3-phase diode bridge - 6 separate windings

[Nix2]

Hi.
I have a old Bester Magster 160 MIG machine, in which I will replace the main transformer.
The problem is that the new transformer (photo on the right) has the simplest winding system, it has a secondary winding connected in a star (so 3 windings), while the original transformer had 6 secondary windings ((photo on the left) , connected to a specific diode bridge.


The question is: is it possible to use (maybe with some modification of the connections inside the bridge) this specific diode bridge for the new transformer?

 

[eddie49]

I do not understand how the 6-secondary-winding circuit works, or why it was used. I never did learn any AC theory.
If you can access the individual diodes, and insulate any possible incorrect connections from studs to heatsinks, the original weird rectifier could be converted to a conventional 6-diode three-phase bridge rectifier, with three input and two output terminals.

 

[Nix2]

Of course I can do it, but I don't want to overwork myself BTW I think it's a 12-pulse bridge, but the diagram is not very intuitively drawn, and such circuits are not often encountered...

I think it was used because of the 2 x smaller ripple (more stable current) compared to the standard 6-pulse bridge.

 

[Barking Mat]

It's three phase, hence L1,L2,L3.

 

[Neilj]

This has intrigued me as I also don't understand how the 6 secondary winding circuit works! It looks to be something along the lines of a 12 pulse bridge but it's not that exactly as a 12 pulse bridge requires 12 diodes, not 6.
https://www.motioncontroltips.com/faq-what-is-a-pulse-rectifier-and-what-kinds-are-there/
I think it's trying to create the same outcome as the 12 pulse bridge, in that it's attempting to create a more stable DC voltage with less distortion. If you have a conventional transformer with 3 windings as a replacement then I would agree with @eddie49 and say that you'd need to start looking at splitting out the original diodes to reconfigure them as a standard 6 diode bridge.

 

[Nix2]

Yes, I have already removed the bridge and I will modify it to create the most common 6-pulse bridge circuit.

BTW, have you ever seen such a transformer winding arrangement?

And one more thing: The original transformer has one column (phase) shorted, so I can't connect it, but I was going to connect an oscilloscope and see for myself whether I see 12 pulses or 6...

 

[voipio]

I can't see how it's possible to generate 12 "pulses" from three phases with a simple diode arrangement, unless the three capacitors marked C4 are quite large in value (100s, possibly 1000s of microfarads, and non-polarised), sufficient to generate a significant (60 degrees) phase shift during load conditions. I suspect the capacitors are much smaller in value (perhaps @Nix2 could confirm, and are there to prevent the diode switching transients from causing RF interference.

 

[Seadog]

Does load affect the phase shift?
Surely, it's either a capacitor or it's not.

 

[Nix2]

@Nix2 could confirm, and are there to prevent the diode switching transients from causing RF interference.

That's exactly right, IMO they are just some kind of filters/anti-interference circuits.

 

[voipio]

Does load affect the phase shift?
Surely, it's either a capacitor or it's not.

Yes, phase shift is affected by capacitance, frequency and load.

 

[Nix2]

This is what the windings look like, I have described them from A to F.