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A Dual MOT spotwelder

Bob Engelhardt Sept 19, 2007 Here is my version of a microwave oven transformer (MOT) spotwelder. MOTs are commonly used for DIY spotwelders as they are readily available and have decent power (1.5 kva +-). Mine follows the usual path of removing the original secondary winding and replacing it with a few turns of very heavy conductor. Mine offers a feature that I am notaware of in other MOTSWs - the use of aluminum plate for the winding conductor. This posting is mostly show-and-tell and not a how-to for building a MOTSW. You could build one by following what I've done, but understand that it's based upon using the materials that I had rather than an optimized design. Some design criteria - be able to weld 16 ga mild steel. One test welded 2 pieces of 1/8" mildsteel (about 11 ga). The input power is 17A @ 240v = 4 kva. I had intended to use a single MOT, but it just wasn't enough power. It could weld 16 ga, kinda (not reliably). - be able reach to the middle of the longest seam that my press brake can make (22"). The reach is 9", which is within 2" of the center. Close enough. - small and portable. My shop space is taken. The spot welder will sit on thebench in use and on a shelf when not. It is 18" long, 5" wide and 10" high. It weighs 28 lbs. - minimize buying of parts and stock. I bought a piece of welding cable for $9 and a piece of copper bar for $4. Everything else came from the dump or my junk box. Fabrication I am a true believer in KISS. KISS means easier fab and greater likelihood that the designed object will work. MOTSWs are reallysimple to start with, so KISSing was pretty easy.

All the parts except the windings:

The MOTs were tack welded together. The magnetic shunts were removed along with the secondaries.

The arms are aluminum, 'cause that's what I had. They are .75 x .7", which is equivalent in resistance to a copper rod .67" in diameter. The ends of the arms were drilled and tapped for 1/2 - 13 threads, forsome bolts that I had that I thought were copper. They weren't copper (brass, I suppose) and in the first welding trials their tips got red hot, but not the material. So I bought a 3/8 x 1/2 copper bar for $5.25 / lb (!!) and made tips that press fit in the drilled-out holes. The copper works very well. I never considered using aluminum for the tips - it might work. Or not - the oxide coatingmight be too much resistance.

The contacts open by pivoting the upper arm, of course, to allow positioning the work and then are tightened by the pressure lever. The pressure is maintained by an adjustable spring.

The upper arm and its return spring are insulated from their support tower by nylon bushings and from the lever by its nylon cup. It would have been a lot easier to insulate the lowerarm! The mounting bolts for the lower arm pass through over size holes, to allow minor adjustment for aligning the tips.

Commonly, the replacement secondary winding is made from copper welding cable, e.g., #2, which I didn't have. My original intent to not buy material led me to use aluminum. Not aluminum wire, which I didn't have either, but 1/2" aluminum plate.

At the 2 volts in thesecondary winding, insulation can be minimal, so I used paint. A catalyzing enamel that I happened to have, which I hope will be strong enough to avoid wear-through. I used duct tape first but really hated doing it. The conductor is 1/2" x 9/16", which is equivalent to copper .46" in diameter (0000 ga). In addition to being very low resistance, the winding totally fills the space in the transformercore. I've heard that this is good, but I don't know why. To keep the total secondary circuit resistance as low as possible it was really important to make good connections to the aluminum. This meant scrubbing off the oxide and coating with an anti-oxidant wherever there was a connection. Including the copper tips in the arms. And the connections were tightened very securely. When I found that I...