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three phase scr controller

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norberto fines

New Member
i have aquired a n engine driven welder with an scr current controller. The input is a three phase and then rectified with a bridge diodes. with three negative ground scr. The scr controller is lost. How to make a simple three phase scr controller?




norberto
 

john1

Active Member
Hi norberto,

Could you post more details about the arrangement?
I would think it more usual to control the field windings
on the generator.
Are you sure the SCRs aren't controlling the field current?

Is the welding current AC or DC ?
Is it a rod welder or a Mig/Tig type unit ?

Can you see the name of the manufacturer, or is this unit
a home-made assembly that somebody has put together ?

John
 

norberto fines

New Member
john,

this is my first time in forum. I wonder if my reply to you by using personal message is the right thing to do. or should i use the post a reply on the lower left corner. Any way this is the diagram:
 

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Lunatic

New Member
You need to control the phase angle / duty cycle of the SCRs. Basically, control at which point the SCRs should turn on.

The later in the AC cycle that you turn it on, the less average power will make it to the welding electrode. If it is an 80VAC generator, and you get 80VAC out of it when it's running under full load, you can build a circuit to put the voltage into a comparator(using rectifier and a voltage divider) and then set the turn on level by changing the 'reference' value of a potentiometer on the other half of the comparator. You should be able to use one potentiometer to control all 3 comparators with each comparator being fed from a different winding on the generator.

The other method would be to use a micro-controller to watch all 3 windings and do the same thing basically, I think it's too much effort for a phase controller.

At the most, since the SCR's will turn off as they drop past 0 volts on the way to the negative side of the AC cycle, you will be able to control your power between 50 and 100%
 

john1

Active Member
Hi norberto,

I agree with 'Lunatic'
control down to about half should be ok.

With just a triple pot, you will find that
you can control down to about half, then nothing.

I feel that i would prefer not to allow the pot
to go right down to nothing,
because one will always drop out first,
and one will always drop out last.

I would worry about overloading the odd one

I would put a limiting resistor in,
so that it goes down to nearly cutting out.

But thats up to you.

I assume this is what Lunatic had in mind,
it doesn't get much more simple.

Best of luck with it.
John
 

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john1

Active Member
Hey,
Sorry,
I made a mistake with that last drawing.

And i couldn't figure out how to remove it.

So heres the revised one,
Hope i got it right this time ...
John
 

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norberto fines

New Member
lunatic,
Is there any circuit designed to control such scr? Maybe I can make one if i have something to copy. I cant design it myself
 

fosterm

New Member
Hey noberto
From my experience with these units, it's much more easier to buy a burst firing control card from a used/scrap welding unit for a couple bucks or directly from miller mfg or similar. :D
They are generally all the same with a small 3ph xformer conn to the generator output, secondary winding conn to the anode's of three TO-220 7Amp scr's, on the control card controlling the 1000A-2000A scr's it may save you the stress, if you search for them on the internet.
 
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