Author Topic: 3phase to single phase conversion with transformer for equal line currents  (Read 1823 times)

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Offline ZeroResistanceTopic starter

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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
 

Offline sibeen

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I don't think you can end up with a totally balanced three phase supply. Have a look at the Scott transformer connection.
 
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Offline Brumby

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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.
 
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Offline IanB

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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?
 
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Offline ZeroResistanceTopic starter

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Thanks for all the replies.

I found some other transformers apart from Scott as suggested by sibeen.

1. Scott T
2. Open Delta
3. Le-Blanc

The 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.
 

Offline ZeroResistanceTopic starter

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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?

I 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.
 

Offline ZeroResistanceTopic starter

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The simplest method would be a motor-generator setup.  A 3 phase motor driving a single phase generator.

Thats a novel idea, I doubt if the manf will totally revamp his setup though.
 

Offline sibeen

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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.
 
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Online tautech

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OK, so have you monitored current draw or mains voltage sag when spot welding ?
How many amp supply is your single phase outlet ?
What mm3 is 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 switchboard could lift results to another level of performance and be substantially cheaper than any other solution.
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Offline IanB

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The story so far is I purchased a single phase 8KVa spot welder
...
Now I want to move to a 10KVa, 2 or 3 phase input transformer.

Firstly, 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.
 
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Offline ZeroResistanceTopic starter

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OK, so have you monitored current draw or mains voltage sag when spot welding ?
How many amp supply is your single phase outlet ?
What mm3 is the outlet supply cable ?

Firstly, 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.

What 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.
 

Offline ZeroResistanceTopic starter

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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.

Currently 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?
 

Online tautech

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OK, so have you monitored current draw or mains voltage sag when spot welding ?
How many amp supply is your single phase outlet ?
What mm3 is the outlet supply cable ?

Firstly, 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.

What 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.
As 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 6mm3 cable then just short sections of 2.5mm3 to 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 !  >:D
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Offline hussamaldean

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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
 


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