Invertor welder went bang

RF Lighting · 9 Jun 2020

I was doing some welding in the workshop today using a cheap invertor welder.

It’s just a 13A plug in stick welder job.

Anyway I’m welding away and all of a sudden there was a loud bang from the end of the welding rod and all my electric went off.

Where the bang had happened there was some orange staining almost rust colour, and what ever happened managed to blow the plug fuse, trip the C32 local breaker and also the C50 submains breaker, as well as welding the MK socket switch in the on position. Fortunately it didn’t take the cutout which I think is an 80A BS88.

I’ve not taken the welder in bits yet but I’m wondering what could have caused this? I guess a biiiig current must have flown to take out all that circuit protection. Possibly somehow the welder has presented full mains voltage at the output cables? I’ll have to measure IPFC at this socket, and see if it’s high enough to take the magnetic trip on a C50.

bernardgreen · 9 Jun 2020

Probably the isolation between the mains input to the invertor and the low voltage output failed thus putting the Live mains directly to the welding rod. A close shave for you.

JohnW2 · 9 Jun 2020

RF Lighting said:
Where the bang had happened there was some orange staining almost rust colour, and what ever happened managed to blow the plug fuse, trip the C32 local breaker and also the C50 submains breaker, as well as welding the MK socket switch in the on position. .... I guess a biiiig current must have flown to take out all that circuit protection. Possibly somehow the welder has presented full mains voltage at the output cables?

Frightening! Yes, as you and Bernard have said, that would seem to be the most likely explanation - or, alternatively, I suppose, simply a sudden L-N or L-E fault on the mains side of the inverter (for whatever reason) - but that would seem far less likely in view of what you observed at the welding electrodes.

RF Lighting said:
I’ll have to measure IPFC at this socket, and see if it’s high enough to take the magnetic trip on a C50.

Yes, that would be interesting, albeit rather academic, because we know that a C50 (and a C32) did trip magnetically. I suppose that a C50 can trip magnetically with 'as little' as 300A, and that would only need a Ze of ≤0.77Ω at 230V, and in the 'worst case' of 500A being required, that would 'only' require ≤0.46Ω - neither of which is beyond credibility.

It will be interesting vto learn what you discover - I wait with bated breath!

Kind Regards, John

Taylortwocities · 9 Jun 2020

May be safer to use that socket up in the loft?

RF Lighting · 9 Jun 2020

I’m not an electronics expert so I’m not sure what’s happened. Inside the only damage I can see is a little flash mark on these components.

They seem to be on a mains input board which supplies a bridge rectifier.

JohnW2 · 9 Jun 2020

RF Lighting said:
I’m not an electronics expert so I’m not sure what’s happened. Inside the only damage I can see is a little flash mark on these components. ... They seem to be on a mains input board which supplies a bridge rectifier.

No 'tracking'/damage on any of the PCBs?

Have you measured 'continuity' between mains input and the output of the inverter? ... and what about the L-N and L-E resistance at the mains input?

'Enough' PSC to magnetically trip a C50, at least!

Kind Regards, John

RF Lighting · 9 Jun 2020

No I can’t see any sign of any damage at all apart from what’s in the photos!!

No I haven’t checked. I will do and let you know.

Old Salt · 9 Jun 2020

Where is it stored? Could it be damp?

RF Lighting · 9 Jun 2020

No, no damp at all. It had been working fine for a bit before it spat it’s dummy out.

ericmark · 10 Jun 2020

With an inverter two things can cause high current, one the capacitor goes, and two it stops oscillating, as you know it turns AC to DC charges a capacitor and then turns DC to high frequency AC to allow a smaller transformer and smoother welding, if it fails to turn the DC to AC then the transformer becomes near a direct short.

RF Lighting · 10 Jun 2020

Just done some more looking. It looks like something has flashed over here. I thought they look like MOVs but the symbol on the PCB looks like a capacitor? I really don’t know enough about electronics

Also I’ve been at it with the megger and everything tests >200MΩ L>E and N>E on the input, and also L input > both output terminals, and N input > both output terminals.

Also I have noticed that the plugtop fuse did NOT blow, just the MCBs!

Also I’ve managed to give my self a nice dose of sunburn today while using my good old transformer welder

Harry Bloomfield · 10 Jun 2020

RF Lighting said:
Also I have noticed that the plugtop fuse did NOT blow, just the MCBs!

That often happens, MCB's respond much quicker than fusewire in a cartridge fuse.

magicmushroom666 · 10 Jun 2020

They do look like MOVs to me, should be able to identify the voltage rating from the side of the least burnt one. Cheap enough to replace. They short the supply by design to prevent voltage spikes, but I've found them to blow if the fuse doesn't trip soon enough.

I've repaired an inverter welder before and it was the output IGBTs which had failed shorted and had to be replaced.

davelx · 10 Jun 2020

They certainly look like MOVs to me, though as you say the legend on the PCB suggests they are capacitors, but I suspect that's not the case. I f you can confirm they are MOVs then this might be a suitable replacement

https://cpc.farnell.com/epcos/b72205s0381k101/varistor-13-5j-385vac/dp/RE03669?st=MOV

but check the quench current ratings of the original, a higher quench current might be needed. If so, look here:

https://uk.farnell.com/w/c/circuit-...-suppressors/tvs-varistors/prl/results?st=mov

JohnW2 · 10 Jun 2020

magicmushroom666 said:
They do look like MOVs to me, should be able to identify the voltage rating from the side of the least burnt one. Cheap enough to replace. They short the supply by design to prevent voltage spikes, but I've found them to blow if the fuse doesn't trip soon enough.

MOVs certainly often look like that (and, as you say, hopefully some markings might clarify) - although that obviously doesn't prove that they are! As you say, that could explain the trip, but it wouldn't explain what RF appeared to be describing as having happened at the business end of the welding electrodes - maybe that was just a red herring?

Kind Regards, John

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