I just came across this site looking for answers on some questions I have concerning microwave oven transformers (M.O.T.). I am currently going into my third year as an E.E. student and I am trying to design a DC stick welder using M.O.T. I have been researching this topic for a week or so, but haven't found all of the answers that I need. The question I currently have is, if I separated two M.O.T. cores, removed both of the I portions of the E.I. cores, and stacked the two E parts to make an E.E. core, would I need to rewind the primary coil or will it still be usable?
Because welding machines need low voltage-high current, the secondary is going to be rewound with a 4 AWG wire that has a current rating of 95A. Due to the thickness of the wire, it is impossible to get 20 turns in the M.O.T. core as it currently is. The diameter of the wire is 5/16" and will wrap side-by-side in the core twice. As the transformers currently sit it is possible to approximately get 4 layers of wraps for 8-9 total turns on a single E.I. M.O.T. core. To get an amperage low enough for the cable rating, I think I need approximately 20 turns. (At school I rarely go over 95 A using 1/8" rod and probably won't be using more than a 3/32" on my DIY welder. (welding sounded like a cool thing to do with my summer break)). However, if I combine two practically identical E cores from the M.O.T., I calculated that I can fit 22 turns around their cores with them put together.
From what I have put together, this sounds to me like a good idea to reduce saturation (still trying to figure out the physics behind this because our semester was shortened), copper loss, and to allow for the thicker gauge wire to fit in. I thought about using magnet wire, but to get to a 4 gauge there it appears that I would have to use a Litz wire. This would be my first time using magnet wire, but I did try to salvage some from a core that I cut open and twisted 8 16 gauge wires together to get an overall 7 gauge (it was too stiff to use in one core.) A side note on that, molten NaOH (sodium hydroxide) will do work on the enamel that coats the copper wire.
I have enough M.O.T.'s that I can stack two cores and have two transformers to run in parallel on the mains (120v), and series on the secondaries (unless someone wants to talk me into running the mains in series and plugging them up to a 240v outlet). I intend on using a 300A 1600V bridge rectifier to convert to DC then a choke/inductor and maybe 3 6000uF 50V Electrolytic Capacitors that are available to smooth out the ripple. Once I make it to this point I would like to install a power control circuit to control the power input to the primary coil variably limiting the output on the secondaries. Then I feel like a 555 timer would make a nice duty cycle but I've only watched two videos on that and still don't have anything to use it for yet.
Thanks in advance.