how to create 415v single phase from @420/240V?

[John Paton 1Participant @johnpaton1]

I have been given a bandsaw blade welder off a three phase machine. The welder’s transformer is obviously single phase and designed to connect across two phases of a three phase supply.

 

I shall be using it on smaller blades than it is rated for so tried it on 240v, but that doesn’t give sufficient ‘grunt’ and the anneal function is not hot enough either.

I thought about a 110 to 240v step up transformer (the manufacturer of that transformer said he thought it might work if I isolate the centre tapping) but that trips the ring circuit even with no load attached so I am assuming it has too few windings on the primary to limit the current flow.

New step up transformers with adequate capacity cost way more than this thing is worth to me and I feel my 240 top 415v inverter won’t like being loaded in this way.

 

Can any of the knowledgeable electrical engineers here suggest a cheap solution? I don’t mind having a go at rewinding a transformer if that is an option?

 

TIA

John

[Robert Atkinson 2Participant @robertatkinson2]

Hi John,
It’s a bit dodgy, but if you disconnect the earth from the center tap of the 115V tool transformer you could then connect the secondary in SERIES with the mains supply giving you about 350V which may be enough. So the transformer primary (240V input) connects between supply live and neutral as usual. One end of the now isolated (because the center tap earth has been disconnected) 115V output connects to the supply live. The load connects between the other end of the 115V secondary and supply neutral. Don’t forget to connect tha supply earth to the welder.
There are two ways around to connect the 115V secondary. One will “Buck” and give you 120V out. The other way will “Boost” to 350V. You will have to try it ans see which is correct.

It’s dodgy because somone may try to use the transformer for it’s normal use and be electrocuted. To be safer completely remove the yellow 115V output connectors and label it as earth removed.

Robert.

[MacolmParticipant @macolm]

Robert Atkinson beat me to it, but you might find find something useful in the following.

In principle, the rated voltage of the primary winding of a transformer is the maximum that can be applied. The AC voltage causes a magnetic flux in the iron core, supplied by a “magnetising current”. An efficient design will seek to maximise this flux just short of saturation (with some safety factor) for the rated voltage, so excess volts will drive the iron hard into saturation with the consequential magnetising current increasing excessively. So applying more volts is a no-no.

What you might try with a site transformer (ie with a 110v isolated secondary) is to connect it in series with the primary, thus making it into an auto transformer with 240+110 volts thus available, unfortunately still only 350 volts. Many site transformers have a plastic case, but you would need to disconnect the centre tap from earth. Worth a try if you already have the transformer, but not worth buying one. A spot welder takes a large current for a short pulse, and the transformer may well further reduce the available power.

Another possibility is to look for a suitably rated transformer with a 240/415 primary which can then be used as an auto transformer. Finally, check if the spot welder transformer has perchance a 240 volt tap (possibly not connected to anything). There are various peculiar supply arrangements in the USA.

[John Paton 1Participant @johnpaton1]

Thanks Guys – makes sense and I very much heed your cautions.

Pretty sure 350v output would be sufficient to weld my smaller blades as the welder is designed for at least 3/4” blades. Most of mine will be 3/8” with the occasional 1/2”

I will try this but adopt my normal policy of feeding it off load through my variac initially to check the voltages produced.

There is indeed a spare tapping on the welder’s transformer and its value/use is not declared by the manufacturer. I can investigate that too while using a low AC voltage from the variac.

[MacolmParticipant @macolm]

A quick search turned up a list of electrical supplies in the USA, though some may be obsolescent. The 240/120v single phase seems to be a common domestic supply, but I suppose the welder would be made compatible with whatever 3 phase supply  the motor would use. I hope this is of some use.

240-volt 2-wire with end ground
277-volt 2-wire with end ground
480-volt 2-wire with end ground
240/120-volt 3-wire 1 phase with end ground
480/240-volt 3-wire 1 phase with end ground
416/240-volt 4-wire wye with grounded neutral
416/240-volt 4-wire wye with ungrounded neutral
416/240-volt 4-wire wye with grounded phase
480/277-volt 4-wire wye with grounded phase
480/240-volt 6-wire delta with one center tap grounded
480/240-volt 5-wire delta with one center tap grounded
480/240-volt 6-wire delta with one corner grounded
830/480-volt 4-wire wye with grounded neutral
120-volt 3-wire grounded delta
120-volt 3-wire ungrounded delta

[Robert Atkinson 2Participant @robertatkinson2]

The standard domestic and low power commercial supply in North America is 3 wire 120/240 V This is not delta (or wye). It is split phase with the center tap neutral connected to ground. The two phases “lives” are 180 degrees out of phase so each is 120 V to neutral and 240 V between phases. a 3 phase system has 120 degrees between phases. The american system is the same as the 55-0-55 V “110V” site tool supply provided by safety transformers but at higher voltage.

Robert.

[Chris Pearson 1Participant @chrispearson1]

You need to put a different transformer in the machine. How much power does it draw?

ETA: why is it “obvious” that the machine’s transformer is single phase?

[Chris Pearson 1]

On Chris Pearson 1 said

You need to put a different transformer in the machine. How much power does it draw?

…

I probably agree!   What voltage is required by the welder?  I guess about 30V and a few tens of amps.   A lash-up to get up to 400 odd volts to work an existing transformer doesn’t feel like ‘best practice’, but hey, I’m a cowardly lion.

I’m surprised the 230/110V transformer doesn’t work off-load.  What’s its rating and how exactly is it wired?  Something wrong perhaps.

Macolm’s comments about saturation didn’t ring any loud bells with me.  Might be because it’s a long time since I swotted up on transformers, or maybe I misunderstood the book in the first place.   Wouldn’t be the only example…

Dave

 

[Robert Atkinson 2Participant @robertatkinson2]

The transformer in a band welder will be a custom item and almost certainly designed with leakge inductance to limit the maximum current. Replacing it is unlikely to be practical.
As I read between the lines the supplier sold the OP a 230-115 V step-down isolation transforer with the expectation that he could run it “backwards” as a 230 to 460 V step-up. Of course the core saturated when 240V was appled to the 115V winding.
The “correct” solution would be to use a 240 V to 415 V transformer either auto or asolating. The only one I found with a quick search is this https://www.cef.co.uk/catalogue/products/1906697-400va-230v-415v-to-12v-230v-ac-ac-multi-tapped-panel-mounting-transformer?
It would need to be run backwards but does have a suitable (240 V) winding.
Using the 115V isolation transformer the OP already has in a boost configuration seems a reasonable compromise.

[John Paton 1Participant @johnpaton1]

Thanks all for the further advice.and suggestions.

To answer Chris’ question first – the welder only has two wires so no mechanism to connect to a third phase. Strictly it is connected to two phases of 220v but that’s the same as single phase at double the voltage.

I was given the welder, it came off a Startrite 3 phase bandsaw and comprises a 415v welder with custom made transformer with tappings on the primary winding to give the required welding current (ie varies the voltage to give that current) to suit different blade widths. (1/4” up to 3/4” blades)
The cost of a replacement ‘special’ transformer or a 240v one from the German manufacturer (they do make that option I believe) would be way outside my budget as would a custom wound step up transformer or import from USA.

 

I had been thinking along the lines of an ex equipment transformer if they are available cheaply or ‘abusing’ a readily available site transformer f that would step the voltage up from 240v to 415v required.

The plate suggests that power is only about 6 amps at 415v so a 3kva unit should suffice given that the welding cycle is only a couple of seconds.

The secondary ‘welding’ winding is a very heavy flat copper strip about 10mm wide and I suspect welding voltage will be somewhere around 5v and a current of maybe in the order of 100 to 150amps.

The saw blade fuses with heat from current flow rather than an arc, with the blade reaching bright yellow heat.

The mechanism lets the two blades move towards each other under spring tension and power is cut off when sufficient material has ‘fused’ together.

You the grind off excess thickness along the weld line and then put back in the clamps and reconnect at lower power to gently heat the blade to bring the temper to the correct blue colour.

[MacolmParticipant @macolm]

Assuming this is a resistance welder, the voltage of the secondary will be of the order of one volt, and the current in the thousands of amps. That is, the power throughput will be several kilowatts for a fraction of a second. The transformer secondary will be one turn only, consisting of bundled copper strips perhaps 0.1mm thick, 15mm wide and the bundle several mm thick. This bundle needs to flex when clamped to the saw blade each side of the break when the weld is formed.

The blade is clamped with the two sides of the break abutting, and pressed together by a strong spring force. To make the weld, a timed pulse is passed through the break in the blade, and the material at the break is heated to melting point, and fused by the pressure. After cooling, a lower current is used to heat the join to annealing temperature. The flash is then ground back to give a uniform cross section.

The transformer is rated for a low duty cycle, perhaps 5% or less. In a similar garage spot welder, the transformer is soon very hot after a few welds, and is not suitable for fast continuous use. Welders in a car plant are liquid cooled. A band saw welder benefits from being able to cool during the longer time between welds.

Thus the transformer is a specialist design where the flux in the core is pushed as high as possible so as to need fewer turns, which can be thicker wire. A voltage changing transformer would almost certainly need to be bigger in overall size than the welder transformer itself. The welder transformer primary could possibly be rewound, but this is unlikely to be a simple matter. The wire will probably be square or rectangular section, and needs to fill the space to preserve the power rating.

[John Paton 1Participant @johnpaton1]

Spot on Malcolm – well described. Welding voltage and current obviously a good deal lower and higher than I had guessed!

with a voltage at that level it is clear why surface contamination will interfere considerably!

Maybe if I really clean all contact points I may be able to weld small blades even with 240v input if using the maximum blade size tapping?

[Chris Pearson 1Participant @chrispearson1]

Thank you.

If the transformer and so on came from a 3-phase machine, how do you know that it was connected line-line as opposed to line-neutral?

[Robert Atkinson 2Participant @robertatkinson2]

John,

If you use a “boost” transformer it does not have to be rated at 3kVA. If the load is 6A the secondary of the transformer needs to be rated at 6A. So if using a 115V “site” transformer that is only 690VA. Given the low duty cycle a 500VA transformer should be adequate.
See circuit (if image post worked)

Robert.

[MacolmParticipant @macolm]

A useful feature of spot welders (and friction welders) is that once the weld line becomes liquid, the pressure ejects the original surface layers, and the weld forms from clean metal. However, sometimes it is difficult to get enough contact to start the current, but this rarely impairs the final weld.

As Robert says, a boost transformer only supplies the excess fraction beyond the mains part, so a site transformer will be adequate. Given that the welder uses taps on the primary to vary weld power, there seems a good chance of success with lighter blades even though the applied voltage is a bit low.

Incidentally, I started typing into the reply box, and eventually the page just disappeared, loosing the text. Lesson relearned – prepare the text in a reliable word processor and paste it in.

[Chris Pearson 1Participant @chrispearson1]

I am still struggling to understand the design voltage for this device.

If it trips the MCB with no load, could there be a short?

[Robert Atkinson 2Participant @robertatkinson2]

It was connecting a site transformer backwards that tripped the MCB.

[Stuart Smith 5Participant @stuartsmith5]

John

Have you tried running it from the 240v single phase supply? I presume you have, but I couldn’t see any mention of this      

Sorry, just noticed you said you had tried this.

The other thing about using a site 110v transformer to add to the existing voltage is that the instantaneous current might be high and cause the output voltage of this transformer to dip considerably. (Assuming that the welder works like a spot welder)

Stuart

[Stuart Smith 5Participant @stuartsmith5]

John

This company do 420v to 240v transformers at ratings up to 10kVA.

https://www.ete.co.uk/product/transformer-420v-to-240v-50va-multi-voltage/

They are not cheap, but presumably you could use one in reverse.

Stuart

[MacolmParticipant @macolm]

These are isolating transformers, and no doubt would work with primary and secondary reversed. However, this does mean the transformer capacity must be the full requirement of the spot welder. The “site transformer” arrangement suggested only needs to supply roughly one third of that much.

You are correct that any transformer used to boost the voltage will absorb power, and it needs to have enough rating that the welder still works correctly. Use of a welder such as this is very low duty cycle, and no doubt it will be designed with this in mind, and thus unsuited to continuous use. The boost transformer could be similarly rated, but this would  need a non standard higher output to the welder to get the correct voltage under load.

[John Paton 1Participant @johnpaton1]

Thanks guys and top marks Robert and Malcolm – I took your advice and with careful steps running the voltage up gently using my variac at each stage I was able to determine the correct phasing of the two windings. The saw blade welder works a dream at 380v!! (All clamping surfaces need to be spotlessly clean)

not sure how to attach a photo of the ‘unground’ weld but I will try and upload it for interest.

again huge thanks to all for helpful and encouraging input.

 

I can now use my bandsaw again!

[Robert Atkinson 2Participant @robertatkinson2]

Hi John,

Thanks for the update and glad you got it working.

Robert.

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