Question About SCR Control Miller TS250 Conversion to Single Phase

Lurked in the forum a while and converted a Miller TS250 welder to single phase about a year ago. It's worked great and I've had no problems with it. I have attached the series of photos of the conversion for how I did it. I followed the Haas-Kamp method as found on the Practical Machinist forums.

So now I want to make a change. I would like to use an SCR to control the voltage by a knob on my wire feeder, but I'm not exactly sure on which way to proceed.

I have a single-phase 10kW SCR that I can use in the system before the transformer and capacitors. It is undersized, since this system should run up to about 12kW, but should at least tell me if this will work. The question I am mainly struggling with is if I should completely avoid that and obtain a 3-phase SCR and install it between the capacitors and the transformer.

Any help, advice or berating is appreciated.

Thanks!

01 - Wiring as found:




02 - WYE Jumpers as found:




03 - Fan motor wires as found:




04 - On/Off switch wires as found:




05 - Delta secondary wires as found:

Below are the next photos from the conversion.

06 - WYE Connection of secondary after wiring change:




07 - WYE Connection of secondary with mechanical protection of connection:




08 - Capacitors with bleeder resistors soldered on:




09 - Capacitors wired in and mounted inside welder:




10 - Capacitors shown as mounted inside welder:

12 - Schematic of welder as found before conversion:



13 - Schematic of welder as completed after conversion. Colors in schematic correspond to the colors of the wires used as seen in the photos of the conversion:

EngineerTex, I just noticed this is a Miller CP-250TS conversion. So it worked well with 80uF on each side of the middle leg? How was short arc performance?
Also, not sure if you're still looking for a remote voltage option, but maybe it would be easier to use a motor on the crank assembly you control from the feeder?

What are Q1 Q2 Q3 on the output ?

Those are SCRs instead of the output diodes. They need to be triggered to conduct like a diode. Using those instead of diodes replaces the function of the output contactor.

jjohn76 said:

Those are SCRs instead of the output diodes. They need to be triggered to conduct like a diode. Using those instead of diodes replaces the function of the output contactor.

Click to expand...

That's what I thought I saw....So the OP wants to add SCR's and their already there.