I have a small toroidal power transformer, perhaps 20VA, about 7cm outside diameter; core cross-sectional area is about \$16 \times 22 mm = 350 mm^2\$. I expect it's a wound silicon steel core.
I got a handful of these for $3 USD each at the local electronics surplus store (Halted in Santa Clara, CA) and am using them to experiment with winding my own transformers. I want to reuse the primary if possible since at about 13 turns/volt it's liable to be 1500 turns or so and so far I'm winding toroids by hand.
It has two primary windings and three leads, labeled 0-115-220.
I'm not clear whether the design intent is to use the BK-OR winding by itself for US 120 VAC or the BK-OR and OR-BR windings are intended to be wired in parallel for that voltage.
Doing the arithmetic on the label specs, the OR-BR winding is for 105 VAC. The two windings do indeed have different resistance and different inductances, \$28.7\Omega\$ vs \$52.1\Omega\$ and \$13.0H\$ vs. \$11.2H\$ (@100 Hz) respectively.
I conclude from the difference in inductance that the turns of the two primary windings are roughly proportional to the voltage drop each is designed for (115 and 105 V respectively, considering \$L\$ proportional to \$N^2\$). I conclude from the resistance measurements that the OR-BR winding uses thinner wire; it must have fewer turns but has higher resistance.
So I guess I'm answering my own question in that the thinner wire seems to clearly indicate the OR-BR winding is intended to be used by itself and only when needed, and the 115V winding is just over-designed when used as part of the 220 V winding.
But just out of curiosity, what would happen if the two primary windings were wired in parallel across 120 VAC? I'm thinking it's not a particularly good idea but not quite sure what behavior one could predict.
Also I'm curious if this is a common transformer design practice. It seems like it would be more economical to wind a bifilar winding of the same number of turns, say for 120/240 VAC and then just use them in parallel or series depending. Why would a designer prefer this configuration instead?