in their home. Better yet, illegalize stairs immediately.I encourage you to reread my post, as I have already covered the fact that this is a high-energy project and it is dangerous. If you want to contribute please post something useful, maybe safety or design tips. Otherwise there is no need to reply.Also please refrain from typing in all caps. I am sure you are a very knowledgeable individual, but it makes you seem like a crazy person. Bear in mind this is the internet and maybe not just OP will be reading this.Overall cs.dk summed it up really well: it will work, but it won't perform beautifully. It would probably be much better suited as a spot welder.Sorry this thread went a little south OP. Let us know of any more questions!

author: tautech
author_rank: Super Contributor
timestamp: December 26, 2015, 11:33:51 am
content: Quote from: sjg0 on December 26, 2015, 10:56:04 amAnd I encourage you to read the thread:I quote the OP:I couldn't find enough information to properly design one+I have no experience in designing circuits thus requiring help.As you think you have the experience to guide the OP through the design of a welder and an appropriate control system, be it Triac, SCR's, Mosfets or Choke controlled please do so.Please instruct also in safe connection methods of a scope to ensure any active control of the incoming mains is balanced on both + and - cycles of the mains sinewave.Members will be watching with interest.Been there done this and not about to share with those without sufficient experience just so they think they can build a welder on the cheap.

author: oldway
author_rank: Super Contributor
timestamp: December 26, 2015, 01:54:12 pm
content: Quote from: sjg0 on December 26, 2015, 10:56:04 amOnly a stupid guy can write such a bullshit...4 posts in 2 years, you have not showed you know something in power electronics.Security is not a domain for joking@OP: first learn with low voltage (<50V) low current circuits how triacs and phase control work.

author: nucleopolymere
author_rank: Contributor
timestamp: December 26, 2015, 02:43:10 pm
content: Hi everyone and thank you for your answersJust have another question:What is the safety issue you guys are talking about, is it about the main ? because I dont think the secondary has enough voltage to kill me, does it?By your experience you think i should not do the arc welder. Ok, I respect that. Is the spot welder would be much safer for a project with the main ?If so I'm going to do my research about it and build one .I have a lot of projects in my head but there will always be a risk of electrocution.I 'm not a know-it-all, really I don't know much that is why I've been posting on this site. If you guys tell me that i should not do this then I won'tI am going to do some other projectQuote from: oldway on December 26, 2015, 01:54:12 pmI think you just missed the point, he did not meant the arc welder is not dangerous , in his post, he even does not recommend to me to build one. but for whatever project with the main or high power application the risks can be avoided for the person who knows what he is doing ( which is not my case) Anyway I do not want to fight with any of you, just saying.But again for the spot welder with MOT, do you reccomend it to me ?And what does OP means ?

author: nctnico
author_rank: Super Contributor
timestamp: December 26, 2015, 03:07:35 pm
content: Mains connected circuitry has many safety issues like using the correct parts, isolation distance and at some point you will want to debug (make measurements) as well which means you really need to know proper measurement techniques (no, an isolation transformer will only make matters worse!). It is better to acquire knowledge about making low voltage (<48V) circuits first and how to measure them.

author: nucleopolymere
author_rank: Contributor
timestamp: December 26, 2015, 06:21:48 pm
content: Ok thenIll guess Ill ve to do some other low power, low voltage stuff.Topic closed