Three phase transformer

I have acquired a big(4kva) three phase transformer. the primary is a star
configuration and the secondary a single voltage delta. it is wound on three
limbs of an iron core.
I can separate each of the primary and secondary windings to give three
pairs each wound on a limb of the core.
I suspect this is perfectly normal - but I've never looked at one before.
Playing (on paper) with the turns ratios, my application of voltage step up
from 240 to 415 would be satisified by putting each primary in series across
the mains and putting each secondary in series. each winding would get less
voltage than originally designed thus not exceeding the original turns per
Provided I connect each winding start to finish in the right order, is there
anything to stop me using it in this way?
I've done a bit of a search on three phase transformers but not
surprisingly, I've not found anything about using them for single phase
I'd be grateful for any info.
"why is is that I always want to use things for purposes that they were not
designed for??"
Reply to
Bob Minchin
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In theory you are quite correct. The only other thing that springs to mind, which you may already know is to connect them so they are syncronised. That is to say that as it's AC the ploes must all align the same way, or else you may get one coil reducing out the effect of the other two. Whilst this will still work it will only produce 1/3rd the magnetic flux.
Hope this helps.
Reply to
Simon Cochran
Open Delta configuration for single-phase operation from a three-phase stack is quite normal, I'll try and get the formula for working out the voltage. We are just using that on one of our chargers to provide an isolated 240V supply for cleaners on a rail depot job.
Leave the input as Star but don't both with the neutral if you have three-phase of the correct voltage.
Peter -- Peter A Forbes Prepair Ltd, Luton, UK
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Reply to
Prepair Ltd
The solution has been given to me by a colleague at work and I've just tried it out successfully.
Wire both primary and secondary as star (make sure you isolate the two star points!)
Connect the outer limb primary windings in parallel. This junction becomes one terminal and the third connection becomes the other terminal. Repeat with the secondary.
So the centre limb is put in series with the two winding on the outer limbs.
Make sure the star points are formed from the all the starts or ends of the windings, NOT a mixture of starts and ends.
To test the result, put the primary in series with a high power lamp across the mains. if the lamp lights up check the wiring.
Total power handling of the arrangement will be half of the maximum VA rating of the original three phase transformer.
Don't exceed the voltage rating of the original windings. But for non continuous applications it may be acceptable to exceed the current rating of the windings and approach 75% of the original VA rating.
If anyone wants a diagram, please let me know.
Reply to
Bob Minchin

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