cas@mtcchi.uucp (2668-Charles A Sherwood(Z550900)0000) (10/11/90)
Hello people Is there anyone out there that has build a welder using a surplus aircraft generator. I have acquired a 400amp 30 volt aircraft generator and I am considering building a welder out of it. Burdens Surplus also sells surplus aircraft generators for this purpose and they provide plans, which I have a copy of and it goes like this: |------------| ||-|generator |A-------------arc stablizer-------- |------------| | work | F | gnd--- Gnd | rheostat | | ----------- The rheostat is used to control the welding current. The arc stablizer prevents the arc from burning the generator's brushes and provides a smooth weld. This has lead to a few questions. 1) Is it worth all the effort? Will it work as well as a regular welder? 2) An arc stablizer is a large coil of wire. Burdens sells them, but can I make my own? What is the correct wire size? How much wire is needed? What is it wound on? Iron or air core? 3) How much horse power is required to drive the generator? If I use a 5HP electric to drive it(5hp motors are cheap and available). 5HP x 746 watts/HP = 3730 watts Assume 70% generator efficiency, 3730 x .7 = 2611 watts. 2611 watts /25 volt = 104 amps of welding current. IS this enough?? or is a larger drive motor required? A larger electric drive motor would not be cost effective. A 7.5HP single phase electric motor retails for about 400, which would by a nice welder, with accessories. A 8HP gas engine would be an alternative, but is a little noisy. 8HPx 746 x .7 / 25 = 167amps (more than enough) chuck sherwood
jgd@rsiatl.UUCP (John G. DeArmond) (10/11/90)
cas@mtcchi.uucp (2668-Charles A Sherwood(Z550900)0000) writes: >Is there anyone out there that has build a welder using a surplus aircraft >generator. I have acquired a 400amp 30 volt aircraft generator and I am >considering building a welder out of it. >Burdens Surplus also sells surplus aircraft generators for this purpose >and they provide plans, which I have a copy of and it goes like this: >The rheostat is used to control the welding current. >The arc stablizer prevents the arc from burning the generator's brushes >and provides a smooth weld. >This has lead to a few questions. >1) Is it worth all the effort? Will it work as well as a regular welder? Jeez, you're bringing back old and somewhat horrible memories. The short answer is No and No. Sell the generator and spend the money on a nice DC buzz box. It'll weld better and you'll have time to actually weld instead of futzing with this thing. Before I explain why these things don't work well, let me quickly outline what happens when you stick weld and what characteristics a power supply must have. The process of stick welding is actually quite complex and very dynamic. To pick an arbitrary point in the cycle, assume the arc is struck, a weld is in progress and we want to take a series of snapshots. An arc is flowing in the plasma formed by the ionized air and the vaporized flux. A drop of molten metal forms on the end of the rod and as it grows to about the size of the flux coating, a combination of forces propels it toward the work. As this drop travels across the gap, it momentarily shorts the electrode to the work and extinguishes the arc. Then the trailing edge of the drop breaks with the rod, the arc reestablishes and the process repeats. Now let's look at the electrical requirements. When the arc is flowing, the power source must supply whatever voltage it takes to sustain the arc at the designated amp level, normally 15 to 50 volts or so. As the drop of metal starts flowing across the gap, the gap is narrowed and thus the impedance of the arc decreases until at the moment of short circuit, the impedance is zero. At that point, only parameters outside the arc environment limit the short circuit current. This job falls on the power source. If the power source is not current limited, for all practical purposes, an unlimited amount of current will flow during the short. This has several very negative consequences. The strong magnetic forces involved will tend to distort the weld puddle and blow it and the plasma out. And as the short breaks, the extremely high current cause a violent arc to establish which forcibly blows much of the filler metal out of the joint. To address this problem, stick welding power supplies are designed as constant current sources. In other words, the power supply will supply its setpoint current into a wide variety of loads ranging from a short circuit to an established arc. In reality, most power supplies are not perfect constant current sources but a range of variation as much as 2:1 is usually not noticed by the weldor. For inexpensive line operated welders, this constant current characteristic is obtained by designing the transformer for high flux leakage and quick saturation. For more expensive sources, the regulation is done with a combination of magnetic amps and electronics. For your basic generator set, the constant current characteristic is achieved by designing the generator as a compound differential generator. In this scheme, the field consists of both a series and a parallel winding and they are connected to buck each other. Therefore as the load increases, the field actually become weaker as the series winding bucks the parallel winding and as a result, there is less voltage to force current through the circuit. This extremely poor voltage regulation is just what the doctor ordered for welding. The second major consideration is arc stabilization. Because the weldor's hand is not perfectly steady and because local conditions in the arc can vary, the voltage necessary to restrike the arc after the metal drop breaks the short is highly variable. It is desirable not only from a restrike standpoint but also from an initial strike standpoint to have a fairly high voltage available instanteoutly. This is normally achieved by including some excess inductance in the output path. This inductance induces a "kick" voltage on top of the transformer/generator voltage as a function of the dI/dt through it at the moment of break. For basic line operated welders, this inductance is typically built onto the transformer core but magnetically isolated from the main winding. For generator type welders, the inductance is typically in the form of a stabilization coil of the type you mentioned in your post. Generally the more inductance the better and I've actually added more inductance to marginal welders by winding a toroidal inductor from copper tubing placed around the stator of a fractional hp motor and insulated with fiberglass tape! Too little inductance makes the arc hard to start and very hard to maintain. Now let's analyze the generator you have. Your generator is most likely a shunt field generator which means the field consists of many turns of fine wire, draws relatively little current and operates connected across the armature output. This configuration is designed for good voltage regulation - exactly the opposite of what you want in a welder. The aircraft system required a regulated voltage not only for consistent operation of its loads but also for controled charging of its batteries - just like a car. What you'd see if you build a welding power source from this unit is an arc that is impossible to regulate and which splatters and spits and blows metal all over the work. The stabilization inductor will mitigate the severity of the problem by virtue of an inductor's tendency to resist changes in current. The fact that the field is driven from the armature lead will also help a tiny bit. But because of the electrical and magnetic time constants of the field assembly, any correction from this mechanism is too slow to regulate the arc and tends to make the voltage oscillate. Now it IS possible to melt two pieces of metal together using one of the "scratchie-stickie" rods like a 6011 but you won't be proud of the results, it won't be strong and you'll develop a chronic "I'll never be a weldor" complex. Trust me, I know. Your best bet is to sell the generator, perhaps to someone else with the welder gleam in his eye (If your concience would allow you to - mine won't) and perhaps add a bit of money to your proceeds and buy a DC buzz box. I've got a very nice one like I used to sell from my shop for under $300. You can't even start to touch a gasoline engine for that amount of money. And a brand name like Miller or Lincoln won't cost much more. Then if you want to heliarc, you can add an outboard high freq arc stabilizer. Or you might want to look around for a nice used gasoline powered unit. I consider a 230 amp gas welder just about the optimum unit if you have any remote work to do at all. I rarely paid more than $150 nor got much more than $300 for these units even mounted on small trailers. Welding duty is pretty easy on engines so they should last a long time and even after the engine gets weak, the govenor will compensate. >3) How much horse power is required to drive the generator? If I use a > 5HP electric to drive it(5hp motors are cheap and available). Most commercial welders use gasoline engines in the range of 12 to ~20 HP. I'd consider an electric motor to be economically impractical for this application. Hope this helps, John -- John De Armond, WD4OQC | We can no more blame our loss of freedom on congress Radiation Systems, Inc. | than we can prostitution on pimps. Both simply Atlanta, Ga | provide broker services for their customers. {emory,uunet}!rsiatl!jgd| - Dr. W Williams | **I am the NRA**