Electronic – Connecting a 230 V power supply to a center tapped transformer

Transformers are usually rated (volts in / volts out) based upon full load current. You are measuring during no load.

So you measured the no load (unloaded) voltage is 17.5 v = 122/6.97 v.

The Un-loaded voltage is a more accurate measurement of the turns ratio.

Fully loaded, you will get lower voltage than 17.5 volts, which results in your transformer's rating of 15 volts output.

Fully loaded means that there will be a voltage drop on the internal resistance of the transformer,

This type of multi-tap transformer primary was common before the days of regulated DC power supplies. The user was supposed to select the appropriate tap according to local mains voltage. Variations from 210V to 250V were common, in European radios, TVs, tape recorders and the like. Old test equipment like scopes also had this type of voltage selection.

I've seen a 1957 valve radio with such a tapped primary, and it was set to the wrong one. The secondary AC voltage was low, and that meant that the rectified DC was low, too. We changed to the correct tap, and the voltages came up to levels much closer to the nominal voltages in the schematic. The radio worked a lot better like that!