# Transformer 220/380/440 V 24 V explanation

I have a circuit that uses three-phase electric power (without neutral). There is an old transformer in this circuit (see image). It converts 220-440 V to 24 V.

The input has the handwritten numbers "0", "220", "240", "380", "415", and "440" on it. Two cables, L1 and L3, are connected to "0" and "380".

I want to use this transformer in a circuit with single-phase electric power. Should I use the same wiring at "0" and "380" for N and L?

What do the numbers actually mean? Obviously they are voltages, but is there any guide for different wirings?

That looks like it has a tapped primary and you connect the input voltage between '0' and whichever of the taps matches your input voltage.

simulate this circuit – Schematic created using CircuitLab

Changing the tap changes the turns ratio between the primary and secondary, which changes the voltage ratio. For example using the 440 tap will include twice as many turns as the 220 tap.

Be careful around it when it's connected as all of the primary terminals will have voltage on them, and the voltage can be rather high because the primary side will also act as an autotransformer. If you connect the correct voltage to any of the taps the others will all have around the voltage they are marked as on them and you can get a nasty shock if you touch one.

• Thank you for the detailed explanation and for the security advices! Commented Nov 3, 2022 at 22:00

Transformer is currently connected with two live phases of three phase supply to 380V input. Which means your single phase voltage to neutral must be 220V.

Single phase input of 220V should be connected L to 220V and N to 0V.

• +1 This is very important and some of the other answers overlooked that there is no neutral in the original configuration! Commented Nov 4, 2022 at 22:58
• Probably, 380V connection was chosen because it would unbalance the 3 phase supply less? Commented Nov 4, 2022 at 23:18
• @rackandboneman If the original device had 3 phase supply without neutral then there is no other options to choose how to connect it. Commented Nov 4, 2022 at 23:26

The transformer in your picture appears to be Italian and appears to be designed to cover both phase to neutral and phase to phase connection on a variety of supplies found historically in Europe.

As Justme has explained the primary of the transformer has multiple taps and you connect one pole of your supply to the "0" terminal and the other to match your supply voltage.

To explain the voltages the manufacturer has chosen.

The phase to phase voltage of a three phase supply is $$\\sqrt{3}\$$ times the phase to neutral voltage.

$$220\mathrm{V} \times \sqrt{3} \approx 380\textrm{V}$$ $$230\textrm{V} \times \sqrt{3} \approx 400\textrm{V}$$ $$240\textrm{V} \times \sqrt{3} \approx 415\textrm{V}$$

220V phase to neutral and 380V phase to phase was historically the nominal voltage in mainland europe, while 240V phase to neutral and 415V phase to phase was historically the nominal voltage in the UK.

At some point the EU standardised the nominal voltages at 230V phase to neutral and 400V phase to phase. Whether the typical real-world voltages were actually changed to match is another matter. I would guess that your transformer predates said standardisation.

As far as I can tell 440V is used on board ships. Phase to neutral loads are not commonly used on this system, with smaller loads being supplied via step-down transformers.

The transformer primary has several taps (connections) to allow operation from different voltages.

You would connect one input wire to "0" and the other to the appropriate terminal. If your supply voltage is 440 V, connect the second wire to the "440" terminal. If the supply is 380 V, connect the second wire to the "380" terminal.

If you connect the input to the correct terminals, you will get 24 V out.

• Thanks for the answer. In my country the single phase electricity system is 220V. So in order to use this transformer to a 220V single phase circuit, should I put N and L in 0 and 220 inputs? Commented Nov 3, 2022 at 21:57
• The answer is yes - use the 0 and 220 inputs. Commented Nov 3, 2022 at 21:59
• @Dimitris Are you really sure about that? I have no idea where in the world you are, but just in case you're anywhere in Europe, be advised that mains voltage got standardized to 230V/400V over the last few decades. People still tend to talk about "220V" sometimes, but while that was true in some countries 20 years ago, now it's just a misleading old habit. (However, since your transformer doesn't have taps for 230/400V, the output voltage will then be either a bit more or less than 24 V depending on which tap you pick). Commented Nov 4, 2022 at 11:20
• @TooTea I am not very familiar with the terminology. I know that in my country when we use 1 phase (neutral-phase-[ground]) we got (line to ground voltage) 230±10V, when we use 3 phase (3phase-[neutral]-[ground]) is (line to line voltage) 400V. Commented Nov 4, 2022 at 13:03
• @Dimitris 230±10V should give you a range of 24V to 26.2V if you use the 220V tap (9.1666... ratio), or 22V to 24V if you use the 240V tap (10:1 ratio). Commented Nov 4, 2022 at 15:19