Cable suitable for spot welding machine  - Page 1
Forum section: electronics / general

author: fventura
author_rank: Newbie
timestamp: October 30, 2021, 02:11:30 pm
content: HelloI'm not sure if this is a electronic or (probably) electric issue... But please, if you know the answer, give me a chanceI decided to try to make a spot welding machine from a microwave transformer.I saw several projects on youtube and I'm going to do a different one (like Frankenstein).I saw people using 30mm2 and 25mm2 cables in the secondary, with only one complete turn.I think that I don't need to generate as much current, so I used a 16mm2 with two turns (I also saw who did that). It got 2.2V output and should have 360A (I'm waiting for an amp meter to check).I'm going to install a voltage regulator and timer that will allow a maximum current of up to 1s (is it too much?)I need help with the following:I'm going to make 3 types of end tips for the machine: free (positioning wherever you want), fixed parallel, and clamp.Using the 16mm2 cable is not practical, as I want freedom of positioning the ends. BUT it also seems more than too much, since at most there will be current for one second (360A).How can I find the minimum section for a use of this type? (What is the proper break time?)Is there a formula I can use that tells me, for example, how long does it take for a certain cable to reach a certain temperature (in the conductor)?Thank you in advance for the help you can give.

author: jpanhalt
author_rank: Super Contributor
timestamp: October 30, 2021, 03:10:10 pm
content: @fventura,Welcome to EEVBlog.For continuous duty welders, there are tables relating gauge, length, and maximum current. For a spot welder, in a non-industrial setting (my guess), I used #4 AWG fine stranded (i.e., very flexible) welding cable. Mine is a CD welder and each is about 4' long. I suspect you could go smaller (e.g., #6 AWG), but I wouldn't go much lower because of the voltage drop at the high current expected.

author: fventura
author_rank: Newbie
timestamp: October 30, 2021, 03:42:48 pm
content: Thank you, it is not a formula but it is a pratical information. Less work hereI don't know ao long you use in each spot.But, if I need 1 second long (max), with wires with only 1 ft long, would be enough the #8 AWG wire? (In the transformer secondary I use #6 AWG, with 1/3 ft to the case)Once again, thank you for your assistance!

author: Doctorandus_P
author_rank: Super Contributor
timestamp: November 12, 2021, 11:03:00 am
content: combination of high currents and low voltage output make a lot of calculations difficult because of lack of data, and experimentation is the easiest path. Calculating wire resistance from wire length and diameter is quite easy, but the resistance of the spot weld itself has a high uncertainty. The coupling between the primary and secondary winding of the transformer is also a mostly guesstimate.For a battery spotwelder and those thin nickel strips you don't need much power.If you also want to weld steel plates (up to several mm thick) then the efficiency of the whole thing is a lot more important.Simple guidelines:For a battery spotwelder you've got plenty of power.If you want to optimize, then use the most copper for the windings that fit.If you want to use flexible leads then you can use short pieces of thinner wire for that, but they will heat up of course. If you know the resistance of the wire, the current and the time, then you can calculate the energy input. And the energy input translates easily to a temperature difference by deviding it by the thermal mass of the copper in your wiring.I recommend to use silicone insulated wiring. Those are generally the most flexible, but they can also take a lot of heat. I think silicone can go up to 220 celcius.Also, put some thought in the total form factor to keep the flexible wires short."XT60" is a popular and cheap high-current connector, and putting it's two contacts in parallel makes it an even higher current single wire connector. (And there are bigger versions of this connector type)If you want to weld steel then the mechanical part is also important and often overlooked.You want to regulate the clamping independent from the current switch and you need enough clamping force to push your plates firmly together. And you should keep them clamped immobilized as long as the welding spot is still red.You can easily test a good weld by pulling the plates apart after making a single spotweld. For a good weld you tear a hole into one of the plates if you separate them. If the weld itself breaks then it is a bad weld, even if you think it was "strong". Those bad welds can vary a lot in strength.