Welder diagnosis - please

[Memmeddu]

Replacing any bad diodes won't be an option as they'll be pressed into the seat sinks, not stud mounted.
Plus they are best 'balanced', or more will likely fail.
I suspect the OEM assembly will be fearsomely expensive! You may find an economical solution using a generic rectifier, connected only to the the high output dinse socket that you need, e.g. https://www.ebay.co.uk/itm/1PC-DQFI...225620?hash=item3b45451494:g:vMgAAOSwAJxe1bme

Or you could make one for £5.

Replacment Rectifier for MIG Welder

I have a 250 Amp single pahase Mig Welder which had a diode short circuit in the rectifier. The replacement rectifier was available but...

www.boatdesign.net

That wouldn't work
We need a three phase rectifier

He needs three of those two connected in parallel between for the 190A side
Ad just One for the 100A side

1PCS MDS 150A 1600V Three-phase Diode Bridge Rectifier Module Board MDS150A L | eBay

Type:MDS 150A 1600V. Can be used to PWM inverter electric current input,battery charging,etc. Welding structure,excellent power cycling and temperature features. Size:L W H/80 40 33mm/3.14 1.57 1.29".

www.ebay.com

 

[OldRigger]

Ok iv'e taken a picture. I didn't exactly know where to put the voltmeter probes but moving them around areas A and B i was getting some AC voltage. Best i could get was about 32 volts

 

[Memmeddu]

Ok iv'e taken a picture. I didn't exactly know where to put the voltmeter probes but moving them around areas A and B i was getting some AC voltage. Best i could get was about 32 volts

View attachment 276193

Block A
That's where power arrives from the windings
These are inductors and they are on the AC side
Set your multimeter in V~
And check voltage between
1st and 3rd lug bolt from the left
1st and 5th one
3rd and 5th one

You should read way over 32V ,at least 70 or more volts between each of those connections

If you read lower values than 50 ish Volt AC ...
Disconnect the 2nd the 4th and the 6th cables from the left of your picture

This way you're going to insulate windings from the rectifier bridge
Check again voltages between
1st and 3rd lug bolts
3rd and 5th lug bolts
5th and 1st lug bolts

You should read around 50-70V AC
If you still read way lower values your windings are damaged

Try this way and let us know

 

[Memmeddu]

Edit you might need to disconnect the cables from the rectifier itself prior to claim the windings damaged

 

[OldRigger]

Thanks Memmeddu, I will have a look at the voltages this evening.

 

[OldRigger]

Ok, here are the results
1 - 3 61.6
1 - 5 61.8
3 - 5 62
I disconnected 2.4 and 6 but it made no difference

 

[the snooper]

One thing you should take into account with those voltages, is the machine running at the correct speed

 

[Memmeddu]

Ok, here are the results
1 - 3 61.6
1 - 5 61.8
3 - 5 62
I disconnected 2.4 and 6 but it made no difference

Those are fairly good voltages .
I'm pretty sure that rectifier bridge has to be replaced.
Windings should be good.
Now you can try to connect 1-3 and check voltages .
If they still roughly the same you might try to weld at roughly 60A
If it does work your diodes connected to 5 are gone
If your voltages becomes way Lower than 60V ,try again disconnecting 3 and connecting 5 .
And again if the problem still ,connect 3 and disconnect 1 .
That's to find which diodes are gone.
Remember prior to switch the welder off prior to connect and disconnect your connections

 

[Memmeddu]

One thing you should take into account with those voltages, is the machine running at the correct speed

It will but here we have doubled the voltage so diodes have surely failed

 

[OldRigger]

So I undo the number 1 lead and join in on to number 3?

 

[the snooper]

So I undo the number 1 lead and join in on to number 3?

Just test your rectifier and see how that goes

 

[OldRigger]

i'm not ignoring you all. Something has cropped up so I can't do anything on it for a week or so.

Had to do a covid test yesterday. Rubbing the swab on my tonsils made me gag nearly as much as when i got my jim'll fixit badge!

 

[OldRigger]

So iv'e removed the rectifier and checked the diodes. My voltmeter has a selection for diodes which i use for testing continuity. All the diodes are consistant in their readings 1 for one side and 420 ish the other, i'm guessing thats ohms. Now one thing I have found is one of the blue disc things had fallen off and was in the base of the machine (see photo).
1. Would that cause it to stop working?
2. What is it?
3. Where can I get a new one?
4. what specification do I need?

 

[Seadog]

The blue thing is not essential for the welder to work. It is a protection device. There should be writing on it that will allow you to find a replacement.

 

[OldRigger]

Well thats not good news! back to the drawing board.

 

[a111r]

That's a MOV.
It won't stop the rectifier working, it's only for electrical interference suppression.

Beaten to it... They often don't have meaningful markings. You could just extend the leads and re-affix.

 

[eddie49]

The values that you are seeing on the Diode Test range are probably "1." which means infinity Ohms, i.e. no conduction, and "0.420" which is the typical voltage drop across a forward-biased silicon diode, i.e. when it is conducting. In other words, all the diodes pass current in one direction and block in the other direction, so they are all good.
As Seadog said, you can ignore the blue thing for the moment. It is a Varistor, or MOV, or VDR - a voltage-dependent resistor, wired across the rectifier output, to absorb voltage spikes that might damage the diodes. If it has simply fallen off, you could solder it back on.