catchtwentytwo said:
You can spot an installation using because one of the three pole mounted XMFRs is usually physically smaller.
Many years ago, I was sent to take over a small industrial installation . . . Pulling into the parking lot, I saw the "smaller" can on the pole and knew it would be interesting.
On a Delta system, with the center-tap of one transformer grounded, there should be two smaller transformers and one larger, symmetrically, not the other way around, as Paul states below.
The only time I've seen a high-leg system with one smaller transformer is when it's an open Delta, in which case there are only two transformers, and the high-leg transformer is the smaller one.
big john said:
Would this be the transformer from which "B" phase is derived? I'd guess it's smaller because unlike the other two phases, it doesn't usually supply any single phase loads?
Remember, on a closed Delta system, each line is connected to two transformer secondaries, not one as in a Y system. The open Delta, on the other hand, does supply the high leg from a single transformer terminal, but never line-to-neutral loads.
Builder said:
An off topic question,
On a open delta system, is there a sine wave produced on the open side of the system, where the missing transformer is?
Yes, and in theory (i.e., in a no-impedance world), the system should behave exactly as if it were a closed Delta. Also in theory, if you took a closed Delta system and opened the point where two secondaries connect, the voltage between the two conductors should be zero.
Paul Allen said:
Just for the record, in a delta configuration, the way you can spot it by looking at one on an overhead pole is one large transformer and (usually, closed delta) two smaller transformers.The larger transformer is the one that has the neutral and is sized for the single and three phase loads it will see, The other ones are sized for line to line and three phase loads only. Delta services are usually used only when there are a limited amount of 120 volt loads needed. if the majority of the load is 120 volt, 208V/ 120 service is much more practical.
Actually, the open Delta is also suited for a larger 240/120v load and smaller 3-phase load proportion.
ndc81167 said:
when I put my tester across the center leg to ground i get nothing
is this normal
Only with a corner-grounded Delta system ,either intentionally grounded (most likely) or a floating Delta and you just happened to find the accidentally-grounded (faulted) phase.
For the record, the Delta secondary with the grounded center-tap is exactly the same as the 240/120 single-phase system in your home. The open-Delta system most likely started out as an economical way to convert an existing single-phase system to three-phase.
A primary (pardon the pun) advantage of the open Delta system over a closed Delta is that a third primary conductor is not required. In fact, only an open Delta system can be derived without three primary conductors.
Not only that, the primaries must be connected single-ended, meaning each uses the system neutral as one of its conductors. That's the only way to get the phase timing differences from two hot wires.
It's a lot like getting either 208v 1-ph or two 120v 1-ph circuits from only two conductors of a 208/120v system; the neutral must be used to get anything other than a 1-ph line-to-line voltage.
The 208/120v Y system, and its larger cousin, the 480/277Y system, are relatively new kids on the block. Y-connected primaries, aka single-ended, on the other hand, have been around longer, but there are plenty of Delta primaries around, too.
