3-phase diode bridge - 6 separate windings - Page 1
Forum section: electronics / projects

author: Nix2
author_rank: Newbie
timestamp: September 21, 2024, 11:11:55 am
content: Hi.I have a old Bester Magster 160 MIG MAG machine, in which I will replace the main transformer.The problem is that the new transformer (first photo) has the simplest winding system, it has a secondary winding connected in a star (so only 3 windings), while the original transformer had 6 secondary windings (second photo) , connected to a specific diode bridge.And here I wonder - this is a very interesting, but non-standard and rarely seen circuit (Google is also silent on this subject) and the bridge diagram/circuit is hardly legible... Is this a modification of the classic 6-pulse, 3-phase bridge rectifier?Regards

author: schmitt trigger
author_rank: Super Contributor
timestamp: September 21, 2024, 03:26:11 pm
content: Would you have close up photos of the original transformer?The schematic unfortunately does not show whether the windings which I have highlighted in yellow are actually magnetically coupled to the main windings, neither do they show the dot (polarity) relationship.To me this appears like some sort of hum bucking winding. But without the missing information I can't be sure.Ome thing is obvios, though: the primary has fewer taps.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: September 21, 2024, 05:19:20 pm
content: From IRC:Code:Kuschelpony: if someone would forward this to the forum, that one circuit looks totally clear tome [link: this topic]Kuschelpony: its two star-connected rectifiers connected in series, with the pair of transformerwindings arranged that the single-ended magnetic flux cancels outKuschelpony: the clever part of the circuit is that it needs only 1 diode voltage dropKuschelpony: if you have further questions im happy to explain moreFor my part, it would also work if half the windings are independent chokes (or maybe flux linked too), but all six as windings is clever. Reminds me of the Circlotron amplifier topology.Tim

author: schmitt trigger
author_rank: Super Contributor
timestamp: September 21, 2024, 06:46:06 pm
content: Tim;now that you describe it, it starts to make sense, let me see if my poor brain has got it right.Taking as an example windings 114 and 188 are each rectified by its own individual diodes, and the resulting pulsating DC is fed in series to the auxiliary winding 117, which "fills the valley" left over from the rectification of the two other phases.Brilliant indeed.

author: Xena E
author_rank: Frequent Contributor
timestamp: September 21, 2024, 08:40:30 pm
content: Quote from: schmitt trigger on September 21, 2024, 03:26:11 pmJust my 2 cents.If, phase pairs serve the positive output through the auxiliary windings (snubbed), from the third phase, and this is done in an approximate 30% ratio to the main windings, it is indeed a buck winding... the diagrammatic arrangement would suggest that.It would serve to emulate averaging of the output voltage to simulate an approximation of a lower voltage six phase DC output.Reason?I'd speculate the OC voltage from the choke (L) wouldn't rise off load as much as if it were the only smoothing device.I couldn't find any online manual providing references eluding to special features for this machine, that would give a clue to this being the reason for the design.

author: schmitt trigger
author_rank: Super Contributor
timestamp: September 21, 2024, 11:43:23 pm
content: I believe that your 2 cents are worth at least 50.The key with this would be the ratio between the main secondary and the auxiliary windings, to obtain the necessary bucking ratio to work effectively as a ripple reduction.Very clever circuit indeed.Anyway, going back to the TS’s question, most likely the replacement transformer will result in a higher output ripple.

author: trobbins
author_rank: Frequent Contributor
timestamp: September 22, 2024, 05:25:16 am
content: Perhaps google 6 pulse rectification with six windings, and see if the circuit can be reconfigured to be the same as a common arrangement.Many industrial rectifiers and battery chargers included a choke to alleviate ripple current to suit the load application, especially if it avoided the need for capacitor smoothing.

author: Xena E
author_rank: Frequent Contributor
timestamp: September 22, 2024, 10:01:55 am
content: Quote from: trobbins on September 22, 2024, 05:25:16 amYes, choke input filtering is a good method, though I suspect my theory may be correct and the higher ripple from a standard 3ø bridge may cause the output voltage to be higher off load by the nature of the inductor.Whether important to the application, (Perhapsnotblowing holes in the work piece at the start of a weld run on thin material!), IDK.It's a complication that the manufacturers thought worthwhile,Yes, it is quite clever if so...Pure speculation on the limited evidence, I couldn't say for sure unless I had the transformer to inspect.X

author: Nix2
author_rank: Newbie
timestamp: September 24, 2024, 02:11:47 pm
content: This is what the windings look like, I have described them from A to F.

author: Xena E
author_rank: Frequent Contributor
timestamp: September 24, 2024, 04:32:07 pm
content: Thanks for posting the pictures.It kind of looks as I'd have expected, when you have installed the new transformer there may be some differences from the way the original performed, I'm convinced that it maybe not so able to handle thin material perhaps.It's a shame that there hasn't been anyone from the welding plant manufacturing industry chimed in, but line frequency transformer equipped gear is in the minority now.Good luck with your repair.

author: Wolfram
author_rank: Frequent Contributor
timestamp: September 25, 2024, 12:29:06 pm
content: Interesting topology on the dual winding one. The diagram is a bit confusing, but if I read the schematic correctly it's basically a 3x3 matrix halfwave rectifier sitting between two star-connected transformers. The diodes on the diagonal (U-U, V-V and W-W) are not populated, as they wouldn't be passing any current.

author: trobbins
author_rank: Frequent Contributor
timestamp: September 26, 2024, 01:53:05 pm
content: The image hosting with imgbb.com is not working for me.Typical secondary side rectifiers connect the diodes to one or two common nodes, such as one or both dc output polarities, in order to simplify heatsinking and load busbaring.In contrast, the above topology locates the diodes in between two WYE sets of windings, with each winding connecting to a pair of diodes, and the two WYE neutrals are made the DC polarities for loading. And the primary is a configurable floating WYE.I haven't come across that sort of configuration before, and it is not one of the normal configurations defined in IEEE standard practices. Perhaps it provided a simpler interconnection and diode heatsinking arrangement for that application.

author: Nix2
author_rank: Newbie
timestamp: September 27, 2024, 11:11:33 am
content: I have only just noticed that my copy does not have a factory-made... choke marked in red. There is one on the diagram, but I have found out that the diagram actually comes from a Magster 250 MIG machine, and my version is a Magster 160 (the semi-automatic looks the same, the transformer is wound the same, 12 ranges on the primary winding, characteristic 6 secondary windings). Unfortunately, I do not have a diagram strictly for my Magster 160 and I think it is impossible to obtain.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: September 27, 2024, 06:22:01 pm
content: Since a 6-pulse rectifier delivers uninterrupted DC, an output filter choke isn't terrifically important, and a mains reactor would do the same job. That or transformer leakage ought to be useful?Tim