MOT spot welder (120 VAC)

[SnifMyBack]

Hi everyone,
I'm writting here because I'm very close to drop my project completely because of my non working MOT spot welder. I've tried to look for any possible threads, videos, blogs, etc... that cover DIY MOT spot welding, without solving my issue. Right now the spot welder just doesn't do what it is supposed to do: solder.

Informations:
- Run on a 110/120 VAC 15 A line (I guess this is the problem)
- MOT is rated 750 watts
- MOT drived by a 20 A continous SSR
- SSR controlled by Arduino with adjustable time from 0 to 250 ms for pre-weld, pause, main weld.
- MOT secondary coil is re-wired with 4 AWG car subwoofer cable (I think it's copper plated aluminum, so that's clearly not the best cable) with 2.5 turns (I've tried 1.5 before without any succes too)
- Shunts ARE removed from the MOT
- From a reading of a current clamp meter, the MOT is consuming around 13 amps continuous.

So what happen:
I've tried pretty much every combinaison of delays for welding and it always land to these two options:
1. Nickel strip just have little spot (from the pressure of the copper rods) = no weld at all
2. Nickel strip get orange around the copper rods = little to medium welding ( I can detatch them by hands)

My next possible solutions:
1. Remove insulation around the 4 AWG wire (secondary coil) for a certain lenght and re-wrap it with electrical, so that the diameter is smaller (which leave less unused space in the core). Once done, I would cut the cable in 2 and put the in parallel inside the core.
2. Add a second identical (at least in wattages) and put it parallel on the primary and in series on the secondary.

If someone have an idea, I'm interested knowing it so I can complete my DIY eBike for this season.

P.S. English isn't my mother tongue, so if something isn't clear enough, I'll try to do my best to clear that out.

 

[john61ct]

self-censored

 

[SwampDonkey]

You need some kind of pulse timer and probably a capacitor bank. It needs a JOLT of current.

 

[SnifMyBack]

You need some kind of pulse timer and probably a capacitor bank. It needs a JOLT of current.

Thanks for the reply.

When you talk about the capacitor, thought mean one on the 120v side right?

Everyone is saying that it take a lot of current to melt the material, but in reality, current is voltage decide how many amps go through the nickel strips. So do you have a clue on how much amps it would take to solder? (The minimum value for good welds)

Thanks

 

[SwampDonkey]

You need some kind of pulse timer and probably a capacitor bank. It needs a JOLT of current.

Thanks for the reply.

When you talk about the capacitor, thought mean one on the 120v side right?

Everyone is saying that it take a lot of current to melt the material, but in reality, current is voltage decide how many amps go through the nickel strips. So do you have a clue on how much amps it would take to solder? (The minimum value for good welds)

Thanks

Go with low volts (12 or less) and as many amps as that thing can handle. I use 850A at 7V with electrodes tapered like pencils. Im no electrical engineer, but my welder is set up with a 100Ah car battery (in place of a MOT) then to a MOSFET bank that is timed by an arduino. You could simplify this by using a simple pulse timer. Im actually building a 12-14V spot welder right now using This board:
https://www.ebay.com/itm/12V-24V-Pu...m=262569487083&_trksid=p2045573.c100667.m2042

 

[krlenjuska]

It depends what thickness of nickel you want to weld. For 0.1 you need about 12-15J (IMO) and for 0.15 or 0.2 about 20J.
Now when you create MOT welder you need bigger transformer in order to get more power. I managed to weld with 700VA transformer but now i have 1750VA MOT and its much better because i have better control.
You dont have ideal voltage - you can weld with 1.8v but IMO 2.5 to 3.5 is best (my tests so far).
I have plans to test 5-6V but i dont have wire for that right now (and im to lazy to get it).
I can recommend some timers if needed. Chinese electronics but cheap and ok for small projects.
I wanted to use 2 same MOTs in series but i decided to get bigger mot.

 

[kdog]

I'd try pure copper cable for the secondary winding. I found with my first mot welder that I had to keep total R down as much as possible on the secondary/welding side. You're trying to drive ~1000amps with only a few volts...any resistance is going to make lower amps. Maybe the aluminium has too much resistance.
Fat copper all the way to short copper electrodes.
Also make sure that the cross section of your electrode tips is small.

 

[fechter]

I'd try pure copper cable for the secondary winding. I found with my first mot welder that I had to keep total R down as much as possible on the secondary/welding side. You're trying to drive ~1000amps with only a few volts...any resistance is going to make lower amps. Maybe the aluminium has too much resistance.
Fat copper all the way to short copper electrodes.
Also make sure that the cross section of your electrode tips is small.

+1
You need pure copper cable for sure. Something like welding cable. Aluminum is about 2x the resistance of copper. Also need more turns. You should be able to fit 3-1/2 turns of 4ga on a typical MOT core. 4-1/2 turns might be better if you can fit it. More turns will give you more voltage to make up for resistance losses in the secondary. The primary will try to draw as much as it needs and will be way over the rating but since the pulses are short, the wire won't get too hot. The primary should be fine.

 

[krlenjuska]

I use 35mm2 copper wire and everything works. Voltage is 1.8 because i dont have more space for one more turn. Tried today to weld 0.1 nickel and works very good with small MOT. Also i can weld 2 strips together without issues. Cant split them at all. Electrodes must be sharp and pressure must be good.

 

[flangefrog]

If you want some flexible copper wire have a look at these listings:
https://www.aliexpress.com/item/AWG-Silicone-Line-Ultra-Flexiable-Test-Line-Cable-1AWG-60-200-Degree-High-Temperature-High-Voltage/32877467918.html
https://www.aliexpress.com/item/High-Temperature-Wire-1-2-3-4-6-7-8-10-11-12-13-14-15/32875076705.html

6AWG is probably enough and works on my 12v lead acid spot welder
I used 25mm2 (3AWG) welding cable with the outer insulation removed (it was double insulated) for my MOT spot welder but it was very inflexible.

 

[krlenjuska]

6 AWG wire is not enough for MOT secondary. Simply there is no enough current. Maybe 25mm2(3 AWG) but im not sure. Remember current is important here more than voltage.

 

[john61ct]

If you want some flexible copper wire have a look at these listings:
https://www.aliexpress.com/item/AWG-Silicone-Line-Ultra-Flexiable-Test-Line-Cable-1AWG-60-200-Degree-High-Temperature-High-Voltage/32877467918.html

Is that $43 for one meter?

 

[flangefrog]

That's 1AWG. The lighter gauges are cheaper.

 

[krlenjuska]

If you want to pay 40$ for meter of secondary wire - you should buy kweld. MOT spot welder should be cheap.

 

[SnifMyBack]

Sorry for the very late reply...

So I'm happy to see all the support you guys provided! I appreciate it. I've tried differents solution to solve my issue without resolving it. So here my solution: go with bigger MOT.

As much as it sounds evident, it's THE solution that just work without any problem. I've found a used 1100 watts rated microwave. The sticker on the back was saying 1500 watts of consume power. I'm not sure that the MOT is really consumming 1500 watts of power but it work anyway... My welding time setup is 30 ms pre-weld / 200-250 ms pause / 50-65 ms main weld.

The next steps is too add detection circuit for the crossing point AC wave so I can make it more precise and consistant.

So all I can say for newbie, don't waste time trying to make a small MOT work, just get the biggest and remove it's shunts and everything will work flawlessly. 750 watts MOT just doesn't give enough power for 0.15 nickel plated steel.

 

[krlenjuska]

Nickel plated steel is much easier to weld - you should be able to weld it with your 750 MOT without any problems.
I can weld 0.15 pure nickel with my 700VA MOT. I guess your problem is secondary wire thickness or electrodes.

 

[SnifMyBack]

Nickel plated steel is much easier to weld - you should be able to weld it with your 750 MOT without any problems.
I can weld 0.15 pure nickel with my 700VA MOT. I guess your problem is secondary wire thickness or electrodes.

Well that's weird, because as soon I've switched to a more powerful MOT, it worked perfectly. 4 AWG wire seems plenty for me, so I'll guess that the way my electrodes are mounted is the problem.

Anyway right now it works without problem so I'll leave it like that, but maybe I'll try again with the old one.

Thanks for your input on the subject!

 

[krlenjuska]

Where in europe can i find C14500 tellurium copper for electrodes?