On Fri, 15 Feb 2008 21:16:12 +0000 (UTC) Michael Moroney
| email@example.com (Michael Moroney) writes:
|>>On Thu, 14 Feb 2008 20:15:45 +0000 (UTC) Michael Moroney
|>>| One weird setup I've seen, and I'm not totally sure exactly what it is:
|>>| 1 large 3 phase transformer (single can) with a small transformer can
|>>| below it on a pole. The small transformer has NO MV connection nor even a
|>>| MV bushing! It sits below the drops to the homes/businesses. The only
|>>| thing I can think it could be is the big can is 240V delta, and the high
|>>| leg powers the small can, with is a 208V/240VCT. The big can powers half
|>>| the residential services plus any 3 phase services, the small can powers
|>>| the other residential services. It's sort of Scott-T relative to the big
|>>| can. But this is a guess, I actually don't know how it's wired. I even
|>>| took a photo to try and figure it out. There are quite a few of these
|>>| setups in Philadelphia.
|>>If the big can is already 3 phase, why would anyone need to also have a
|>>Scott-T? That doesn't make sense. It must be something else. How many
|>>MV bushings on each can?
|>3 on the big can, 0 (yes, zero) on the small can.
|>I called it Scott-T like because I'm guessing the small can is fed from
|>the 240 delta high leg to neutral, which would be 90 degrees shifted from
|>the 240VCT winding. Assuming my guess at what it is is correct at all.
|>> How many secondary lugs?
|>4 on the big can, I believe 4 on the small can.
| I looked at my photo again, and it is understandable what is going on
| after all. See for yourself:
| It's not clear the large transformer has 3 MV bushings, but it does.
| One connection from the large transformer goes only to the small one, 3
| others (1 connected to a bare neutral) go below it and two additional
| leads from the small can go downward, for a total of 5 leads that run down
| the pole.
| There's another (probably) 120/240V service going left to right (with a
| drop to the upper right) that's appears not to be involved or connected,
| except for the neutral.
| It appears to me, if the large can is delta with the lead to the small can
| the high leg, that this is a 5 wire 2 phase circuit, or what many would
| call 4 phase, 4 poles spaced at 90 degrees and a neutral. Can anyone
| suggest an alternate possibility?
It looks to me like what is going on is that the first can provides common
And then the second can turns one of the phases into 120/240 like:
Most of the loading is 3 phase where 208Y/120 serves the need, but some
load needs genuine 240 or split phase 120/240 and they didn't want to
hang a 2nd can up high to do it directly from MV (and can't bring the
MV down to where the 2nd can is now).
Just a guess.
| Phil Howard KA9WGN (ka9wgn.ham.org) / Do not send to the address below |
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