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  • #16
    Not sure why you are worrying about VA when intended to run some low voltage hand tools and the transformer weighs 200LB'S!!!!.
    BTW, In new designs I use Toroidal type, very easy to modify or add a small overwind etc.
    Max.

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    • #17
      Frequency ...

      Core size is also influenced by frequency. At higher frequencies the core can be smaller. Part of the reason that the military uses 400Hz in aircraft and ships.

      Different frequencies are also used in foundries for melting different materials in induction furnaces. 60 Hz for iron, 400Hz for aluminum. Not exactly applicable to the topic, but thrown in for free...

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      • #18
        Originally posted by kf2qd
        Core size is also influenced by frequency. At higher frequencies the core can be smaller....

        This is true and causes us the occasional problems in our 50Hz domain when US made transformers although rated for 50Hz run hotter than one might like and die a sudden death if the supply frequency sags (which it only does when on generator supply).

        Perhaps if you are in a 60Hz area and a 50Hz tranformer comes into your hands that would be a fine candidate for experimenting having more leeway in the form of the extra iron.

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        • #19
          Maxheadroom, you misunderstood. The transformer is from an impressed current cathodic protection system, removed from a building. It put out about 40 amps at up to 40 volts, through a selenium bridge rectifier. I could not figure how to adapt it for any useful purpose and it sure was overkill for my purposes, AND it was over 50 years old. I know, 50 years is nothing hell, I am over 50!
          Duffy, Gatineau, Quebec

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          • #20
            Many years ago I made a scratch-built high power audio amplifier. I needed +-68vdc for the power output stage, and +-12vdc for the rest of the circuit. I don't recall the current required. But I calculated the winding ratios needed to convert 110AC to the values needed (two secondaries), then calculated wire gauge necessary for the current.

            Now I knew the window size of the IE transformer and in essence, the core size. I rummaged around my stash of parts, found an old TV transformer, cut off the old copper, and rewound it to my design. It worked perfectly.

            I think that was in 1984 or so. I never finished that amplifier because I could not find one of the components in the parts list - I still believe that parts list was in error. The chassis is in my shop to this day awaiting my reborn interest in it. I have a 100W Fender amp now, though, so except for curiosity...

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            • #21
              I kind of got spirited away last night. The girls wanted to go dancing, so I didn't get home till after 1 am. Too late to play with transformers, so here I am today, after having slept in quite late, crawling around on the basement floor looking for hunks of iron.

              I have a pair of transformers I can use, though they are a little larger than needed. That will be fine- if I need to reduce the idle current I can add a little winding to put in series with the primary.

              It would be nice if I didn't have to disassemble the cores at all. Usually, when I want a low voltage, high current secondary, I cut a few strips from copper sheet, insulate one side of the strip with scotch tape, then just pass that through the slots. Each of these becomes one 'stack', or disc, and then I wind on another one beside that. I try to shoot for four stacks side by side, since that allows me to wire it up as a center-tapped winding. With care, I can get a decent fill ratio and low copper losses, and it's a lot easier to wind a flat strip fairly tightly than it is to pass heavy gauge wire through the core and end up with a tight winding. If I've scotch taped the inside of the copper strip, there's little chance of that becoming damaged while feeding it through the core slots.

              Cutting out the copper strips from sheet is problematic, though. Without a custom machine to cut the strips equal width and roll off the burrs, it's just another time-consuming part of the total project. And if the copper strips are relatively thin, the scotch tape takes up a significant portion of the available winding space. An ideal width for the strip is something just less than the width of the scotch tape, so there are some limitations between thickness and the effective gauge of the strip. Sometimes it works out ok.

              Just did some checking- the scotch tape I use measures .002 thickness- the copper I have on hand measures .006. 25% of the winding space is taken up by the insulation. A strip of this copper just under an inch wide is roughly equivalent to 10 gauge wire. Of course, when you flat-wind this, there is no air space between windings as there is when you lay round wire side by side, so this isn't really a bad situation.

              I watched one video where is shows distribution transformers being made. It shows a flat strip being wound as one of the windings, probably the secondary. It's pretty much the width of the core. Nice that it leaves a very flat surface upon which to wind the higher voltage windings. Then I see the core being inserted- this was the interesting part, as it looks as if the core is U shaped pieces being inserted from either side of the winding bobbin, and it looked like they were thin. This would of course mean that several individual pieces would be nested together before the core is complete. There must be a way that they make a good edge to edge contact in order to maintain a high density flux path through the core. It seems no small feat to be able to achieve this for every piece of the core that is inserted, while keeping all these steel strips in contact as one 'lump'.

              The other way to do this, and the one that interests me the most, is to have this steel strip as a length, which you then wind through the bobbin. Normally, you would wind two strips through the core, making two doughnuts. Essentially then, you have two torroids passing through the winding bobbin, and it should end up being very efficient. Not only electrically, but mechanically as well, since you can avoid having to pass miles of wire through the hole in a doughnut. It's the procurement of this steel strip that is problematic for me- where would you go to find a hobbyist quantity of suitable steel of a suitable width?
              I seldom do anything within the scope of logical reason and calculated cost/benefit, etc- I'm following my passion-

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              • #22
                If you are interested in the most efficient way to build special transformers look into cut cores. You wind the primary on one bobbin and the secondary on another. You can take them apart and try alternatives. The only downside of this approach is that you have to buy the cores rather than recover them from scrap.

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                • #23
                  Does burning the varnish off the old laminations leave them in a soft state?
                  I don't know. I'm not up on the annealing procedures for silicon iron, which is what transformer laminations are made from.
                  Free software for calculating bolt circles and similar: Click Here

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                  • #24
                    I've only done this once, but it could happen more often in the future- I had all my laminations removed from the old transformer, gave each one a pass over sandpaper laid on a flat surface, blew them off, then laid them all out closely and gave them a coat of lacquer. We had the spray booth operating, so it wasn't a big deal for me to go in there on a weekend and do this. I gave each side one coat.

                    I should have bothered to measure the thickness before and after, but I didn't. We had lots of laminate laying around, so I made up my own bobbin from that. We also had lots of backing laying around, so I cut some of that into 2 inch squares to mix epoxy on. The tricky part was holding the bobbin pieces in good square relationship while the epoxy cured- the hardest part was cutting a nice and neat rectangular hole in two of the pieces where the core would pass through. Laminate tends to chip and crack, etc. A laser or water jet cutter would sure be nice for some of this work. Next time I build my own bobbin, I'm going to make it in two parts so there are no enclosed holes to cut.

                    I've just finished completely destroying the secondary windings on one of the last two transformers I need to wind. I've measured the turns per volt and now need to figure out my best way to cut this copper sheet into strips. I'll need to use three strips soldered up in series for each half of the secondary winding. One thing about using flat strip- it's super easy to add breakout taps to the winding- just lay another piece of strip across and solder. Makes a very low profile junction. At the beginning and ends, I just make a fold at 45 degrees and there's a connection tab.
                    I seldom do anything within the scope of logical reason and calculated cost/benefit, etc- I'm following my passion-

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