transformer wiring causing brain melt down

[chokesrule]

I have come by a transformer which has 2 primary windings of 115 volts . There are 3 wires to the primary windings. 2 brown and 1 white. I take it the white is a shared o volts or ground /center tap. I live in the UK where domestic mains is 230/240v. I am trying to figure out how to wire this to the mains.If both primary windings have a common ground , I cant see how I can wire the two coils in series?? The only possible way i can suppose is to wire both brown wires one to live and one to neutral and the shared white to mains earth???? Is this right?surely not? as each coil would alternate at 240v , which each is not rated for.I am totally confused can any one help?

[multisync]

first, ohm out the coils

[nigelwright7557]

I would guess the white is the center tap.

For 115 volts the two browns go to live and the white goes to neutral.

For 240 volts the live goes to one brown and the neutral goes to the other brown with white left floating.

[richie00boy]

Maybe it's a foreign transformer where earth is a centre tap, so you only ever get 115V max above earth for safety reasons. If so then you can connect it to mains but you must insulate the white wire, if you connect it to earth it will blow the transformer and house electricity supply as you will be putting 115V onto house earth.

[chokesrule]

well white to either brown is 14 ohms resistance:brown to brown is 28 ohms . I guess white must be centre tap. So it is left floating, i can see why now :the white wire is basically the end of the first winding AND the start of the second, ie to connect to 240 mains if you isolate the white wire the two windings are already in series. thanks for the help.

[leadbelly]

Originally Posted by chokesrule

well white to either brown is 14 ohms resistance:brown to brown is 28 ohms . I guess white must be centre tap. So it is left floating, i can see why now :the white wire is basically the end of the first winding AND the start of the second, ie to connect to 240 mains if you isolate the white wire the two windings are already in series. thanks for the help.

Whoa, that's a lot of resistance for a primary. Are you sure you're not confusing the secondaries with the primaries? How many and what resistance are those?

[kjlonergan]

It would be unusual for a 115/230 volt primary to be center tapped. For 115V operation the primaries must be in parallel, with the "start" of winding 1 connected to the "start" of winding 2 for proper phasing. For 230V operation the windings are connected in series, with the "start" of winding 2 connected to the "finish" of winding 1. This means all four wires need to be brought out. The center tapped configuration is much more common as a secondary winding.

[AndrewT]

what is printed on the manufacturer's label?

[Steerpike]

Seems like it could be just a 0V-110v-220v winding - nothing too odd. Brown+white for 110, brown+brown for 220. At 110V only one half the winding will be used, which is inefficient, but not that uncommon. The colour coding is odd though - they would normally be 3 different colours.

[AndrewT]

could it be a 55,0,55Vac (=110Vac) isolating transformer for industrial and outdoor tool use?