A friend gave me a transformer from an APC UPS that supposedly was capable of \$1300 VA\$ (I have my doubts, but it is very large and I am not going to be using nearly so much power, so OK). He doesn't have any other info about it, so I thought I'd experiment. I have a good idea of how these devices work, so it was easy to identify "mains" side vs "secondary" side (of course they would be backwards when using the batteries). The mains side has three wires: white, yellow, and black.
I figured that the this was to accommodate a \$120/240 V\$ switch, but on testing them, I get \$16.4V\$ from the white/black (expected), but then \$18.4V\$ from the white/yellow, which was not expected. If my conjecture had been correct, I would have expected to see half that voltage from the white/yellow mains inputs (I would have been putting \$120V\$ into a tap wanting \$240V\$ to return \$16.4V\$).
So what gives? Does anyone have experience with these transformers that could point me to an explanation?