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Spot welder help needed

Hi, Gang: I'm trying to home-brew a spot welder for 26ga sheet steel and not having very good luck. I started with a huge transformer (11" x 11" x

14") with the core in the shape of a hollow square and four coils (two on one side and two on the opposite side). The secondary coils are wired in parallel and the primarys are set up with four terminals, equally spaced, with copper shunt bars so you can arrange them to be either series or parallel. There are NO data plates, the only writing on it is "Birdsell Welder". The core has a movable center piece and is marked with rod sizes, the further you withdraw the center piece the bigger the rod you can use. I removed the movable center piece completely, wired the primarys in series, and wound a new secondary from #4 stranded copper, five turns around one side of the core. This gives me 120v in and 5v out. My ampmeter only goes to 100 and it pegs instantly under load. I made tongs and tips from Brass (The only thing I had). This thing makes very poor welds. Ten seconds (Way too long) with two layers of 26ga steel are just barely stuck together and break apart easily, leaving just a small pimple at the contact point.

Where did I go wrong?

Any insight appreciated

Roger in Vegas Worlds Greatest Impulse Buyer

Reply to
Roger Hull
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Reply to
tomcas

But at 60 Hz, the coupling is awful. You would do better to put the primary in parallel and use more turns on the secondary.

A transformer operating at several kilohertz with an appropriate core material for several kilohertz can get away with a very small number of turns - it's one reason the "inverter welders" can be so small. A 60 hz transformer needs a much larger number of turns to get good coupling.

Reply to
Ecnerwal

No it isn't, and no you wouldn't. If the transformer was designed for use with 60 Hz, the coupling will be fine. What you *don't* want to do is use more turns than absolutely necessary, and what you *do* want to do is use the heaviest gauge wire possible. In commercial spot welders, the secondary is normally a *single turn* made of very heavy copper bar stock.

The idea is to minimize resistance in the secondary because you want very high current (thousands of amps) at very low voltage (1 or 2 volts) to do spot welding. Even a small amount of secondary resistance will cause the secondary to heat excessively, and drop the secondary voltage enough so you can't force sufficient current through the joint to be welded.

Not true at all. You'll get lots *more* flux leakage at high frequencies. Unless you use a closed toroid made of a suitable material (powdered iron or ferrite depending on the frequency) to minimize losses, the total losses will be intolerable. Capacitive coupling turn to turn will also reduce efficiency, so you want to use the fewest number of turns possible, spaced as far apart as possible. That also requires a core material of the highest possible permeability in order to achieve a reasonable turns per volt ratio.

By contrast, 60 Hz is simple and forgiving. All you need is a closed silicon steel form, E I construction is satisfactory, to contain the flux, and the frequency is low enough that the capacitance between turns won't lead to high losses, so you don't need a core material with extremely high permeability, and you can get away with a smaller turns per volt ratio.

The advantage of a high frequency inverter is that the transformer can be relatively smaller and lighter than a 60 Hz transformer of the same VA capacity, but that comes only through very careful design to minimize the much higher losses inherent in high frequency transformers. This is not something you'd want to try to do at home without a lot of training and core material manufacturer support.

Gary

Reply to
Gary Coffman

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