Transformer 220/380/440 V 24 V explanation

Question

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?

Answer 1

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.

Answer 2

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.

Answer 3

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.

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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.

Answer 4

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.