So I decided to test again, this time with the assumption that:
- Black-Blue-Red as the primary (0-110VAC-220VAC)
- Yellow-Yellow as the secondary
Turns out CaptDon, wraper, and perieanuo are right.
I connected black and red to mains (218VAC), with no load on secondary. Just like before, the circuit breaker didn't trip.
This is from 5 minutes test:
- The primary (black-red) uses 26.34mA to 28.15mA of current.
- The output from secondary winding is 10.14VAC to 10.33VAC.
- And no smell.
I disconnected the primary from mains, and touched the transformer: not hot, not even warm.
Then I repeated the test, the result is similar:
- Primary uses 25.97mA to 28.21mA.
- Secondary output 10.07VAC to 10.32VAC.
- Still no smell.
I disconnected the primary again and touched the transformer, this time it get just a little bit warm.
(Sorry about the "hot" and "warm", I only have two multimeters, and both are being used in this test, so I cannot use the temperature probe.)
I still don't understand how the result could be different from when I first tested it (heat up and smelled), but now it seemes that everything is fine.
The yellow wires are certainly the secondary.
And no windings are shorted.
Thank you for helping out, everyone.