3 Phase Transformer - Connecting as single phase

[Vernonv]

Hi All,
I wasn't sure where to post this, so here it is....

What I have is a 3 phase transformer that I want to use in an RPC to convert 240 single phase to 415 single phase.

Physically the transformer looks kind of like this:


Electrically it looks like this (i.e. it's a 380 to 415 auto-transformer):


Now I'm pretty familiar with electronics and single phase gear, but not so for 3 phase. Anyway I tried wiring the transformer like this:


However where I expected to get 424V out, it actually measured (no load) at around 70V (from memory). I rewired it so that the N of two of the cores where connected and adjusted a few other things and managed to get a nice 415V out of it, but as soon as I loaded the transformer, the output dropped to around 40V (but no heat or smoke).

Anyone have any suggestions of how/if I can get close to 415V out of this transformer?

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[woodbe]

You're a brave man Vernon.

(No, I'm not talking about the electrickery)

Hope you get it sorted, but for pete's sake be careful!

woodbe.

[Vernonv]

Well it looks like I'm not the only one stumped by this.

I've been searching the web to see if I could find any info on anyone doing anything similar, but I'm not having any luck.

I think I'll just have to keep searching.

[rrobor]

Your theory is OK as in essence you are only adding an overwind to boost voltage. But if you look at the transformer, the physics of the thing suggests that the flux in the core will be in the wrong phase for your need, so rather than increasing the voltage, you are reducing it.
Its a long time since I did my theory and I got arthritis so i get the right hand and left hand rule thingy not quite correct. But you will find that is the issue. It may not work with that type of transformer, but if it does I believe you will need to reverse the phase of the central limb.

[Vernonv]

... but if it does I believe you will need to reverse the phase of the central limb.

Thanks Rob.
I have tried wiring in both directions. When wired as picture above, it only gave about 70V. When I reversed the wiring I managed to get almost exactly 415V out .... that was until I loaded the transformer and the voltage crashed to about 40V.

One other thing I tried was to connect the 240 across the centre core's 0-380 taps and then measured the voltage out of the 0-415 taps of the two outside cores - they measured about 125V each ... bummer .