Stumped by transformer application

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DragonWell

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Ventura, CA
I visited a bank branch yesterday and on the roof I saw that all eight gas/electric packaged air conditioners had (2) single-phase transformers attached to the top of each AC unit, and I can't figure out why or how they wire wired.
Each transformer was 120/240V pri, 12/24V sec, single phase, 3.0 kVA. The power feeder came up from the switchgear, to the line side of the disconnect, and the load side wires (all 3) routed up to the transformers and then back down to the AC unit. So it appears the AC units are completely fed through these transformers. I could not get access to the transformers' wiring compartments so I have no idea how they're wired.
The power supply from the switchgear is 208Y/120V three-phase.
The AC unit nameplates call for 208/230V three-phase supply. I did not have a voltmeter on me.
I'm completely stumped.
Who is studly enough to explain this one?
 
Most likely these are connected in a 'buck-boost' autotransformer configuration for going between 208V and 236V(nominal 240v). this would have been done to match the units to the available source.
 
jim dungar said:
Most likely these are connected in a 'buck-boost' autotransformer configuration for going between 208V and 236V(nominal 240v). this would have been done to match the units to the available source.
Click to expand...
That was my first thought too. People often get confused by the nameplates on Buck-Boost transformers, because they show the "real" votlages they were designed to work on. But for Buck-Boost you are just using the winding ratio.
 
DragonWell said:
I visited a bank branch yesterday and on the roof I saw that all eight gas/electric packaged air conditioners had (2) single-phase transformers attached to the top of each AC unit, and I can't figure out why or how they wire wired.
Each transformer was 120/240V pri, 12/24V sec, single phase, 3.0 kVA. The power feeder came up from the switchgear, to the line side of the disconnect, and the load side wires (all 3) routed up to the transformers and then back down to the AC unit. So it appears the AC units are completely fed through these transformers. I could not get access to the transformers' wiring compartments so I have no idea how they're wired.
The power supply from the switchgear is 208Y/120V three-phase.
The AC unit nameplates call for 208/230V three-phase supply. I did not have a voltmeter on me.
I'm completely stumped.
Who is studly enough to explain this one?
Click to expand...

Do you normally go up on the roof of a bank when you visit it?

Only place I'm allowed in the bank is the teller window and loan officer's office!:p
 
Jraef said:
That was my first thought too. People often get confused by the nameplates on Buck-Boost transformers, because they show the "real" votlages they were designed to work on. But for Buck-Boost you are just using the winding ratio.
Click to expand...
Thanks guys!
This led me in the right direction, and I think I found the wiring diagram: http://www.jeffersonelectric.com/s3/site/catalog/buckBoost.pdf. It's wiring diagram #12.
Looks like they went from 208/3 to 230/3.
 
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