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Lincoln Welder Voltage Erratic

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Graveltech

New Member
I noticed there are some welder experts on here. Awesome site!

I purchased a used Lincoln CV 250 Mig Welder (same as CV 300) at auction. I made a big mistake and read the voltage selection guide, but didn't realize that it was 3 phase only. So, I built a 10 hp phase converter. Seemed to work, but as I learned to Mig weld, something never seemed right. I realized that the voltage was all over the map, but usually high. I could turn down the heat setting, and it seemed to work but then when the welder was shut off and back on again, the display would display a low reading (as set) but the output was much higher.

I realized that there was a right way and wrong way to hook up the three phases. One connection worked better then another, depending on where the manufactured phase was.

Anyway, I figured it was just a bad connection internal to the welder. But found none, and after cleaning all internal connections and checking for obvious problems on the PCBs, no improvement.

So, I started taking measurements and thought I was on to something. The tip voltage was much less then the displayed voltage. And yes, with the phase connected improperly, it was MUCH worse. I see that two phases at about 13 Volts are fed into the Voltage Control Board. So it is important, that the fake phase not be one of these. And by the way, for what it's worth, this is a Constant Voltage Welder.

Before I did anything else, I loaded the welder onto my truck and took it to a friends with a large 600 volt 3 phase generator, possibly to take my phase converter out of the equation. But alas, same thing. Now, I should add, I NEVER did take readings while welding, only no load readings.

There is a big cap and resistor on the output, by the way.

Well, I thought I was onto something. But before I left, I thought out of curiosity, I would measure one of their welders. It was another single phase lincoln, around 250 amps, but not constant voltage. To my surprise, it behaved exactly like mine, when comparing no load panel voltage to measured tip voltage! Oh, and I used a Fluke True RMS digital meter.

Now, I am totally confused and would like some input.

One other thing. The MOVs on the 600 volt line were blown up when I got the machine. They are totally absent, and I am curious what values to use. They must be different, depending on the chosen operating voltage, I would guess.

Thanks
 

Hayato

Member
Usually welders are constant current, not constant voltage.

The Displays shows the welding current, for example 50A, 150A ... and a maximun of 350A (your welder model).
And when you measure the voltage in the tip, yes, it will show low voltage values, because metals have low ohmic resistance.
Usually there will be no more than 40V on your welder terminals.
 

Torben

Well-Known Member
Usually welders are constant current, not constant voltage.
Usually, yes. Not in this case, though. It's a constant voltage machine.


The Displays shows the welding current, for example 50A, 150A ... and a maximun of 350A (your welder model).
The Lincoln CV-250, according to its manual, has a selector switch which allows you to choose whether to view voltage or amperage.

And when you measure the voltage in the tip, yes, it will show low voltage values, because metals have low ohmic resistance.
Usually there will be no more than 40V on your welder terminals.
I'm not 100% sure on this one but it would seem that you are describing a constant current welder, not a constant voltage welder. I'm more familiar with constant current stick welders than CV MIGs though.


Torben
 

Torben

Well-Known Member
Hi Graveltech,

Since you bought the machine at auction, I wonder whether you received the manual and service guide with it. If not, I think I may have found it: http://content.lincolnelectric.com/pdfs/products/navigator/im/IM505.PDF
Let me know if that's the right one.

Anyway, it includes schematics and a troubleshooting guide which might help you narrow down your options.

If you can't make out the MOVs you need from the diagram which is supposedly included inside the machine, then you should be able to contact Lincoln Electric and find out what you need.

If you've already gone through the manual and done all that, let us know and folks can start thinking further afield for the answer. We have a few users who may likely be able to help you out better that me. :)


Good luck!

Torben
 

Hayato

Member
Usually, yes. Not in this case, though. It's a constant voltage machine.




The Lincoln CV-250, according to its manual, has a selector switch which allows you to choose whether to view voltage or amperage.



I'm not 100% sure on this one but it would seem that you are describing a constant current welder, not a constant voltage welder. I'm more familiar with constant current stick welders than CV MIGs though.


Torben
Sorry. Now I see the "CV" in front of the CV-305 model, on Lincoln's Site...

Yes, I was describing a constant current welder, like stick or TIG.
 

Graveltech

New Member
Thanks Guys. I'm delighted at the response. I suspect that I will learn MUCH about welders before this is done, and despite the agravation, that is always the benefit at the end of the day! Then you die, taking all that wisdom and knowledge with you! Kind of a shame! Oh well!

Yes, the welder is a constant voltage model. Is this like a regulated power supply? If that is the case, I would think that the voltage would be accurately regulated under load or no load. How does that compare to taking the readings (as I did) on that non constant voltage lincoln mig?

On mine, by the way there is a panel meter that can be switched from amps to voltage. The tip voltage is always much higher, as it was on that other aforementioned welder. If I set my heat setting to minimum, my panel meter may read 9 point something volts and the tip voltage when the trigger is pulled reads twenty something. At 20 volts, I may get 40!

Now let me ask you guys something. How does a mig work anyway? I thought it was DC, but it isn't. . . . is it? A stick welder utilizes a carbon rod to strike an arc, but how is this achieved on a Mig? Shorting out DC with a thin steel wire should just short and stick to the work! Is it a case of high frequency? Please fill me in.

Lincoln has good schematics, (available in PDF, On Line) but not of the PCBs. A voltage control PCB with micro, one quad OP amp and a bunch of descreet transistors. Then there is a rather simple pulse PCB that drives the SCRs.

As far as the MOVs on the 600 volt line goes. They don't show the values, that I can see anyway, and I know the answer (or suggestion) if I call them!
 

smanches

New Member
Mig welders are DC. When the wire touches the metal you are welding, the added resistance right at the contact point causes it to melt and create a small arc. This is done repeatedly at a fairly quick rate, which is why the welder should have a nice buzz to it when it's working properly.
 

Torben

Well-Known Member
Thanks Guys. I'm delighted at the response. I suspect that I will learn MUCH about welders before this is done, and despite the agravation, that is always the benefit at the end of the day! Then you die, taking all that wisdom and knowledge with you! Kind of a shame! Oh well!
Well, let's not get ahead of ourselves. With any luck we'll all have a long time to enjoy the knowledge. :)

Yes, the welder is a constant voltage model. Is this like a regulated power supply? If that is the case, I would think that the voltage would be accurately regulated under load or no load.
Yes, "constant voltage" means that it's going to try its best to maintain the same output voltage regardless of the current draw. Obviously it can't do this for all cases as it's limited by its supply and components, but that's the general idea.

How does that compare to taking the readings (as I did) on that non constant voltage lincoln mig?
I wouldn't think it would be all that useful to try to draw useful comparisons between a CV and CC rig.

On mine, by the way there is a panel meter that can be switched from amps to voltage. The tip voltage is always much higher, as it was on that other aforementioned welder. If I set my heat setting to minimum, my panel meter may read 9 point something volts and the tip voltage when the trigger is pulled reads twenty something. At 20 volts, I may get 40!
I'm afraid I'm not really the guy to guide you on this. We have others here with much more experience in this area.

Now let me ask you guys something. How does a mig work anyway? I thought it was DC, but it isn't. . . . is it? A stick welder utilizes a carbon rod to strike an arc, but how is this achieved on a Mig? Shorting out DC with a thin steel wire should just short and stick to the work! Is it a case of high frequency? Please fill me in.
Someone else already explained the general MIG process (arc, burn off, arc, burn off , etc). I just wanted to correct one thing: stick welders don't use carbon rods, they use steel rods (for welding steel), aluminum rods (for welding aluminum), etc. Carbon rods are used for cutting or gouging, often with assistance from a stream of forced air to help clear the cut. In a pinch you can just crank the heat up way past what you'd weld at and use a regular rod. Messy, but it works. (Many times while learning, I have accidentally had my heat too high for the material and inadvertently cut it this way.)

Lincoln has good schematics, (available in PDF, On Line) but not of the PCBs. A voltage control PCB with micro, one quad OP amp and a bunch of descreet transistors. Then there is a rather simple pulse PCB that drives the SCRs.
I think a lot of the time if the problem is traced to the board, the shop will just swap out the board and either trash the old one or send it back to LE for repair.

As far as the MOVs on the 600 volt line goes. They don't show the values, that I can see anyway, and I know the answer (or suggestion) if I call them!
Calling them would probably be the best solution in this case. Not having used a CV-250 and not having experience with one, I wouldn't want to hazard a guess. Sorry about that.


Good luck!

Torben
 

Hayato

Member
On mine, by the way there is a panel meter that can be switched from amps to voltage. The tip voltage is always much higher, as it was on that other aforementioned welder. If I set my heat setting to minimum, my panel meter may read 9 point something volts and the tip voltage when the trigger is pulled reads twenty something. At 20 volts, I may get 40!
The welder will adjust the voltage according to the load.
It's the same as a the "Voc" in a Thevenin circuit.

And the voltage will vary before the tip to be the always the same on the load/tip. (You got that?) -> The arc is like a resistor, longer the arc, higher the resistance, and vice-versa, the welder has to vary the supply voltage to keep always the same voltage on the load.

And yes, 40V is a normal reading when you are not welding.


Now let me ask you guys something. How does a mig work anyway? I thought it was DC, but it isn't. . . . is it? A stick welder utilizes a carbon rod to strike an arc, but how is this achieved on a Mig? Shorting out DC with a thin steel wire should just short and stick to the work! Is it a case of high frequency? Please fill me in.
The most common and used is DC, but there is AC, as well.
Striking a arc is pretty much the same in all electrical welding processes.

When you do not have a HF ignitor, you have to touch the the piece to be weld and lift the electrode very fast, to get an arc. -> This is used in electrode welding (Like Torben said, you can have carbon electrodes to cut a piece, or shielded metal electrodes - to weld). And in Lift-Arc TIG process.

TIG and MIG usually have an HF ignitor, a high voltage and high frequency electrical current will flow from the tip of the torch to the piece to be weld when you pull the trigger (like those sparks from the oven), and this spark will ionize the shielding gas to form the arc.

The advantage of MIG welding is that the welding machine feeds automatically the filling wire. The opposite from TIG and Electrode, that you must always control the position/distance of the filling rod (TIG) or the electrode (electrode welding). And MIG as TIG, uses a shielding gas to protect the weld. (In electrode welding, the shielding gas comes from the burnt "cement" that shields the metal electrode).
 

Graveltech

New Member
Thanks All. The plot thickens. So . . . does EVERY Mig welder have a HF igniter? Failing this, I still don't understand why the filler (.035 wire) would not simply weld itself to the work upon contact and commence to heat to incandescence!

I also don't understand, why if a CV machine is like a regulated PSU, why it would be normal for the no load voltage to approach full output voltage.

As for carbon arc. As a particularly destructive youngster, I remember removing the carbon rods from D cells, sharpening them in a pencil sharpener and connecting them to a car battery. Then proceeding to destroy thin metalic objects. That and smelting Lead on the kitchen stove! Ah, the good old days! I always figured, that there was carbon in stick welding electrodes, to facilitate a good hot arc. Shows, how wrong one can be.

Sorry about spelling errors. My spell check vanished from the top of my screen, some time ago.
 
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Hayato

Member
Well, when you are welding correctly, you'll have a smal "pool" of molten metal, so your electrode won't stick.

Stick occours when you do not have that molten metal pool, if you have ever welded with electrode, then you'll already notice that sticking mostly occours when you start the arc (the electrode and the piece are not heated enough). It occours because the tip of the electrode is smelted, but the piece is cold, and then the smelted tip is cooled by the cold piece ---> Stick!

Whit HF, there is almost no danger of sticking, you do not need to touch the piece before you have a nice arc.

Graveltech, do you know the Thevenin theorem?
Almost every electronic system can be reduced to an ideal voltage supply and a resistence.
Now lets imagine that for your welder, the voltage supply = 40V and the resistence = 0.10 ohm.
If you plug a multimeter (internal resistance = 1 Mohm) on the tip, you'll read about 39.99V.

But now imagine an arc and a piece of metal, that you have total resistence of 0.09 Ohm.
Total resistance = 0.09 + 0.1 (this is from the internal) = 0.19 ohms.
40 / 0.19 =~ 210.5 A
Voltage at the piece + arc = 210.5 * 0.09 =~ 19 Volts.

Can you see that? When you load your circuit with a very high impedance load, you tend to read always the "Open circuit voltage."

Ofcourse, things are not so simple, your welding machine will adjust the internar resistance or the voltage so that it always remains the same on the output, while you are welding.
 

Graveltech

New Member
No, I've never even heard of it! Interesting stuff though. Thanks.

So how do I determine if my machine employs HF.

And secondly, how can I make a determination that the welder is working correctly? I've used it a few times over a year or so and it is erratic, but once you fiddle with the heat setting, you can eventually get a setting that works good. But it's lost, when the machine is shut off! And the display reading of voltage is virtually meaningless.
 

Hayato

Member
Most of the HF ignitors sparks when you press the trigger. (In some you can even hear a sound like a deffective TV flyback (a high frequency buzzing sound))

Have you ever welded (for real) before you had this machine? It's hard to say without seeing the machine.
Try to weld things that are easy to solder, like 1/8 metal plates, or something thickier. If you are trying to weld thin plates, then it's a hard weld to start.

After how much time of welding it shuts down? The best thing to do, is to weld with another, equivalent, machine, to see if the same thing happens.
 

Graveltech

New Member
I've done a fair amount of stick welding and used this machine a little over the last year or so. Just fiddled with it, until I could use it for the given task.

Yesterday, I was sooooo pissed, I have trouble describing my mood. the lesson? Never assume anything!

I figured out how to make an accurate test. Apply a one ohm resistor accross the output and measure the voltage rather quickly. The voltage read on my fluke, corresponded nicely with the panel meter. A resistance of 1/2 ohm would cause the filler wire to become red almost immediately, so 1 ohm gave me enough time to take a measurement.

I welded just a little and all seemed well, so I went and did something STUPID! I put everything back together, which is no small task. Then I welded somemore. For a couple of minutes, everything seemed fine. Then the heat shot up again! I had the machine set at about 20 volts, and with my measurment technique, measured 30! Exactly, the same symtom! If I shut the machine off and back on again, all is fine immediately. So, it doesn't seem to be a case of something heating up.

I had a long talk with Lincoln today, who were helpful and nice beyond belief! Most of our thoughts returned to an issue with the phase converter and possibly noise being generated and upsetting the microprocessor. He (Lincoln) thought, it might be freezing up the micro, but then realized this was not the case either, as I could still adjust the voltage.

One other thought of mine, was the possibility of an SCR conducting more then it should. This is rare, but has been known to happen and is difficult to diagnose. They will send me detailed information on how to test for this.

In the meantime, I will load the machine on my truck once again and connect it to a three phase 600 volt generator, actually weld with it and take measurements, again!

If this works well, then I will focus my attention on the phase converter, although I'm really in the dark there too. Lincoln says that running such a welder on a Rotory converter is VERY challenging for a number of reasons, but if you can pull it off, it works as well as on regular three phase.
 

tcmtech

Banned
Most Helpful Member
I suppose I should jump in here about now. :p

I worked as a service tech for a local welding supply shop and I am very familiar with the Lincoln line. I also build custom phase converter systems for applications such as what you are doing also.

The lincoln system looks at the incoming voltages between the phases as well as the phase angles and whats going out to the welding power side.

Most likely you are using the standard lame ass spin a three phase motor and hope it works as a rotary converter setup. The third line voltage it creates varries wildly with the load and solid state electronicaly controled welders can not run off of it without some additional components to stablize the voltage and phases. :)

These wide swings of both voltage and phase angles messes up the controller. You are basicaly having the voltage and phase go too far outside of the operating peramiters and the control system is getting messed up from it. ;)

One phase is the primary power phase for the actual control systems inside the welder. Thats most often L1 - L2 on the line conection.
That one MUST be the same single phase line as whats feeding the converter as well. Thats the only way you will get a stable enough referance voltage for the control circuits to work right.

The rotary converter for a welder of that size should not be any less than 15 hp or it wont be able to keep the second and third phase lines stable in both voltage and phase angle. ;)

You also must use AC power factor correction capacitors on the second and third phase lines to balance the voltage and phase angles. IF you are running on a 240 volt system they would have to be no less than 200 uf per leg, and 300 uf would be best. And a minimum of 370 Volts AC rated.

Your actual power draw is going to be around 100 - 130 amps at 240 volts when you have the welder turned up. So make sure your electrical system feeding it is actualy capable of running a load that big with no more than a 5% voltage drop at the welder under that level of load!

This should help get you started towards having a good solid welding system. :)
 

Graveltech

New Member
Thanks to all so far.

Well, It didn't take me long to realize that how the phases were connected made a BIG difference. And I may have mentioned above somewhere that two phases go into the voltage control board. So it's important that it isn't the FAKE phase.

The rotory converter is made with a ten HP 600 Volt motor. It's all I could get (free) and also a 230 - 600 volt transfomer. So the phase is made at 230 and put through the transformer. It does work well, I would say. It has two running caps, 20 MFD and 40 MFD.

The whole thing runs off a 60 amp breaker. Now, I, well not me but a friend that has grown up in a welding shop has used my machine to weld some pretty heavy pipe for me, like 14 inch heavy wall, and nothing quit or seemed lacking in power. In fact, my friend seems to run any welder he uses WIDE OPEN! With the problems I have been having, this was obviously not a problem for that job, and everything performed VERY well.

So, I don't know what to do from here, except for starters, check the welder back on good 3 phase power and take it from there.
 

Graveltech

New Member
Well, I finally tried the welder back on proper three phase and it worked perfectly. Very stable and output corresponds with display reading.

So now, I can focus on the rotary converter, but I don't know where to start. If the capacitor values are load specific, how can I overcome this?
 

tcmtech

Banned
Most Helpful Member
The load values are less critical. Its the output voltage, phase angle and power stability that makes it work or not work.
The problem I have ran into countless times with home made rotary converters is when a undersized motor can not maintain the amps load on the third line and stay within a reasonable phase angle and voltage. The third line voltage will drop off and the phase angle will drift one way or another but not stay at 120 degrees from the other two. It will drift and one phase will be 150 degrees and the other will be 90 more or less.
That phase angle drift makes the phase to phase voltage go way low on one phase and high on the other.

The purpose of the capacitors is to work as power factor correction devices by creating a LC tank effect on each phase. When its tuned to the line frequency the spinning motor rotor is what holds the three phases at the correct 120 degree separation.
The capacitors and the motor windings make up a pair of phase shifting auto transformers of sorts.

And like any motor or transformer you have to obey the laws of physics that determine the loads they can carry. :(
Wattage is wattage and compound loads take up more due to efficiency losses.

A properly designed rotary converter can run with a transformer and have little phase angle or phase to phase voltage drift but it will need to be bigger than the direct run type.

As I mentioned before given the estimated load and approximate amp draw I would start with at least 200 uf per side of the third line and I would not be surprised to see it be far more than that.
And given you are using a step up transformer between the converter and the welder you have to factor in that it will take even more power and more counter balancing capacitance. It has inductance that has to be counteracted in order to keep the over all system tuned properly.;)

I honestly would not try to build one for your application that was less than 15 hp. Other wise you will fry the motor!:(

And balancing the added inductance of the transformer could likely push the capacitor values into the 300 -400 uf per side of the third line.

I would be happy to help you any way I can. I do have much experience and understanding of how to do what you are attempting! :)
I have built dozens of converters just for people to do what your doing.

My first welder was three phase and I had to run a converter on it too.(and I still have it for a back up welder.) The standard designs that you will find on the INTERNET do not work for welder type loads. Most of them hardly work for motor type loads! :p
 

Graveltech

New Member
Thanks so much. I may well take you up on your offer of help. I guess, first, it would be interesting to know what values of caps I used. My notes appear lacking! Of course, the unit is nicely mounted and I cannot readly get at the caps. Why do things, nicely finished, always require more work?:( When the wires are flying in the breeze, things seem to work great! The odds of things not working seem proportional to the number of tye raps you used!

It's a hard thing to take readings when you have to weld and draw the required power. Not only that, but the 600Volt scares me. Maybe I should wire the welder for 230 Volt (3ph) just for testing purposes.
 

tcmtech

Banned
Most Helpful Member
If the welder has a 230 volt connection capability do it! The transformer is not helping the system efficiency.
I did think about it and your 10 HP should carry it under most circumstances. But it would be wise to install a simple thermal sensor or thermal switch on the motor windings and have it shut down the whole system If you get it to hot.
I have done a couple of converter systems like that before. I just used a bimetallic thermal switch from a old 160 watt fluorescent lamp ballast and epoxied it to the motor windings. Then used it to interrupt the main contactor coil power. I also put a small neon bulb in parallel with the thermal switch. That way when it did over heat and shut down it would light up the indicator to tell the person it shut down to protect itself.
 
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