MIG Welding, CC Inductor, and bypass thereof

This is in regards to an old Hobart CyberTIG welder that I modified by putting in a new microcontroller that I programmed in BASIC.
http://igor.chudov.com/projects/Welding/11-New-Rectifier/
Basically because I have a separate SCR controller, and separate module for controlling it, I can program it to do anything I want, to supply any voltage and any current, within my power limits, of course.
So it is basicallly an arbitrary DC power supply, with a few extra relays that control high frequency arc starting and gas valve.
This thing works just fine in constant current mode, letting me stick weld as well as TIG weld.
Note that it has a big inductor that helps with constant current welding.
Now that I am done with CC mode welding, I started exploring MIG welding, which requires constant voltage. Found a LN-25 wirefeed unit on Craigslist.
The issue at hand is that the output of my welder goes through a large inductor that is helpful in CC mode, but counterproductive for wire feed welding. This inductor weighs perhaps 50 lbs. (?)
So, for nice wire feed welding, I would need to bypass it. I see two ways of doing it:
1) Make a simple bypass circuit using a big contactor and big cable (to be turned on when MIG welding is being done).
2) Instead of bypassing the inductor, install a capacitor that would "cancel out" the effect of the inductor.
Can that be done at all, is this an insane idea or not? My hope is that it would be done with a smaller "work envelope", utilizing smaller wires and a smallish capacitor, instead of a bulky contactor and big wire, etc.
Any thoughts on bypassing the inductor?
thanks
i
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On May 14, 9:46 am, Ignoramus20083 <ignoramus20...@NOSPAM. 20083.invalid> wrote:> 2) Instead of bypassing the inductor, install a capacitor that would

In order for the capacitor to cancel the current going through the inductor, the current through the capacitor would have to be as large as the current going through the inductor. So a lot of capacitance and large wire.
Dan
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Not true actually, the inductor is there to smooth current fluctuations due to change in arc length, etc, and keep the arc going, so the capacitor would only see action when welding parameters change -- hopefully only a fraction of the time. I may very easily be mistaken, though.
i
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snipped-for-privacy@krl.org wrote:

Putting an L-C circuit with an arc, you might end up building a power oscillator. Look up "Poulsen arc transmitter" in the usual places. An arc can exhibit a negative AC resistance: this was used in the very early days of radio, to build CW transmitters before tubes were available (at that power level, anyway).
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Martin H. Eastburn @ home at Lions' Lair with our computer lionslair at consolidated dot net TSRA, Life; NRA LOH & Endowment Member, Golden Eagle, Patriot"s Medal. NRA Second Amendment Task Force Charter Founder IHMSA and NRA Metallic Silhouette maker & member. http://lufkinced.com /
David R Brooks wrote:

Before tubes was hardly radio. That was massive RF noise bursts hoping that someone got the interference. Tubes refined to bands and channels. Transistors to sub channels while tube and transistors had sidebands.
When high power is needed, tubes are still used. Solid state is catching up but RF power is still complex for solid state for big stuff.
Martin
-
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Ignoramus20083 wrote:

The capacitor cancels the inductor impedance, only at one frequency. I don't think this approach is feasible for this application. I would be thinking in terms of splitting the inductor winding and connecting the halves in series for stick or tig welding and in parallel (1/4 the inductance) for mig welding.
But you might just be able to short the inductor with a low voltage contactor, similar to what is used for cranking motors. As long as you don't open it during welding, you don't have to worry much about its voltage rating, and the current rating will be pretty conservative. http://www.mouser.com/catalog/630/1640.pdf
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Yep, you are 100% right, a cap won't work. I realized it too. :-(

Yes, I have a few suitable contactors, one three pole motor control type contactor with 65 amp rating on each pole and 110v coil. I would parallel all poles. I also have 24v coil, 400A rated DC contactors.
I collect contactors and motor controls.
Thanks. I will just do that bypass -- shoould not be a problem, it is simple work.
i
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Ignoramus20083 wrote:

Did you *ever* look at the parts list of a MIG/MAG welder before making that clueless claim? I bet not!

Oh yes, and voltage is measured in gallons, amperage in miles...
Nick
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Why not just install a separate output terminal not including the inductor for the MIG?
Don Young
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Don, that's another great idea, the simplest so far. The minor disadvantage is that it would not go along well with the polarity commutator that I have, but it is surely no big deal.
i
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On Mon, 14 May 2007 11:46:11 -0500, Ignoramus20083

Note that MIG/MAG welders still have inductance in the output, they just have less than stick welders. Decent ones also have variable inductance.
Can you rebuild the inductor to make part of the core movable?
Mark Rand RTFM
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Just peeking at the parts lists on lincolnelectric.com indicates the difference between CC and CV welders seems to be capacitance in the output circuit at a casual look.
I'd imagine the inductance would be smaller than that in a CC welder too, but adding in switchable capacitance should smooth output voltage variations to some degree.
The question, of course, would be: how much capacitance and how to wire it so you can switch between modes easily.
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Yes. A point was well made that the capacitance value would depend on "frequency", but since this is not something where there is a fixed frequency, we do not know the needed capacitance.
Practically speaking, I should simply bypass the inductor by adding one more output that bypasses the inductor. It is a $10 solution and is totally bulletproof. All I need it bolt a welding cable connector to a piece of thick insulation board and mount it into a hole on the welder, and connect it properly to the output of the SCR rectifier.
i
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On Tue, 15 May 2007 08:30:03 -0500, Ignoramus10518

The frequency issue was WRT "cancelling" the effect of the inductor, not really pertinent to your DC application. The function of stabilizing the output voltage against fast arc impedance fluctuations suggested by John is not the same and I recommend you give it serious consideration.

The suggestion of looking at how other machines do the CC/CV switch was also a good one IMO.
And don't forget the adjustable droop :-).
G
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Glen, I am a little lost, John talked about using only a part of the inductance?

Good idea. I looked at the Miller Deltaweld 451 manual:
http://www.millerwelds.com/om/o232s_mil.pdf
Please see page 23.
Its schematic is really very simple.
It has a three phase transformer, three phase SCR rectifier and a control board -- all like I have in my current welder. If you recall, I threw away the six phase rectifier and put in a regular SCR rectifier with a modern SCR controller.
Now look at the top right corner of the picture. The first thing you see is a fuse-protected capacitor bank C6. (part number 29, 6*16000uF). Then to the right, you see a transformer Z1, with its primary being the positive welding lead and secondary leading into "CR1". That "Z1" is called a "stabilizer" and is part number 22.
CR1 is a 24 VAC, definite purpose contactor (!)
I am not sure WTH this contactor is doing here.

G, I will definitely work on the droop and constant power welding in general. I need to make some changes that I am afraid to make. That is, make the voltage and current adjustment pots be inputs to the BASIC controller, as opposed to being inputs to the SCR firing controller (as it is now). I am afraid that a bug in BASIC may make the welder fry something or someone by outputing a lot more than it was asked for. Now, at least, I know that output will be no more than what I dialed.
i
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When the contactor activates, the second winding on the inductor is shorted, which will cause the inductance to become very low. Shorting a winding rather than the main winding allows one to use a relatively small contactor.
Joe Gwinn
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On Tue, 15 May 2007 13:13:55 -0500, Ignoramus10518

<clip>
Pardon the slow response, I have been in an internet free zone (Adirondak mountains).
Consider what is going on in the arc in GMAW welding: There are variations in arc impedance which occur on a fast time scale due to the motion of metal droplets within the arc, and there are variations on a slower time scale due to changes in average arc length due to the difference in wire feed rate and burnback rate.
As the wire gets closer to the puddle you want the current to increase and cause the wire to burn back faster; as it burns back you want the current to drop so that the arc does not get too long. Thus the wire feed rate determines the average current and the power supply voltage determines the average arc length in the CV mode used for GMAW - nominally.
On a faster time scale however a true constant voltage characteristic would cause problems due to excessive current fluctuation. Consider the 3 "modes" of GMAW: short circuiting mode at low power for thin materials where the liquid metal droplets form repetitive short circuits, droplet transfer modes at medium powers where fairly large droplets periodically break off and transfer without shorting, and spray mode at high powers where there is a continuous spray of fine droplets in the arc. (Not that there is a sudden transition between modes as power is increased, it is actually a gradual and continuous decrease in drop size as power is increased.) The variations in arc impedance due to droplet position in the arc are obviously worst in short circuiting mode and reduce in severity as power is increased. But in all cases you do not want to allow droplet position to have a big effect on arc current, which should change more slowly with average arc length - at a speed which cam be handled by your SCR controller.
The variations due to droplet movement are way to fast for your SCR controller to respond to, and you don't want the arc current to fluctuate that fast anyhow, so on the fast time scale you want the arc characteristics to approximate constant current. Hence an inductor.

It is there to vary the inductance of Z1, as Joe Gwinn already mentioned. When the contactor is closed the current induced in the second winding tends to cancel the flux produced by the main winding, hence reducing the inductance. Presumably CR1 is closed only in CV mode.

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Not easily, no, it is all well bolted down to the case of the welding machine. I thought that I did not need the inductor at all?
Esp. considering that I am rectifying three phase (smoother voltage)?
i
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Iggy sez: "Any thoughts on bypassing the inductor?"
Yeah. Whyn't you program (in basic of course) part of your new microcontroller as a switch to handle the inductive current.
Bob Swinney
This is in regards to an old Hobart CyberTIG welder that I modified by putting in a new microcontroller that I programmed in BASIC.
http://igor.chudov.com/projects/Welding/11-New-Rectifier/
Basically because I have a separate SCR controller, and separate module for controlling it, I can program it to do anything I want, to supply any voltage and any current, within my power limits, of course.
So it is basicallly an arbitrary DC power supply, with a few extra relays that control high frequency arc starting and gas valve.
This thing works just fine in constant current mode, letting me stick weld as well as TIG weld.
Note that it has a big inductor that helps with constant current welding.
Now that I am done with CC mode welding, I started exploring MIG welding, which requires constant voltage. Found a LN-25 wirefeed unit on Craigslist.
The issue at hand is that the output of my welder goes through a large inductor that is helpful in CC mode, but counterproductive for wire feed welding. This inductor weighs perhaps 50 lbs. (?)
So, for nice wire feed welding, I would need to bypass it. I see two ways of doing it:
1) Make a simple bypass circuit using a big contactor and big cable (to be turned on when MIG welding is being done).
2) Instead of bypassing the inductor, install a capacitor that would "cancel out" the effect of the inductor.
Can that be done at all, is this an insane idea or not? My hope is that it would be done with a smaller "work envelope", utilizing smaller wires and a smallish capacitor, instead of a bulky contactor and big wire, etc.
thanks
i
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If you add a capacitor, you are making an LC filter which may strongly affect your waveform. When you want a nice rising edge of current, you might get a sinusoidal pulse followed by ringing. I think the capacitor idea might be doable but it would require a ton of electrical engineering.
Grant
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