Arc welder as cnc power supply for Series 1? Anyone try it?

Looking at HF Inverter Arc Welder - 80 Amp it says it has an open circuit voltage of 60 volts DC. Probably pulsed dc, but maybe not. Haven't looked at it on a scope. At any rate, it can put out up to 80A, way more than necessary, but that rating is for 35% duty cycle, anyway. At 20A or so, I'd think it would run continuously. It's a little nipper, too. Smaller than a lunch box, and $150. Runs on 120VAC. Might have to add some cazapitors to the output if it's choppy voltage. I'd also add a smooth-start circuit consisting of a timed relay and a resistor that lets the cap charge through the resistor for 5 or 10 seconds, and then parallels out the resistor. That's gonna be one big relay, too. Oh, a 30A automotive relay would probably do the trick. Parallel 2 sets of contacts and it would be fine. Looking for a way to power 3 or 4 Geckos on original Bridgeport 8A (run them at 7A) steppers. Anyone try this? Anything wrong with this picture?

cnc1000

New comment: Looks like their TIG welder # 66787 might be a better fit. 220V. Anyone try this already?

Quote:

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Originally Posted by cnc1000

Looking at HF Inverter Arc Welder - 80 Amp it says it has an open circuit voltage of 60 volts DC. Probably pulsed dc, but maybe not. Haven't looked at it on a scope. At any rate, it can put out up to 80A, way more than necessary, but that rating is for 35% duty cycle, anyway. At 20A or so, I'd think it would run continuously. It's a little nipper, too. Smaller than a lunch box, and $150. Runs on 120VAC. Might have to add some cazapitors to the output if it's choppy voltage. I'd also add a smooth-start circuit consisting of a timed relay and a resistor that lets the cap charge through the resistor for 5 or 10 seconds, and then parallels out the resistor. That's gonna be one big relay, too. Oh, a 30A automotive relay would probably do the trick. Parallel 2 sets of contacts and it would be fine. Looking for a way to power 3 or 4 Geckos on original Bridgeport 8A (run them at 7A) steppers. Anyone try this? Anything wrong with this picture?

cnc1000

New comment: Looks like their TIG welder # 66787 might be a better fit. 220V. Anyone try this already?

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I don't think it will work. What you need is a constant voltage power supply. Arc welders are generally constant current, which means that the voltage changes as the load changes.

What you need is a good old fashioned linear power supply. Big transformer, rectifier, and some capacitors. The hardest part will be to find the right transformer. You need about 35A at 60V, which is 2100W. With that much wattage I think I would run it from a 240V supply. If you want to try building something from scrap, I would look for old microwave oven or car battery charger transformers. Cut the high voltage windings off the oven transformers and rewind them with large magnet wire or copper strip. You may not be able to find a single transformer big enough, so you could split the motors between two power supplies. Or you could use multiple transformers in parallel for more current. If you do that though it's better if the transformers are identical so they share the load equally. You could even build a separate power supply for each motor. That might actually be easier to put together from junk parts.


C|

I have the original 3 phase transformer and all the caps. There is some confusion out there about voltage. Some people refer to the rms voltage and some refer to the peak. If I run the standard 3 phase transformer, what is the peak (or for that matter, rms) voltage that I should expect? Running Geckos, so can't go over 70VDC.
Thanks.

You mean use the 3-phase transformer with single phase? You might be able to get that to work, but there are some issues. The simplest answer to your question is, "it depends". It depends on how the transformer is wired, and also the rest of the power supply design is important. Do you happen to have a schematic for the original supply?

C|

Hi,
No, I would use it on 3 phase. You can't use a 3 phase transformer on single phase, so far as I know. I'd put it back in, drive it with 3 phase power, and have 3 secondaries, each with a bridge rectifier and a capacitor. No real gain in summing them, but I could. Otherwise, just run each supply to each Gecko. Running a 4th axis would mean sharing power with one of the axis, which would probably be fine, especially if it were the Y axis, because I would probably never ever use both at once. Yes, I do have the schematic for the supply, thanks.

cnc1000

A welder is going to give you noisy power with a large ripple voltage.

Why not just buy a simple single phase supply and be done with it?

I think this would do just fine: PS-15N70 - 1500W 70V Power Supply - AnTek Products Corp

Not appreciably more expensive than your welder idea and requires no tinkering with capacitors or anything...

That probably looks like a good solution. In sizing the power supply, I'm assuming 7A per phase, but I have two phases and that 7A is the rating for full step operation and in that mode, you always have two phases running 7A at a time. They commutate around, but always have 2 phases on. Since I will be running in chopped mode, which looks like a sine wave, the actual current would be the rms value of 7A, which is 5A per phase, roughly. So each of 3 motors would be taking 10A. If all 3 were running at once (unlikely), then I'd need 30A capability. Since I only expect to run 2 axis at a time at most, 20A would probably be fine, so long as there is temporary capability to exceed the 20A specification for the supply.
If someone has a better technical explanation, I'm all ears.

On the welder, I agree that the DC output is probably nasty, but not as nasty as you would guess, since it is an inverter technology, not simple transformer linear supply. I would put some large capactors on the output, anyway.
Thanks,
cnc1000

Since you're running stepper motors they will be consuming power even when stopped. So it doesn't matter how many axes you have moving, it matters how many you have powered up. If you plan on doing any 4th axis stuff, you will need to have all 4 motors powered up (assuming they're all steppers). I'm pretty rusty on power supply requirements for steppers, so I can't really give you much info as far as actual current requirements per motor.

And that welder will not work as a power supply. The problem is not ripple voltage. The problem is that it's a current supply and not a voltage supply. A current supply changes the output voltage in order to maintain a constant output current level. What you need is exactly the opposite. You want a supply that keeps a constant voltage and delivers varying current based on the load. Putting capacitors across a current supply will not make it a voltage supply.


Since you have the original power supply and 3-phase power available, why not power it up and see what the output voltage is? I'm assuming the original motor drivers are no good?


C|

Quote:

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Originally Posted by cnc1000

Hi,
so far as I know. I'd put it back in, drive it with 3 phase power, and have 3 secondaries, each with a bridge rectifier and a capacitor. No real gain in summing them, but I could.
cnc1000

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3 Phase rectification is quite a bit more efficient than 1ph where the ripple on 1ph is 100% and 120Hz, on 3 ph it is < 30% and 360Hz ripple, so less smoothing in the way of capacitors is needed.
As others have stated, a SMAW welder is constant current - variable voltage, if the voltage were to be maintained without collapsing as it does, it would be impossible to weld satisfactorily, TIG and MIG use a different technology.
If you have 3ph available but require a lower voltage, instead of one transformer, you could use 3 small 1ph connected together, a bridge rect. and small electrolytic and you have a supply.
(A 3ph transformer does not exist, it is really three 1ph on one frame!) ;)
Al.

I gutted the machine pretty much like you do a fish. The transformer has been removed, so it's going to take some fiddling to test that. I was able to get a nice supply working again, so my supply problem went away. The motor controls are all in boxes, now.

Interesting on the 3 phase transformer. I never thought much about it, but there is one common core to it so I thought there might be some reason for that. It's the usual E-core, but has another set of E's or similar. If they are actually independent single phases, then they must be arranged cleverly such that the magnetic fields don't interfere inside the laminations.

Yes, I'm sure the welders are current controlled, not voltage controlled. For cnc, lots of supplies are unregulated and droop down under load, anyway. With a welder that is rated for 100% dutycycle at 80 amps, I think I could reasonably expect it to behave itself at 20 amps. Yes, the voltage would droop some. How much? Don't know.

cnc1000

Im at a lose as to how your plan is .... I guess the main thing is the transformer of smaw would not be the issue in this pic? the transformer if secondary common tap for windings is for balancing of current in windings and the relationship of 120 degree phase rotation difference for 3ph.... the constant current and fluctuating voltage is dependent on loading and is controlled by circuitry after the transformer??? if this is altered it should do what ever you want constant voltage etc... if your using just the transformer, put what ever you want after it and good to go... if your leaving the circuit that is there now then I guess the main concern is that the voltage not increase more the 70 volts??? will your drives work with a tolerance??? if so what. At a stand still drives will most like reduced current after a time limit. The combination of all motors under use at same time and concern for overload would most likely not be a concern!!! as you would have to be hogging like hell...