I believe that I have a three phase delta to single phase transformer 1Kva with input 415 v three phase and output 230 Vac single phase. ( Someone help me to identify what kind of transformer is this?)

I would like to test the transformer. What would happen if inject a 230 vac from a socket directly to the secondary of the transformer? Would I damage the transformer or it's perfectly fine and I would get the 415 v ac at the primary end. I'm just worried the sudden surge of voltage to the terminal may effect the transformer wingdings.

Below is the sample picture what I'm trying to do: (please ignore the immature wiring connection and the exposed fuse) enter image description here

enter image description here Some part of me saying this is a bad idea. What would happen and why is the question now?

Edit: Primary connections: enter image description here Secondary Connection:enter image description here

  • \$\begingroup\$ It says 230V on the label not 240V. \$\endgroup\$ – Andy aka Jan 14 '15 at 9:44
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    \$\begingroup\$ Sarenya, since you are in Malaysia, can you contact the manufacturer and request the data sheet? relcotech.com.my/contact This would help clarify the question greatly. \$\endgroup\$ – MarkU Jan 14 '15 at 23:02
  • \$\begingroup\$ Is the centre coil being used at all? \$\endgroup\$ – Andy aka Jan 15 '15 at 8:39
  • \$\begingroup\$ There seems to be no wire connected to it. Not used then. \$\endgroup\$ – 3.1415926535897932384626433832 Jan 15 '15 at 8:47

Would highly recommend that you try testing first with 2.4vac or 24VAC input. For a simple unloaded transformer the ratio of input to output will be the same. Note that determining proper 3 phase output may require a scope or special circuitry to see the phase differences (if needed). Using a Variac (variable transformer) can help select low input voltages. Other safety measures would include fuses and/or resistors in line with the input voltage.

If absolutely required to have a test system using the full voltage input you might put the whole transformer that is being tested behind a clear shield or enclosure that prevents any test person from accidental contact. Entry to the enclosure or removal of the shield would open interlock switches (opening up all input lines) disconnecting all power to the transformer.

  • \$\begingroup\$ Double pole interlocks, are the wrong tool for this transformer. It is three phase, so you want three pole interlocks. \$\endgroup\$ – hildred Apr 15 '16 at 17:03
  • \$\begingroup\$ Yes, that would be correct if the test input were to use a 3 phase connection. Though in this case the OP wants to input from the secondary single phase side, (and per the first picture). The above has been edited anyway. \$\endgroup\$ – Nedd Apr 16 '16 at 6:42

No, you would not get 415 volts at the primary, at least if this is a regular symmetrical 3-phase transformer in open delta configuration. The turns ratio in that case would be would be (415 x √3)/ 240 = 3. However, if you feed the secondary you lack the 120⁰ phase shift resulting in only 1/2 of the voltage over the individual secondary windings (now primary), therefore 120V instead of 138V. The primary will drop proportionally to 360V.

You may want to check if the transformer is truly a symmetrical 3-phase transformer.


Amended due to original question being misleading.

It appears that you have a transformer that produces a single phase output but has a 3-phase input connection. On the face of it, it's a magical black-box that converts 3 phases to a single phase using one common core. This can't happen with standard magnetics; either the transformer is only using two input windings with the secondary on a third branch of the magnetics or something else is happening that is beyond my understanding.

It looks like (from the top picture) that there are two primary windings and the secondary is receiving the magnetic field from both in the shared central section of the core. IF THIS IS THE CASE, then you have to decide how the primary coils are wound and whether there is a true 3-phase connection - I suspect that there isn't but using a multimeter should uncover this.

This is how I suspect it is functionally: -

enter image description here

If this is true then my original deleted answer basically stands - you should be able to apply 230V/240V to the primary across two particular connections and get something like 127 volts on the secondary.

Please note that the supply out from this device is 230V not 240V as stated in your question.

  • \$\begingroup\$ How do I connect a 240 to the three phase delta input? \$\endgroup\$ – 3.1415926535897932384626433832 Jan 14 '15 at 8:27
  • \$\begingroup\$ It's a single phase transformer (as you said) so just connect live and neutral 240V to where you'd connect the 415v wires. \$\endgroup\$ – Andy aka Jan 14 '15 at 8:30
  • \$\begingroup\$ As you can see in the diagram above the 415 input is R,Y and B. Its a delta input and on the opposite you will get 240 single phase. Im using a single phase transformer. So do I connect the o and 240 V to one of the two phases. \$\endgroup\$ – 3.1415926535897932384626433832 Jan 14 '15 at 8:35
  • \$\begingroup\$ The diagram tells me nothing. Your words said "single phase transformer". \$\endgroup\$ – Andy aka Jan 14 '15 at 8:37
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    \$\begingroup\$ @Andyaka, I fell to the same pitfall originally; the wording should be corrected in the question; It's a three-phase to single-phase transformer \$\endgroup\$ – PkP Jan 14 '15 at 8:39

For a single phase transformer and the transformer is working properly, then it would be OK and you would get 415 VAC to the primary as you suggested. But this is not a single phase transformer.

There are several ways a three phase to single phase converter might be wired. There's some information in this source: http://carroll-meynell.com/technical-3phase. If you introduce voltage to the secondary winding, the primary windings probably end up with different voltages.

At the very least, use a circuit breaker and I'd suggest using a series resistor at the 240VAC end to limit the current. You're not attempting to load (take power out of) the primary coils, are you?

  • \$\begingroup\$ Some of my friends are suggesting that its better to inject the voltage slowly step by step. Would it make any difference. \$\endgroup\$ – 3.1415926535897932384626433832 Jan 14 '15 at 6:56
  • \$\begingroup\$ Sure, if you can. There will be less hazards in case of a malfunction. \$\endgroup\$ – PkP Jan 14 '15 at 7:12
  • \$\begingroup\$ Nope not planning to load the transformer. Just wanted to test if the transformer working according to its rated values. \$\endgroup\$ – 3.1415926535897932384626433832 Jan 14 '15 at 7:29
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    \$\begingroup\$ I'm reconsidering the answer; I'm not sure after all how the thre phases are wired in your transformer. Better be careful. \$\endgroup\$ – PkP Jan 14 '15 at 7:56

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