Single phase / three phase

I've generally assumed that a device is single phase when I see "240 V" on the item's plate. Similarly, whenever I see "415 V" on the spec plate I assume the device requires three phase power. The following struck me as odd. Any thoughts?

I was taken by the spec plate on the side of this welder (415/440v single phase), which kind of destroyed my previous assumption. Facebook Another thing about this one, that you'd notice, is that the input current is very high.

Attachment 528942


For example, here's another welder and the spec plate makes perfect sense to me: Cigweld Transmig 320SP Welder Auction (0021-7045932) | Grays Australia The plate shows 415 V, and it notes three phase, and the final giveaway is the plug design. Simple and clear.

Attachment 528943

I'm going to guess the first one is designed to run off one phase of a three phase power supply. There's 415V between phases and 240V between each phase and neutral.

Welders have a coil that makes them a step up transformer so while you get say 415 volts out from 240 volts in it's still only a single 415 volts you need 2 more of those to make a 3 phase machine run and then they would need to be balanced

I'm just curious about this. I contacted the guy that's selling it, asking what kind of plug hangs off the end of it. That would have been a clue. But he said the plug had been removed. Damn.

It may have been hard-wired.

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Originally Posted by Camelot

Welders have a coil that makes them a step up transformer so while you get say 415 volts out from 240 volts in...

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Can you clarify this please? All welders I have used are step-down. I.e. voltage out is lower than voltage in. Typically 70V. I don't think 415V at the welding rod would be safe!

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Originally Posted by jack620

Can you clarify this please? All welders I have used are step-down. I.e. voltage out is lower than voltage in. Typically 70V. I don't think 415V at the welding rod would be safe!

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There are also 3 phase welders, so they will have 415 volts to deal with, the coil in a transformer type welder as a shunt which you move closer or further away from the primary and secondary windings (coils) to select the required Amperes which can range from 55 to 590

Hi all

You could risking death!!!!!!! Yes

A bit of history first as a retired electrical contractor.

I have seen these welders used before in rural setting in the Snowy Mountains of New South Wales.
The ones I saw were ex Snowy Hydro Models auctioned off after the construction of the Snowy Hydro Scheme had finished, most were purchased by farmers for hard-face welding and most have since been scraped for the copper inside. I think some of his equipment was in some way derived from ex World War Two military disposals, partictualy with regard to equipment used on the Snowy Hydro Scheme.

With the 415 volts AC single phase, opposed to 240 volts AC, the current needed to supply the welder is proportionately less and as such the supply leads can be longer and smaller and switch gear cheaper.
These welders were most often used when a Rural 11,000 volt or 22,000 volt S.W.E.R (single wire earth return) supply was only supply available to a site location.
The supply authorities step-down transformer would be a single phase supply centre tapped to provide usually 220/440 volts. The 240 volts used as a normal supply and the 440 volts used for large current demand machines such as large motors and welders.

To add to this story, while I was living in Jindabyne in the snowy mountains, about 25 years ago I purchase very cheaply a similar old 415/440 welder at a farm clearing sale, planing on refurbishing and using it or to just scrap for the copper. I never got around to restoring it partly because of the expense of installing a poly-phase supply and partly because I often don't finish projects. I passed it onto a semi-retired electrical engineer friend who had just purchased a farming property between Beridale and Dalgety, he had a split phase supply transformer on his property that supplied both 240volts to the old farm house and 240/440volts to the old shearing shed motor.
He later informed me that the welder worked very well producing deep welds in heavy farm equipment.

To answer your question you will need a polly-phase supply to operate the welder, either a two phase supply using both phases or a three phase supply using two of the phases. I note that the rating plate calls for 56 amp supply that's huge and the supply outlet, wiring and circuit protection would be hugely expensive.
I would guess that this welder would be approaching at least 70 years in age and I would be very suspicious of the insulation integrity of this welder without first having it high voltage insulation tested.

You could be risking death using that old welder and most probably the best thing would be to scrap it for the copper.