# What is the purpose of having two coils in a tranformer?

I have been learning about electricity send through long distance from the power companies to people's homes. I know that power is initially send from a power company to a transformer in high voltage, so to reduce current, which then reaches another transformer nearby people's home where current is then increased by using step down transformers. However, what I don't get here, is why do each transformers have two coils? Why can't you just work with one coils or wire where direct current with high voltage is send from power company to nearby substation, where there voltage is then decreased to increase the current that flow into people's homes? The idea of having two coils in transformers just seems redundant to me.

Edit: After, somebody pointed out that the questions is not clear, my question in short is why does transformers need two coils? Why can't it do the same job, or varying voltage and current, with only one coil, where each coils with different number of coils and in different stations, induce each other?

• Your question isn't very clear about how your alternative "one coil or wire" idea would work. I think that you need to think about how to present your question more clearly.
– user93237
Commented Apr 17, 2017 at 18:37
• My question was not meant to give details of how the alternative idea would and should work. It was meant to suggest an idea, which may or may not be correct, that through process of elimination illuminates on the overarching idea of why transformers are set up the way they do.
– TLo
Commented Apr 17, 2017 at 18:45
• Well, in that case it sounds like you're just confused about the basic principles of how a standard transformer works, which is too broad of question to be presenting here. I think that you need to go back and study the subject more thoroughly yourself, and then if there is any particular concept that is giving you problems you can come back and ask a focussed question on that particular concept.
– user93237
Commented Apr 17, 2017 at 18:50
• Can you better explain " where each coils with different number of coils and in different stations, induce each other?" Commented Apr 17, 2017 at 18:57
• maybe you are asking about the autotransformer en.wikipedia.org/wiki/Autotransformer Commented Apr 17, 2017 at 19:10

You can make a transformer with only "one coil". It's called an autotransformer.

However for high voltage power distribution this would have serious safety issues, because the "low voltage" (110V or 240V) household supply would be connected directly to the "high voltage" wiring, and any faults such as a disconnected neutral or earth wire might be fatal, or start a fire because the "low voltage" house wiring was not designed to be safe at a much higher voltage. (Of course it could be designed to be safe, but the cost would be much higher, and every electrical appliance would also have to be redesigned.)

Also, when the voltage is reduced by a transformer, the current is increased in the same proportion. Therefore the low voltage winding is usually made from thicker wire than the high voltage. Joining two different wire thicknesses in the middle of a "single winding" rather defeats the point of having a single winding - you might just as well make both ends of the join accessible, and have two separate windings.

Autotransformers are useful in some low power and low voltage applications, like electronic oscillators - for example the Hartley oscillator circuit - or where the difference between the primary and secondary voltages and currents are fairly small, for example transforming 120V to 240V.

• Yes, autotransformers, more commonly known to those of us who tinkered long ago with CRT TVs as "flyback transformers", were common for generating the high voltages required for cathode ray tubes (CRTs).
– user93237
Commented Apr 18, 2017 at 2:05

Your question is about power transmission but transformers are used in all kinds of applications from tiny little high frequency ones which might have only half a coil with a tap at 1/8th to complicated ones in power supplies of equipment with a dozen coils ! Time is too short to give you the full transformer lecture but the answer above (@alephzero) generally covers the point about electricity distribution transformers. I'll add a bit more..for reasons of economy a distribution system from generating station to home does not want to have too many intermediate transformers so the turns ratios are often large. E.G A primary voltage of 11kV and a secondary of 415V. Around 25:1 . So for 100A in you will be running 2,500A out. Thickness of coil wire somewhat different you see. Another reason , in power distribution, for keeping the two sides of the transformer electrically separate is due to the way fault protection works. You need a fully isolated (i.e. 2 coil) transformer to ensure the 11kV network fault protection works independently to the 415V system, ditto all the way up 33kV / 66kV / 132kV / 275kV networks and finally the generator output. Transformers (with 2 unconnected coils) isolate the electrical systems on either side.