Why is my transformer a Scott-T?

Discussion in 'General Electronics Chat' started by strantor, Jan 18, 2016.

  1. strantor

    Thread Starter AAC Fanatic!

    Oct 3, 2010
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    I have a 6KVA sandpoxy-filled transformer I'm digging out right now (thread on the excavation) and much to my surprise when I finally unearthed the transformer, it has 2 cores.

    scott t.jpg

    It's a Square D model # 6T2F, 480V Delta input, 208/120V Wye output according to the generic info I can find online. The data plate was worn off mine so I didn't notice the odd connection diagram; here's what it would look like though (not my picture):

    IMG_5210.JPG

    It says "Type ST" on the label, there's 2 cores, and the connection diagram seems to confirm a Scott T transformer, for which the only purpose I was aware of, is to generate archaic 2-phase power from a 3-phase supply. But everything I can find online says it's a general purpose 3-phase transformer (to quote the page linked above: "General Purpose - Intended for power, heating and lighting applications").

    I pulled this out of a dumpster years ago at a place I worked. I was an Electrical Maintenance Tech there, and I was familiar with nearly all of the equipment on site. There were absolutely no 2-phase machines on site that I am aware of. I am 99% sure that this transformer was never used to generate 2-phase. If memory serves, I believe I was told that it was a spare for an old industrial trash compacter that was scrapped.

    I went searching for answers as to why anyone would use a Scott T transformer for uses other than generating 2-phase, and found this:

    It seems like this Scott-T transformer should be an odd duck, hard to find, expensive, and used only in a very small, specific niche of applications. But when I search for transformers with a 208Y/120 secondary, half of them are Scott-T types. Why is that? Why are people buying these transformers? Is there an advantage to them that my reference failed to note?
     
  2. Kermit2

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    Feb 5, 2010
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    Did you find any info on the industrial emegency uses?

    Combined with switch gear, the Scott T can create balanced three phase from only two phases. It is used to keep equipment running when utility service drops a phase or otherwise have a problem with one phase of the three phase service.
    Probably not the answer you wanted but I posted it antway. For some reason :)
     
  3. strantor

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    huh? When you lose a phase from a 3-phase supply, you're down to single phase. You saying a Scott T can generate 3 phase from single phase?
    I'm curious now because that document I link to alluded to a similar thing:

    I simply did not understand this blurb. When I read it the first time I thought it applied to "a long, long time ago, in a galaxy far far away" back when 2-phase was used. But now that I re-read it, it seems to be talking about 3-phase. Now I'm totally confused. I thought it was impossible to generate 3 phase from single phase without a RPC or static unit employing capacitors.
     
  4. Kermit2

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    The paper I just scanned through suggested industrial power utility connection often sees a problem with 1 of the 3 phases. Loss of two phases was not even talked about(in that paper)
     
  5. strantor

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    loss of 2 phases would be total power loss
    loss of 1 phase would leave you with single phase, same as your home service.
    When you lose one phase, you don't drop down to 2-phase, you drop down to single phase.
     
  6. Kermit2

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    Okay. I went back and re-read the paper.
    It clearly stated "with the loss of one phase, or more often, two phases..."
    :)
     
  7. strantor

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    Well i guess i have one more thing to add to the list of ways in which i don't understand this transformer. In order for the transformer to do anything at all with only 1 hot leg (loss of 2 phases ), it would require a 4 wire supply with a neutral. And my understanding (was) that one of the only reasons for this transformer was to make a 4 wire output with a usable neutral from a 3 wire supply.
     
  8. Kermit2

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    I gotta go find that paper again... connection diagrams dontcha know. :)
     
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  9. Kermit2

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    Okay. I'm seeing a pattern.
    There are two kinds of Scott T.
    One is a connection and uses TWO individual transformers. Another is a transformer like yours that uses the aforementioned connection(3ph-3ph version) AND is mounted on a special shared magnetic core.
    Using the one you have for anything other than 3 to 3 duty is frowned upon due to large neutral return currents and disruption in magnetic Flux through the central leg of xformer. I fully expect some of the above is not right, but I think it is going the right direction

    Found the paper too. Seems my memory is getting worse every year, but I had the spirit of the text correct. :)
     
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  10. strantor

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    I guess now i need to decide whether to try to make this into a static phase converter or continue on with my plans to make a welder out of it.

    • I've always wanted (but not needed, really) a static phase converter.
    • It's not every day you just "discover" you have a Scott T transformer, half the recipe for a SPC.
    • I need a good welder, now. I'm saving up for a nice one, but my needs are more immediate than my budget.
    • If i chop up the windings that are in it, I'll never get them back and I'll probably never see another scott t.
    • If i chop them up, I'll have two really nice big cores for my welder.
    • If i make a welder out of it, 5 years from now it will sit unused while I weld away with the fancy TIG outfit i should have by then. I'll look at it sitting lonely, no chance of ever being sold, and kick myself for destroying that scott t.
    • But i need a welder and i don't have money.
    ... I'm pretty sure I'm going to murder this bitch and make a stick welder. I can't afford any more crap sitting around waiting for its divine purpose to be realized in the future.
     
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