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Posted by D. Sherman on Tuesday, February 12, 2008 at 16:56:40 :
In Reply to: Re: AC genset to DC welder posted by David N. Lundstrom on Tuesday, February 12, 2008 at 16:11:38 :
I missed the part about wanting a DC welder. The Onan won't give you DC unless you put some big rectifiers on the output. It's an alternator (with a rotating field and brushless exciter if I remember right), but definitely not a DC generator (with a slotted commutator, brushes, and a stationary field) and no amount of clever rewiring will change that. I don't know about Hobarts specifically and am not an expert on welders. I do know something about motors, generators, and how they work, though. I have an ancient Lincoln 300 amp engine-driven welder that I think came out of a ship yard. I haven't tried to fire it up because the engine's seized. On those big old machines that were built before solid-state rectifiers, the DC came straight off the commutator, which is one of the things that made them so bulletproof, but also made them heavier for the same power compared to a modern unit. On a generator like your Onan, there are no brushes and the field, which is on the rotor, gets its current from a rotating exciter whose AC output is rectified by a rotating rectifier assembly. The exciter's field is stationary and is controlled by the voltage regulator. This is a neat setup because it does away with slip rings, commutators, and brushes, and by having the field rotate, it means that the windings that spin carry only the field current, while the high-current output windings are stationary. The disadvantage is you can only get AC out of it. To get DC directly out of a generator you need to have a stationary field, like on an automobile starter motor or DC generator, and have the rotating part carry the full output current of the machine via a commutator and brushes. Obviously, that's an entirely different beast than what you have. That's also part of why these old trailer-mounted welders are so big and heavy. The full 300 amps has to be carried by the brushes and commutator, which is a pretty hefty current for little blocks of graphite, and the rotor has to be would with wire that can carry that kind of current.
As for how they get the AC output, the ones I've looked at (and again I don't know anything about Hobarts specifically) either have a separate winding to produce the 110 or have a transformer to boost some of the AC welding voltage to 110. Usually the AC "convenience outlet" isn't rated at more than a small fraction of the welding power, and it usually isn't regulated very well, either in frequency or voltage, but it's plenty good enough to run a skil saw or a some lights on a job site.
As an aside, there used to be a lot of AC-powered welders that were really M/G sets where an AC induction motor drove a DC welding generator very much like in the mobile gas-powered units. Is still see them at auctions from time to time. They're bigger and in some ways more complicated than the transformer-based ones with solid state rectifiers, but I think I'd trust them more to withstand all sorts of abuse for a long time. Also, when you have control of the field and the armature of a DC motor or generator, you can do a lot of neat things that are complicated do to electronically in terms of changing the torque/speed curve or, in the case of a generator, going from a constant-voltage to constant-current output or anywhere in between. Most of the mine hoists around here still use M/G sets and DC motors to provide smooth variable-speed drives at high power levels that are easy on the power grid and on the hoist machinery. At first glance, it seems like a waste to have a 1000 hp AC motor, a 1000 hp DC generator, and a 1000 hp DC motor, when all you really want is one variable speed 1000 hp motor, but until very recently that kind of control (what they call a "Ward-Leonard" system) was the only way to do it, and it's still mighty nice in terms of simplicity and reliability. There's no belts, no gears, no fragile semiconductors or tricky computer controls, and you can actually make it produce MORE torque at slower speeds if that's what you need to start a heavy load moving. It's also easy to reverse with nothing more than another switch.
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