how does constant volt welding power supply work

[drozone]

i know that the amperage changes with the wire feed speed and also during the weld by the size of the gap. the question is how does it know that the gap is different? the output is controlled by changing the pulse rate of the scr's but is this controlling the voltage or the amperage or both? in otherwords, what is the parameter that the voltage control board and the wire feeder board are sensing to know to change the amp output?

 

[Lewish]

OK, I may be a bit off on my info, but here is what I understand about wire feed welders - most are designed for use with CV supplies. Once you have established the arc, the current is fairly constant. Maintaining the constant voltage is fairly easy. It has a voltage sensor that just measures the voltage and controls it.

 

[drozone]

thanks, lewish,
but the problem is that the p.s is running away with the amperage. i. e. whatever wirefeed speed i run it pegs at 550 amps. it is controlled by a remote wire feed / voltage contraoller.
i've changed every board, relay and component in the loop(luckily the robot has 5 torches and i can swap parts)
i checked the wire feed part of the control and the length of wire changes with different feed rates in the same time period.
i've checked all plugs and cables.
the unit is a miller 451 controlled by a 1DW-1. pretty old stuff.
i figure that if i can find out how the controller knows the gap has changed maybe i can get somewhere. just throwing parts at it until it works doesn't work for me.
everyone responds the same way. with the stuff about controlling the voltage and the feed speed controlls the amperage.
no one out there yet has been able to tell me HOW the controller knows the amperage needs changing during the weld. resistance? inductance? current? it must feed back to a comparitor somehow.
thanks to anyone in advance who can fill in this blank for me.
drozone

 

[Crowbar1]

Sound to me like a possible short in your output lead. The wirefeed speed only affects the output current by varying the conduction time of your wire (electrode). If you feed more wire you conduct more and vice versa. All the while your voltage should remain constant (as per your setting)between open and closed circuit conditions.

Crowbar

 

[drozone]

thanks, crowbar;
the short, then, causing the controller to think the gap is too small? i hadn't thought of that. were you thinking in the feed system or the lead to the feeder? although i'm going to check both the feed system seems more likely to me.
thanks again
drozone

 

[Crowbar1]

I would look in the lead from the power supply to your wire feeder. Also when the current "runs away" does the voltage remain constant per setting or does it increase also?

Crowbar

 

[drozone]

crowbar;
the voltage stays the same. right around the 21 volts i have it set at.
drozone