Today I discovered a very strange thing, this transformer in the picture below, has 3 phases as input, and the output varies as you can see in the picture, the weird thing is that when I measure the voltage across every two phases before connecting them to the transformer I get 380v, but when I do so after connecting them, I get different values, the very same wire goes from high voltage to 0 voltage, is this correct? I couldn't believe it until I touched the wire and didn't get a shock! How does this work? enter image description here


simulate this circuit – Schematic created using CircuitLab

  • \$\begingroup\$ I get different values - how different - please state before measurements and after measurements. \$\endgroup\$
    – Andy aka
    May 2, 2020 at 15:56
  • \$\begingroup\$ After connecting the wires, I get the numbers written on transformer, 0, 220, 380. \$\endgroup\$
    – Shaydzmi
    May 2, 2020 at 16:00
  • 2
    \$\begingroup\$ That is NOT a 3-phase transformer. It is a single-phase transformer with three windings -- a primary that can vary anywhere from 220 to 550 V (+/- 20V) and two secondaries. \$\endgroup\$
    – Dave Tweed
    May 2, 2020 at 16:08
  • \$\begingroup\$ @Dave Tweed, can you please explain this to me in details? \$\endgroup\$
    – Shaydzmi
    May 2, 2020 at 16:13
  • 3
    \$\begingroup\$ Touching the wire to verify 0V is not a recommended method to see if there's still 380V on it. \$\endgroup\$
    – John D
    May 2, 2020 at 17:38

1 Answer 1


After connecting the wires, I get the numbers written on transformer, 0, 220, 380.

It's a single phase transformer.

The primary winding also works like an auto transformer.

enter image description here

When 380 V is applied between terminals '0' and '380', the voltages between '0' and the other terminals will read 20, 20, 220, 280, 440, 500 and 550 V.


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