380V from a 240V supply - Page 1
Forum section: electronics / general

author: artag
author_rank: Super Contributor
timestamp: June 28, 2021, 01:34:19 pm
content: I'm in the UK and have a standard single-phase 240V mains supply.I've got a spot welder marked variously as 380V (on the handheld welder) and 400V (on the dedicated control box). It is not, however, 3-phase : internally it's wired across 2 phases which wil give a single-phase 380V supply. The control box has internal wiring that can be reconfigured to 240V but the welding unit itself is fixed at 380.To run this from my 240 supply I'd therefore require a 240-380 transformer of 5kVA or so, and an outlet capable of supplying about 24A.I could also reconfigure the control box and underrun the welder by 30%. This might be adequate.However, it seems to me that I could replace the 240-380 transformer with a tool transformer configured with 240V in and with the 110V out boosting the 240. Furthermore, the tool transformer will only be providing 1/3 of the total output so can be a relatively cheap 3k3VA unit.I''m sure this sounds hairy but I can't actually see a flaw in it. What do you think ?(Note : I'm aware that the tool transformer has an earth at the centretap. The arrangement described would put this at 220+55 above neutral so it should not be connected to earth. I could connect the 110V winding to neutral rather than live and I would then have a 220-0-110 supply).I'm quite capable of connecting, testing and using this safely, but I'm wondering if I've missed anything.postnote :(I see there is a similar questionhttps://www.eevblog.com/forum/renewable-energy/converting-a-safety-isolating-transformer-to-an-autotransforer-for-460vac/which suggests it's OK, though the answers there have mainly concentrated on creatiung a 400x1.4 DC bus)

author: peteru
author_rank: Regular Contributor
timestamp: June 28, 2021, 03:41:33 pm
content: Jeebus! Would it not be simpler and cheaper to just get a spot welder designed for 240V?

author: mikeselectricstuff
author_rank: Super Contributor
timestamp: June 28, 2021, 05:00:59 pm
content: Using a 110v transformer to boost would be fine, but note that these are typically centre-grounded - shouldn't be a big deal as this can normally be disconnected.

author: artag
author_rank: Super Contributor
timestamp: June 28, 2021, 05:30:55 pm
content: Thanks, MikePeteru : lol, simpler certainly. But I'm notoriously cheap and decent spot welders aren't. And this one, although basically the usual hulking great handheld transformer with copper spikes for jaws, has duration and power adjustments which definitely puts it in a better class. One that I saw dismantled on youtube used a mechanically brushing contact to give a brief pulse. This has electronics and thyristors.

author: peteru
author_rank: Regular Contributor
timestamp: June 28, 2021, 06:00:21 pm
content: Quote from: artag on June 28, 2021, 05:30:55 pmThings filled with magic smoke!I'm all for fiddling with surplus stuff and reusing things, but when you start messing with hundreds of volts and amps, intended to melt metal in a fraction of a second, I'd take the safety inspired approach.Good luck with it.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: June 28, 2021, 06:05:22 pm
content: Yes, boost/buck configurations are fine. As an autoformer, they save space over an isolated (full VAs) configuration. The secondary must be rated for full load current. Do check the polarity with a meter before plugging in the load, don't want 140V accidentally instead.(Not that it would likely do anything this way!)Tim

author: artag
author_rank: Super Contributor
timestamp: June 28, 2021, 07:23:43 pm
content: Quote from: peteru on June 28, 2021, 06:00:21 pmAs always, the semiconductors will sacrifice themselves to preserve the fuse.

author: woodchips
author_rank: Frequent Contributor
timestamp: June 29, 2021, 11:22:24 am
content: A spot welder is a very intermittent load, try connecting a 2 or 3kW three phase motor across the 400V to provide an energy store. Also use a cap to start the motor, just measure the phase voltages and get it close to the same for each phase.I am currently making a single to three phase converter for a 6.5kW motor, which means the transformer needs to be rated at 16A. These are large and heavy!Have you tried it off 240V just to see how it works? Don't forget that over 50% of the weld power is lost in the synchronous reactance. Make the weld tongs as short as possible with the minimum loop area.

author: artag
author_rank: Super Contributor
timestamp: July 03, 2021, 09:39:12 pm
content: There are internal contactors and other devices that require some tap-changing for 240V. It's messy enough that I think it's worth testing on a real 3-phase supply and with the boosted one to see what the power capability is. If it's adequate for my needs it would be worth trying at 240 but I want a benchmark first.