buck boost transformer hookup....

[fastjer]

I have a 45kva GE transformer in the pics https://www.------------/photo?fbid=10218414252692170&set=pcb.10218414261612393 and https://www.------------/photo?fbid=10218414252972177&set=pcb.10218414261612393 here. Its set up for 480 to 208, i need to wire it for 250v to 208v. When I hook it up to the 3 phase coming into my shop (248v) I get 116v at tap 7 across the secondary outputs. or 56v to neutral. Can anyone send me in the right direction? The old timer electrical engineer that works next door is about 90% sure how to do it, but he hasn't done one like this in a long time. I'm using this to buck the voltage from 250 at the plug to 208 for some tile kilns.
Thank you!

 

[Lucien Nunes]

Hi, sorry, your picture links are broken so we can't see your transformer. Could you please upload the pics to the forum?

 

[fastjer]

sorry about that...

 

[James]

You need a 400v phase to phase supply for that transformer to output 208v

There are no input connections suitable for 250v phase to phase input.

 

[pc1966]

As above, not really for that job.

 

[Lucien Nunes]

The taps are only for accommodating minor variations in the nominal supply voltage so they don't span a wide range. The primary winding will be wound with wire large enough for full power operation at the lowest tap voltage, but not lower. Using a significantly lower supply voltage such as 250V would require higher current and hence larger wire for part of the winding, so the makers don't add lower taps unless specified as they would increase the size and cost.

Rather than a buck/boost transformer this is a double-wound delta-wye. In a buck/boost autotransformer the supply and output can be taken from taps on a single winding and the wire can be smaller than either the primary or secondary would be if separate, since some of the current is common to both circuits. There is a Rube Goldberg solution that would in theory create the desired voltage using this transformer, that one could use as a life-saving emergency measure but not as a proper installation to code. That involves rewiring it into an auto configuration with the taps of the primary added onto the ends of the secondary, but it really isn't a practical or safe answer in this case.