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Fun and games with autotransformers

Discussion in 'General Electronics Discussion' started by (*steve*), Nov 21, 2014.

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  1. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    Jan 21, 2010
    I am the proud owner of a device advertised as:

    Device: 12A 12101604
    * AC Voltage Taps 204V,210V,216V,221V,240V
    Size(LBHmm):120 x 120 x 45
    Mounted on a base plate
    Weight: 4.2kg
    ** Note these Transformers are
    new and un-used but have been
    pulled out of equipment.
    The transformer has a mass of leads and the markings on the transformer are not at all helpful in determining how to connect it.

    So what I did was attach an AC source (6.3VAC, but it actually measured 7.06VAC on my meter) to various combinations of taps and found the following:

    • There are 2 separate windings.
    • One winding, with 7.06VAC across red and grey, reads (wrt red) red=0V, brown=3.95V, yellow=4.68V, blue=5.87V, and grey=7.06V
    • The other winding (black and white) is separate and measures 40.5VAC.
    • There are two grey wires.
    • There are two black wires
    • There are an additional 2 thinner red wires
    • There are an additional 2 thinner black wires
    • All wires of the same colour appear to be connected together.
    • The thick wires are labelled as 750VAC
    It took a while to figure out this puzzle, maybe you can do it faster.

    1. Is it an autotransformer?
    2. If not, what is the secondary voltage (assume primary is 240V)?
    3. If so, how should it be connected?
    • The voltages above are how I measured it.
    • Murphy's law applies.
    I'll try to get you a photo as well...

    IMG_6554b (Custom).JPG IMG_6553b (Custom).JPG
    Last edited: Nov 21, 2014
  2. KJ6EAD


    Aug 13, 2011
    So it's a typical power transformer with a multitap primary to accommodate various input line voltages and multiple secondaries? What's unusual?
  3. Anon_LG


    Jun 24, 2014
    Don't you mean: "what's unusual, sir" :D
  4. BobK


    Jan 5, 2010
    I have the answer:

    It is an autotransformer. But there is not quite enough information to determine the connections. We need the phase relationship between the B and W and the R and G coils.

    In any case, here is how to connect it:

    Connect either the B or W to the G so that they are in phase.

    Connect the 240AC to the R and the other of the B and W.

    Now, with respect to the B or W not connected to G we have:

    Brown = 220V
    Yellow = 216V
    Blue = 210V
    Gray = 204V

    Edit: And damn you Steve for wasting 1 1/2 Hours of my time!

    Last edited: Nov 21, 2014
    (*steve*) likes this.
  5. (*steve*)

    (*steve*) ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd Moderator

    Jan 21, 2010
    Bob has it right. Given the time it took him, I'm no longer so embarrassed about how long it took me.

    As for the phasing, it might help if I note that the thick leads have spade connectors on them, and reiterate that there are a pair of grey, and a pair of black connectors, and that there are some thinner black and red wires...

    Knowing the answer it's almost obvious, however I got the phasing wrong the first time.
  6. BobK


    Jan 5, 2010
    In my defense, I actually took much less time than that to figure it out, but them kept messing up my calculations to check it.

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