Bank of Secondary 120/240V, 1PH Xfmr into 208Y/120V

[naminorite]

I'm stumped. A EE specified a bank of three, single phase transformers factory wired 120/240V on the secondary side, but required the contractor to wire it into a 208Y/120V. When I approached the EE, he said that the 208Y/120V can be achieved by tying the neutral terminals and leg terminals together. I presumed he meant one of the legs of each single-phase transformer. As I sat and sketched it out, I find that I'm shorting half of my secondary windings and that when I looked at the turns ratio, ended up with a 480V(L-L). I don't believe my L-N voltage would be the 277V, but that is besides the point. Am I missing something here? Can the specified bank of single phase transformers be wired into a 208Y/120V system?

 

[racobb]

"The last time the boys did that,"

You lost me on that one Bill. I know I am slow so what am I missing?

Thanks

Alan

 

[itsmoked]

racobb; just substitute "the guys I was working with" for

"the boys"

.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[waross]

The boys, the guys I was working with, inadvertently connected the windings into a dead short when changing the connections from 120:240V to 120V. A similar effect to that which my typo may have produced. Surprisingly everything survived.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[racobb]

Thanks

Alan

 

[naminorite]

Thank you all for your time and effort in enlightening me on this matter.

 

[waross]

Yes, that connection will work. It is sometimes called a four wire delta. The downside is that you take a 1/3 hit on capacity.
The three 37.5 kva transformers connected in star will produce 112.5 KVA.
Three 37.5 KVA transformers connected in four wire delta will produce 75 KVA single phase. Calculating the load placed on the individual transformers by mixed three phase loads and single phase loads will take a lot of vectors, particularly if the loads are not the same power factor.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[PHovnanian]

You could waste half the transformers capacity by wiring the secondary center taps together and using only half of each windings for your 120/208V Wye, leaving the other 120V terminal unconnected.

If you need a second 120/208V service at this location, you could use the other halves of each winding.

I'd hesitate to do anything like this in practice. Send a lineman out to do repairs in the middle of the night on such a screwball connection scheme and fireworks may ensue.

 

[apowerengr]

You need to reconnect the internal connections for full transformer capacity. You could make the connections externally but with only half capacity available since you'd be only using half the winding. I've done a hundred of these myself - half day if you are short on coffee in the morning, a couple hours with the caffine boost.

 

[waross]

I'm right with you on the time, apowerengr. Actually the nameplate of this transformer shows the parallel 120V connection. It's a little hard to read in the picture, but if you know what it should be, it is understandable.

Bill
--------------------
"Why not the best?"
Jimmy Carter