3phase to single phase conversion with transformer for equal line currents - Page 1
Forum section: electronics / beginners

author: ZeroResistance
author_rank: Frequent Contributor
timestamp: July 22, 2018, 04:53:29 am
content: Is there a way to convert 3 phase 400V input to single phase, basically the output needs to be used for spot welding, and currently a transformer with single phase transformer is not working out well due to mains voltage dips while welding.I would like to convert 3 phase to 1 phase with equal loading on the 3 phase input (equal current drawn from all lines). The power rating of the transformer would be 10kVA.TIA

author: sibeen
author_rank: Frequent Contributor
timestamp: July 22, 2018, 06:02:14 am
content: I don't think you can end up with a totally balanced three phase supply. Have a look at the Scott transformer connection.

author: Brumby
author_rank: Unknown
timestamp: July 22, 2018, 06:09:39 am
content: The simplest method would be a motor-generator setup. A 3 phase motor driving a single phase generator.An electronic solution would involve taking each input phase, rectifying them to a common DC point and then using this DC to drive an inverter.There was a thread on the "Dodgy Technology" board discussing a "digital electricity" product which would seem to do something of what you asked - but the person who was providing details tried talking to us in "marketing speak" and couldn't give any sort of straightforward answers to some key engineering questions.

author: IanB
author_rank: Super Contributor
timestamp: July 22, 2018, 06:22:33 am
content: If your output doesn't need to be AC, you could use a three phase step down transformer and rectify the output. If course, the size of the required rectifier might be make it uneconomical. On the other hand, how are you switching the current for spot welding? Maybe you could combine switching and rectification in the same device?

author: ZeroResistance
author_rank: Frequent Contributor
timestamp: July 22, 2018, 06:36:02 am
content: Thanks for all the replies.I found some other transformers apart from Scott as suggested by sibeen.1. Scott T2. Open Delta3. Le-BlancThe story so far is I purchased a single phase 8KVa spot welder but it wasn't welding 0.25mm GI strips although it was welding 0.5mm MS strips at full power.This was probably due to excess draw on the mains input and droop in line voltage (this is a guess).I want to move to a 2 phase or 3 phase input transformer so I can get good performance. I may have to ask the manf to redo the transformer and some changes to his thyristor power module.I don't know how many changes will the manf accept and how much time he would take to do it.Now I want to move to a 10KVa, 2 or 3 phase input transformer.

author: ZeroResistance
author_rank: Frequent Contributor
timestamp: July 22, 2018, 06:39:41 am
content: Quote from: IanB on July 22, 2018, 06:22:33 amI think the manf has some thyristor switching arrngement where he controls the cycles delivered for spot welding.?He had 2 knobs on this control box. 1 for % power and another for seconds / cycles.

author: ZeroResistance
author_rank: Frequent Contributor
timestamp: July 22, 2018, 06:40:37 am
content: Quote from: Brumby on July 22, 2018, 06:09:39 amThats a novel idea, I doubt if the manf will totally revamp his setup though.

author: sibeen
author_rank: Frequent Contributor
timestamp: July 22, 2018, 07:01:47 am
content: The problem with the Scott, or Leblanc, is that they are 3 to 2 phase connections and the two phases are 90 degrees apart. You can load up just one of the output phases but the input currents will no longer be balanced.I thought I had a mathcad document lying around on the computer where I had to calculate all the currents for a proposed job, and how the current balancing would work out under different scenarios. The job never came off and now I cannot locate the file.

author: tautech
author_rank: Super Contributor
timestamp: July 22, 2018, 07:15:17 am
content: OK, so have you monitored current draw or mains voltage sag when spot welding ?How many amp supply is your single phase outlet ?What mm3is the outlet supply cable ?All these things can impact on high current draw single phase machines.Just up-rating the supply cable or having a shorter run from the switchboardcouldlift results to another level of performance and be substantially cheaper than any other solution.

author: IanB
author_rank: Super Contributor
timestamp: July 22, 2018, 07:21:06 am
content: Quote from: ZeroResistance on July 22, 2018, 06:36:02 amFirstly, 10 kW should be very possible on a 240 V single phase mains supply. That is only 42 A. Maybe the first thing to investigate is to stiffen up your supply circuit? Have a dedicated circuit from the panel with suitably heavy gauge wire (e.g. 16 mm²) and keep the run as short as possible. Although 10 mm² might technically seem thick enough, it might have too much voltage drop for your application.

author: ZeroResistance
author_rank: Frequent Contributor
timestamp: July 22, 2018, 09:11:31 am
content: Quote from: tautech on July 22, 2018, 07:15:17 amQuote from: IanB on July 22, 2018, 07:21:06 amWhat both of you are saying makes a lot of sense, I didn't check either of those things when unit was spot welding. Also the socket outlet that I was using is located pretty far away from the main input, and may have had 2.5mm2 wiring. I could just see the lights diming off when the unit was spot welding.

author: ZeroResistance
author_rank: Frequent Contributor
timestamp: July 22, 2018, 09:14:25 am
content: Quote from: sibeen on July 22, 2018, 07:01:47 amCurrently I am planning to go in for a 2 phase system, I won't bother with the unbalanced current for now. So 10Kw on single phase is around 40Amps. Will this be distributed in a 2 phase system where each phase will draw 20Amps. Is a 2phase + 1 Neutral system a better idea?

author: tautech
author_rank: Super Contributor
timestamp: July 22, 2018, 09:23:11 am
content: Quote from: ZeroResistance on July 22, 2018, 09:11:31 amAs some example.....years ago before our overhead lines were up-rated, the best/strongest socket in a house was the one on the oven as normally they were fed with 6mm3cable then just short sections of 2.5mm3to the outlet. Simply it was like being plugged straight into the switchboard.Even now we see certain times of the day that power levels are poor and that's more noticeable on single phase than 3. When I had only a single phase welder, sometimes it was better to just postpone a job than struggle with a weak welder due to low power. Now I've got 3ph and 400A I can just wind more on and dish it out !

author: hussamaldean
author_rank: Unknown
timestamp: July 22, 2018, 09:54:04 am
content: IMO, the best way to do it is using three phase full bridge rectifier then an inverter to converter it back to AC. however, there is IGBT DC welding machine which can provide better performance compared to old technology.regards