3 To 1 Phase

[sparks]

I finally found some of my papers about phase conversion.
Maybe someone has already written about it, but heres what i found.

It's easy to get fooled by the magnetic flows i a 3-phase transformer.
Actually it works just like 3 separate transformers, one for each phase.
It's ok to make a 3-phase transformer by connecting 3 identical transformers in Y or D configuration.
In this case we want the secondarys D connected.
Quite a bit easier to see how it works, right?
Since the secondarys form a closed loop the load should be the same on all phases on the primary.
I haven't tried it, but that's the theory as i understand it.

[inventor30]

You are correct, It is worthy to note that you can get single phase from any 2 non grounded, non neutral wires of a 3 phase system. The voltage would vary depending upon if the system were delta or wye connected. Also the voltage to the ground or neutral on a delta connected system usually will be different on each phase, but the voltage to ground on a wye connected system will be the same on all phases. JC

[sparks]

In this case the point was to get one-phase ac from a 3-phase alternator to use for welding.
As long as its dc-welding theres no problem, but if ac is required (Al) one has to find a way to distribute the load over all 3 phases.
This way to solve the problem adds alot of weight to the alternatorwelder so if someone has any bright ideas i really like to hear them.

[Mike Kirney]

Can you not simply use all three phases in parallel? Since you are working with one single resistive load in a welding application, it would be evenly distributed across all three phases.

[larry cottrill]

Can you not simply use all three phases in parallel? Since you are working with one single resistive load in a welding application, it would be evenly distributed across all three phases.

Mike -

I'm no expert, but I think that would be asking for trouble - the reason being that it would be equivalent to shorting the phases to each other for most of the time in each cycle!

Three-phase basically exists for one reason - to provide an efficient system for powering large electric motors. It is true, as has been said above, that you can get single phase power from any two hot wires in a three-phase service, and in fact this is done all the time to provide ordinary wall plug power in buildings that have a 3-phase service coming in. The trick, though, is to try to balance the power as evenly as possible among the three 'pairs'. In a big factory with a huge 3-phase service, just running an individual welder off one pair wouldn't create much of an imbalance [besides which, it's fairly intermittent use], but the story would be different for a small 3-phase supply, where the welder would be a major load. A perfectly balanced 3-phase circuit has zero current in the neutral, and you want to run as close to that ideal as possible.

The real way to use 3-phase for welding would be to use it for exactly what it was designed to do: Hook up a properly sized 3-phase motor driving a DC generator, which would give you your welding current. Doesn't seem too tough to work out, and could be done inexpensively with second-hand parts.

L Cottrill

[sparks]

Actually, when it comes to DC welding with car alternators the best thing to do is to deltaconfigure the windings for max current capacity and replace the rectifier with external diodes capable of handling both the current and the higher OCV.
However, the question was how to get 1-phase AC (needed for Al-welding) from the alternators 3-phase output with equal load on each phase.

[Rossco]

I got all exited when i saw this back on the board.
Nothing has come up tho.
Yes Sparks, your on the track. that is what we'r after. Single phase true ac from an automotive alternator, with the abillity to change to DC.

Ive built several welders since we last spoke on the subject. All DC and work well enough. But what i want to do is build a nice small mobile unit that can weld anything with a tig torch. Which includes alloy, and that involves ac, and lots a current.

Ive been thinking of putting a system together with inline fields/stators, with one stator still carrying three fields and a common rotor. Not so small, or common parts tho. Its the simplest that ive come up with.

I must get into doing something allong these lines to see how i go.

By the way. Ive had no problem with the internal diodes handling the current loads, or voltage. Except when i ran direct half rectified field current. Everything melted down!. Something to do with the exponantial voltage gain i would think. One mean welder before it let out the smoke tho!

Rossco

[sparks]

A friend who welds al-alloys every day for a living is visiting over x-mas, maybe something useful may come out of our late night brainstorms.
At least he can add some do&dont´s when i comes to al-welding.

[Rossco]

Thanks sparks.
Interesting stuff.
BUT.
Ive since talked to one of my high power electrician mates, and he wont tell me! He recons that it will be so dangerous that it isnt worth the risk in encouraging me! ? spoil sport.
Rossco

[Fricke]

Hi!

Abit overworked perhaps... why not make an inverter?? Take the 3phase from the alternator and rectify it in a 3phase bridge to DC and then create 1phase AC like in the welding inverters?

// Fredrik...

[sparks]

Now that would be cheating, wouldn´t it?
Besides, theres not much point in using an electric motor driving the 3-phase alternator, rectify, chop it up to AC again and via a transformer get the volts´n amps at suitable freq.
There is a pricetag on high amp. HF switching components that makes it less fun too.
Old alternators on the other hand is lotsa fun since they cost next to nothing and are very robust.
Not to mention how extremely retro such a piece of welding equippment is.

I hope i´ll get some time soon to do more building and less talking....

[Fricke]

Now that would be cheating, wouldn´t it?

Nope... Not cheating but smarter!