| Author | Message | ||
     David S |
i see many homemade arc welders but they rewind a transformer to make them put out a higher voltage i know i have a transformer that kicks down the voltage so how do i make a transformer kick up the voltage? | ||
| Simon Quellen Field (Sfield) |
A step-down transformer has more windings on the primary than on the secondary coil. A step-up transformer has more windings on the secondary. This means that you can just reverse the connections on the transformer to convert from one to the other. | ||
| wbeaty |
First put a lightbulb in series with the 120V line! If it lights up, yet you've connected nothing to the secondary terminals, then you'd better NOT use that particular transformer in reversed mode... unless you want some flames and melted copper. The turns-ratio is important. But equally important is the thickness of the iron core and the number of turns on either coil. If these were not important, then a 2:1 transformer could use a two-turn primary and a 1-turn secondary. Yet a 2:1 transformer typically has several hundred turns on the primary. Why? Because if it had less, then it would draw current and heat up, even when there was no load connected to the secondary. The complicated explanation: the inductance of your 60Hz transformer's primary coil needs to be huge, so large that the transformer draws nearly zero milliamps when connected to the 120VAC outlet. If it's a step-down transformer, then the secondary will have fewer turns and proportionally smaller inductance. This smaller inductance MIGHT be so small that, if you reverse the roles of primary and secondary coils, then the too-few-turns "primary" will get hot, the insulation will melt, the turns will short, and you might have flames before the fuses blow. | ||
| Alessandro Carcione (alessandro) New member Username: alessandro Post Number: 3 Registered: 2-2005 |
I think you should know that an acr welder steps up the current not the voltage | ||
| John P Becich (johnpeter) Junior Member Username: johnpeter Post Number: 5 Registered: 4-2005 |
I appreciate wbeaty's explanation. I agree with him wholeheartedly. I will attempt to add some more reasons to his worthy admonition to anyone who might oversimplify the design of his own arc-welding power transformer. Transformers are inductors. (In fact, they have both individual and "mutual" inductance.) However, what is important to realize is that as inductors, they are essentially intended for use with alternating current, not direct current. To direct current, any inductor, no matter how big or small, is just a piece of wire with some simple intrinsic resistance. (Conversely, even a straight piece of wire has inductance, but that inductance is extremely small, and begins to present an impedance to current flow only at very high frequencies...perhaps gigahertz.) An wound inductor, however, has the remarkable property of establishing an appreciable magnetic field, if electric current is flowing though it. This happens for both AC and DC flows. However, if that magnetic field is changing, then, by fundamental principals of physics (Faraday's Law), an opposing electromotive force (a.k.a. "voltage") arises to arrest current flow. Magnetic fields change in the presence of alternating current, not direct current. Therefore, transformers are intended for AC, not DC. When does a transformer cease to be useful for AC? After all, common sense implies a limit. Truth be told, an inductor ceases to create a CHANGING magnetic field from a CHANGING current that is imposed upon it, when that inductor SATURATES. Upon saturation, the inductor now looks like a simple wire with some series resistance, rather than an inductor with its impressive ability to present a back-emf that is big enough to prevent self-destruction. If it isn't destroyed during that brief alternating current cycle, during which an inductor has been saturated, then it will desaturate as the current reverses and flows the other way. Regrettably, that inductor will saturate AGAIN, as the current flow surpasses zero in the other direction. The inductor will then heat up, perhaps to be destroyed within a few AC cycles, or perhaps within millions of cycles. Transformers with huge iron cores, and many, many windings have huge inductances, and are able to see large ac currents without saturating. They are also bulky and monetarily expensive! Hence a primary winding of 2 wraps would be a very poor design for any transformer that plugs into a big, stiff voltage like the 117VAC we use in the United States. Another issue arises from mutual inductance. Even with a properly designed transformer, the load on the secondary winding contributes to this scenario. Why does more current flow in the primary, when current is allowed to flow in the secondary? Because current usage in the secondary, through mutual inductance with the primary, causes the magnetic field in the primary to be less effective in opposing current in the primary winding. If this were not the case, then we could all light our homes for free, thanks to transformer isolation. |
