# Fun and games with autotransformers

#### (*steve*)

##### ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
Moderator
Jan 21, 2010
25,506
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...

Last edited:

Aug 13, 2011
1,114
So it's a typical power transformer with a multitap primary to accommodate various input line voltages and multiple secondaries? What's unusual?

#### Anon_LG

Jun 24, 2014
453
Don't you mean: "what's unusual, sir"

#### BobK

Jan 5, 2010
7,682

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
Moderator
Jan 21, 2010
25,506
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
7,682
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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