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Fun and games with autotransformers

(*steve*)

¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
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Jan 21, 2010
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I am the proud owner of a device advertised as:

Code:
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.

questions:
  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?
Notes:
  • 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:

KJ6EAD

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

BobK

Jan 5, 2010
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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!

Bob
 
Last edited:

(*steve*)

¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
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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.
 

BobK

Jan 5, 2010
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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.

Bob
 
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