I recently installed a buck-boost transformer in one of our buildings with 208Y/120v service to support a 1hp submersible well-pump rated for 230v single-phase.
The transformer is a GE 9T51B0158 which is labeled for 208v primary x 240v secondary, up to 500va.
Per the label diagram, I connected 208v nominal (210v measured at the transformer) to X1 & X2, tied H2 to H3, and then took a meter reading at H1 & H4 before connecting to the well-pump control. I measured 259v at H1 & H4, unloaded. Is there some reason to expect this kind of over-voltage? Would you expect the secondary to drop down closer to 240v under some load? Or is it more likely that I have either misunderstood the wiring diagram or acquired a dud unit?
Any thoughts are appreciated.
Thanks,
Dan
Best Answer
That transformer is awful small
Your transformer is not a buck-boost transformer as you assumed it to be -- instead, it's a standard (isolating dry type) transformer with a 120/240V primary and a 208V secondary. As a result, its windings are handling full power, which means that since it's a 500VA unit, it can only handle 2A at 240V coming out. I'd expect it to overheat quite severely trying to handle the 8FLA@240VAC of a 1HP motor.
As to the high output
I would check the input voltage -- it sounds like the input is on the high side, and doubly so because there will usually be a bit of turns ratio compensation on the design secondary side that acts in the opposite direction from what you are describing in a backfed application such as yours. However, it should drop to a more normal value under a reference load.