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Old 07-09-2010, 09:42 PM   #1 (permalink)
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HELP - DIY AC conversion to steady flow DC

Hi ALL!! I'm new here.

My project is not directly related to electric vehicles (so please forgive me), but I'm assuming most of you have the vast knowledge of high amperage electronics that will make the completion of this simple DIY modification possible.

I'm trying to create a conversion for my Lincoln AC-225 arc welder, to add DC welding capability (as opposed to paying an extra $400 for a production unit).

Here's what I have so far, with the addition of a properly sized bridge rectifier, I can successfully convert the AC waveform to DC. But, the waveform is not steady like I need it to be (as seen on the right of the following image)...



For DC welding, the DC current needs to be as smooth as possible, otherwise the welder has to deal with constant arc instability (thus, a crappy weld no matter how great at welding the guy making the weld is).

I have read a lot about adding a capacitor to the output to "smooth" the current to a "ripple" (shown in the next image), but due to the voltage and amperage involved, an inductor might be a better choice. Please post your opinion if you disagree.



So here's my dilemma, I can't seem to be able to find any info (that I can understand) that would help me size the inductor coil to this application.

Here's the current specs:
48-53V @ 30-225A

The voltage is almost a constant.

The amperage is selectable on the AC side of the rectifier via a knob on the front of the welder. Available max amperage can be selected from 30A to 225A. While welding at whatever amperage is selected, the actual amperage can vary depending on many factors, but not to exceed the max amperage selected via the amperage selection knob.

So that's about it.
I could use your help to finish this project.


Last edited by ElectricSquid; 07-09-2010 at 11:30 PM..
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Old 07-19-2010, 08:31 AM   #2 (permalink)
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Capacitors don't worry about amps, just Volts.

48-53V caps are easy to find. The more of them you have and the higher the value, the better you can suppress the ripply in your rectified AC.

I build Vacuum tube guitar amps for fun, I have caps the can handle 500V in the last one I built.

I'd get some and try it.

Look at some TIG welder designs and see how they do it.
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Old 07-19-2010, 01:25 PM   #3 (permalink)
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I've done this in a past life. Forget the maths. Too much inductance is no harm. Find an old crappy arc welder , rip out the transformer , rip out the magnetic shunts and use the secondary as the inductor. job done. Other option , find a junk mig plant. They have output inductors.
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Old 09-27-2010, 02:45 PM   #4 (permalink)
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I'm with Jack on this one.

Sure general-purpose caps in your voltage range are easy to find, but you would need to get caps with enormous ripple-current capability, which would be prohibitively expensive. 5A of ripple for one capacitor is a lot, so if you are talking 200+ amps, that's 40+ capacitors at several bucks a pop. Ripple current isn't something they talk about much, but you can find more about that in caps rated for SMPS duty.

An inductor will smooth out the current nicely, it tries to limit the change in current. This means if you pulled away from the material with the arc still going, the inductor would provide a higher and higher voltage, in an effort to maintain that arc. This could make a long arc if you have a lot of inductance (which you will probably need), and if that's not an issue, get the inductor. Caps won't have that characteristic.

By the way, what crazy diodes are you using? I have some 120A units I thought were huge, yours must be enormous! *jealous* hehe
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Old 09-27-2010, 03:38 PM   #5 (permalink)
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..."rule-of-thumb" for inductor 'sizing' assuming 120Hz (fullwave or bridge, NOT halfwave) rectification:

L => 1100 / R

...hence, if R is very low, then L will conversely be very LARGE.
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Old 02-10-2011, 12:04 AM   #6 (permalink)
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Quote:
Originally Posted by ddunk80 View Post
?
...the original post (OP) is asking about how to "filter/smooth" AC-voltage into pure DC-voltage.

...there are three ways to do it: (1) capacitor (RC) filter, (2) inductor (RL) filter, and (3) combined capacitor/inductor (RLC) filter.

...capacitor filters can charge to peak voltage, but can't "hold" that voltage.

...inductive filters can provide a steady current, but can't "hold" that current for ever.

...combined RLC filter optimizes the two above and provides the best compromise DC-voltage, but there will always be a small amount of residual AC-"ripple."
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Old 02-10-2011, 12:05 AM   #7 (permalink)
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Quote:
Originally Posted by ddunk80 View Post
?
...the original post (OP) is asking about how to "filter/smooth" AC-voltage into pure DC-voltage.

...there are three ways to do it: (1) capacitor (RC) filter, (2) inductor (RL) filter, and (3) combined capacitor/inductor (RLC) filter.

...capacitor filters can charge to peak voltage, but can't "hold" that voltage, ie: BIG capacitor.

...inductive filters can provide a steady current, but can't "hold" that current for ever; ie: BIG inductor (choke, etc.)

...combined RLC filter optimizes the two above and provides the best compromise DC-voltage, but there will always be a small amount of residual AC-"ripple."
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Old 02-10-2011, 08:27 AM   #8 (permalink)
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Quote:
Originally Posted by jackbauer View Post
I've done this in a past life. Forget the maths. Too much inductance is no harm. Find an old crappy arc welder , rip out the transformer , rip out the magnetic shunts and use the secondary as the inductor. job done. Other option , find a junk mig plant. They have output inductors.
We used to have a wall of Miller 3ph welders. Done drooling ? Good.

They used an output inductor as well. I am thinking that a stuck rod would've blown a capacitor to bits.

Any good size xfmr should work. Tape off the input leads. Tie one output lead to the + from the rectifier. Tie the other output lead to the ground clamp wire. Tie the - from the rectifier to your stinger and your ready to burn some 7018++
(Yes, most rods run with + ground)

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