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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?
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%
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.
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.