On Wed, 13 Feb 2008 01:05:22 -0500 snipped-for-privacy@aol.com wrote: | On 13 Feb 2008 05:46:10 GMT, snipped-for-privacy@ipal.net wrote: | |>On Tue, 12 Feb 2008 19:11:46 -0600 Ben Miller wrote: |>| snipped-for-privacy@ipal.net wrote: |>|> One option is to use a three phase transformer where all three |>|> secondary windings are 240 volt center tapped to 120/240 volts. What |>|> you get are SIX separate hot phase wires, and a neutral. I'll label |>|> the phase wires going clockwise as A,B,C,D,E,F. You can have 2 three |>|> phase systems from this, connecting to A,C,E or B,D,F. They would be |>|> 208Y/120. And you can have 3 single phase systems from this, |>|> connecting to A,D and B,E and C,F. Divide the tenants that get single |>|> phase three equal ways and this way they get genuine 120/240. This |>|> is actually a bit more complex than the common systems (for example |>|> you would have 3 separate distributions for the single phase). You |>|> probably can't meter the whole thing at once. |>|> But I'd prefer such a system, myself. |>|>
|>| |>| I am obviously missing something. How do you connect the center taps of |>| three windings together, and then connect them in a wye, and not blow up the |>| transformer? And how do you get 240/120 & 208Y/120 from the same windings? |>
|>Maybe an ASCII art picture will help: |>
|> B C |> \ / |> \ / |>A----N----D |> / \ |> / \ |> F E |>
|>A-N and N-D are both wound on the first core. B-N and N-E are both wound |>on the second core. C-N and N-F are both wound on the third core. A third |>of the 120/240 loads would be served from A-N-D. A third of the 120/240 |>loads would be served from B-N-E. A third of the 120/240 loads would be |>served from C-N-F. Half of the 208Y/120 loads would be served from A,C,E |>and N. Half of the 208Y/120 loads would be served from B,D,F and N. |>
|>You could also get 120 volts from A-B or B-C or C-D or D-E or E-F or F-A, |>but you would not want to. |>
|>You could also ignore the center tap and rewire it for 416Y/240. |>
|>If the windings can be split and wired in parallel, you could rewire it |>for 208Y/120 with double the amperage. |>
|>A transformer with dual secondary 120 volt windings on each of the three |>cores would be quite flexible, being able to be configured for any of |>these three systems (but you would have 12 wires coming off the cores on |>the secondary side). | | I agree with Mr Kelly | | They use delta vee to save money, why would they do this 6 pole thing?
To serve a lot of 120/240 volt single phase loads AND keep phases balanced? It might depend on how large a building is involved. Suppose it is a 10 floor building with 9 floors that have 12 residential apartments, each. Would you in the role of a utility engineer be satisified running single phase service into that building? I would think not. Then would delta with only one side having all the 120/240 loads really be any better? What I would ultimately propose for such a project would be to bring in
480Y/277 to the building, run that to the elevators, and put single phase transformers on floors 3 (serving 2-4), 6 (serving 5-7) and 9 (serving 8-10), fed from diverse 480 volt connections (480 to 120/240 volt single phase dry transformers are pretty common). Various outside HID lights could also be fed from the 277 volts.
The risk of mixing the wiring with 6 phases is probably realistic. But isn't that what color marking of conductors is for? Besides, the only places to get that mixed up is between a common single transformer and the place where the phases split into 3 sets of single phase. If this concept had been chosen a long time ago and been an accepted standard, I'm sure we would have a color code standard, now, for each phase angle.
If anyone does make a three phase transformer that could be wired for either 208Y/120 or 416Y/240, my bet is someone with the know-how could wire it for the scheme I suggested (although it might not have enough outgoing terminal lugs, or enough conduit space, to do it).