| firstname.lastname@example.org says...
|> | <attempt to snip to size>
|> |> That is the same as the six-phase "star" or "6 star". So if you can't
|> |> see any difference, that's because there isn't any. I would not have
|> |> called this a "wye" of any sort since it doesn't look like a "Y". But
|> |> it could be TWO Y's interleaved.
|> | No, not "interleaved" but split. Invert each phase, as in a center-
|> | tapped transformer (I.e. split) and you're there.
|> You mean like having 2 three phase 208Y/120 transformers, one fed 180 degrees
|> out of phase from the other, and combining their outputs with neutrals tied
|> together? That's what I meant by interleaved. If that's not what you are
|> saying, then I don't understand what it is you are saying.
| Either that or CT secondaries. Same difference.
|> |> To make this derived system, you need either:
|> |> 1. 3 single phase transformer cores
|> | Kinda like they do now on the pole.
|> Yes. Except instead of using 120-volt-only transformers (there are such
|> things ... just 2 lugs on the secondary), they need to use the 120/240 volt
|> transformers (plenty of those around) and understand the concept to know
|> how to wire it up.
| IF there is any advantage (doubt it), they'll figure out how to wire
| it up. It's rather obvious.
There is an advantage to having genuine 240 volts. Not all circumstances
make it easy to use separate single phase transformers. The ones that do
would already have multiple transformers. Otherwise it increases cost.
And there are no "6 star" transformers made (yet).
|> Metering that could be an issue. In the case of the apartment building,
|> individual tenants would be metered at 120/240 so it's not an issue. But
|> if this needs to be metered in its "6 star" configuration, it can be done
|> with 3 CTs, each carrying the 180-degree-opposite conductor pairs running
|> in opposing directions just as a single CT would be used for single phase
| Yes, that's why I see nothing unique about your "idea". It's done
| today, except there is no need for the inversion.
Maybe there is a confusion here. CT could mean "center tap" or it could
mean "current transformer". In the above, I mean "current transformer"
for the purpose of metering.
|> |> 2. 1 three phase tranformer E-core
|> | Kinda...
|> It could be done. The manufacturing just has to provide a pair of 120
|> volt windings, or a series 120/240 tapped winding, and sufficient terminal
|> board space to deal with it.
| There is likely a reason they don't. Though in your case of an
| apartment building they do, though likely with individual
| transformers. It's done on 90% of the poles in the country.
What is done in on 90% of the poles? The "6 star" suggest is certainly not.
|> |> Then you need to wrap these cores with 3 primary windings and 3 secondary
|> |> windings. Usually the secondaries go on the inside and the primaries go
|> |> on the outside, so the secondary at lower voltage and higher current has
|> |> less winding resistance.
|> | Ok, I'm confused. This is different how?
|> The difference is that a traditional E-core 208Y/120 transformer would have
|> only ONE 120 volt winding per core bar (times the 3 core bars). Two such
|> windings at 120 volt each, or a 120/240 volt series winding (connected in
|> series inside the winding instead of at the terminal board) would be needed
|> for the "6 star" configuration.
| No different than what happens on 90% of the poles in the country,
| except those are in three cans rather than one.
The "6 star" is not done anywhere I have ever heard of. You must be
referring to a common WYE, which is not the same thing (WYE is a "3 star"
not a "6 star").
|> |> Each of the 3 secondary windings needs to be either:
|> |> 1. a pair of 120 volt windings which you can wire in series
|> |> 2. a single 240 volt winding with a center tap right in the middle
|> | Exactly (is there a difference?).
|> Yes, there is a difference. In #1 you have 4 wires coming from the secondary
|> winding to the terminal board. In #2 you have 3 such wires because the
|> winding is series connected, possibly a continuous single wire, inside the
|> winding itself, and just tapped at a mid-point.
| A distinction without a difference. WHo cares how many screws are
| on the terminal board?
You need more screws for the extra wires. How many depends on if
you want to double up on a single lug or not.
|> It's a difference in construction. Some people might be more familiar with
|> one over the other.
| Sure. If there is no reason to make a widget there is a good chance
| that most are unfamiliar with the widget.
|> |> All of the center points of these windings are wired/bonded together and
|> |> grounded. Then each of the three phases will have two poles 180 degrees
|> |> apart. Some people will call this six phases.
|> | Right; split-phase "wye".
|> Keep the "wye" in quotes, then; it's not really wye. I would never call it
|> a wye at all. It's a 6 pointed radial star.
| "Wye" is usually in quotes, even in a normal implementation.
Really? I see it more NOT in quotes. The term WYE is a longish form
of "Y" which is a depiction of a 3 pointed star configuration, not 6.
The "*" more resembles 6 under most fonts (some have 8, but that's
|> |> Which connection pair do you need to ask about?
|> | I don't. You seem to see a difference between this "star" and an
|> | ordinary 3-phase "wye" that delivers residential 240V split-phase.
|> | I don't see anything new or particularly interesting here, but am
|> | trying.
|> A true WYE is not split phase. That makes a contradiction of terms.
|> But if you know of a manufacturer that makes such a transformer AND
|> calls it "split phase wye" or some such thing, please do point to
|> their catalog reference. I've looked at a lot of transformer catalog
|> info online and have never seen such a thing in a single transformer.
| Certainly it is, on 90% of the poles in the country.
Since well more than 10% of poles have a traditional 3 pole wye, there
cannot be 90% with a 6 pole three phase star arrangement.
|> |> Between A and B you have 208 volts.
|> |> Between B and C you have 208 volts.
|> |> Between C and A you have 208 volts.
|> |> Between A and N you have 120 volts.
|> |> Between B and N you have 120 volts.
|> |> Between C and N you have 120 volts.
|> |> There aren't any other ways to connect, and no way to get 240 volts.
|> | That's funny, because that's exactly how I get the 240V for my house
|> | off the 3-phases on the pole.
|> Which connections give you 240 volts?
| The CT on any leg.
Be specific. First of all you know you need TWO connection points for
a voltage. Simply saying "CT" (which I assume to mean "center tap"
instead of "current transformer") means ONE connection. You get no
voltage from one connection.
So how do you get 240 volts from a 208Y/120 wired transformer bank?
Answer: you don't.
|> If you are getting 240 volts from
|> A-B or from B-C or from C-A, and if the phase angles really are 120 degrees
|> as a true three phase WYE would be, then you are going to get 139 volts
|> at A-N, B-N, and C-N. I don't think that is what you want.
| It's a "wye" not a delta. Three transformers with CT secondaries
| from A-N, B-N, and C-N. You just made you "star" configuration.
What are you labeling A-N, B-N, and C-N? The primaries or secondaries?
Your statement "CT secondaries from A-N, B-N, and C-N" makes no sense.
If there were 3 split-phase secondaries involved, there would be a lot
more connection points than just these three. Even if all the center
taps were connected together, you would still have 6 connection points
to label. Starting at A, that runs to F.
|> FYI, I did find one utility offering 240Y/139 service for some portions
|> of their service area, as a replacement for 240D.
|> Maybe you are getting ONE phase of 120/240 via ONE split phase transformer
|> tapped to ONE phase (connected L-N) or TWO phases (connected L-L) of the
|> primary distribution lines. But just because there is three phase on the
|> distribution does NOT mean you are getting it. You are most likely getting
|> one of: 208Y/120 three phase (my grandfather actually did get this at his
|> home), or 120/240 single phase, or that old 240DCT/120 setup.
| I didn't say an individual home was getting three phase, but it's
| there on the pole. ...including your "star".
It's not there if what's on the pole is 208Y/120, which most poles
supplying three phase power have. The only voltages available are
208 volts and 120 volts. Every possible pairing of 2 connections
on these very common setups gives either 208 volts or 120 volts.
I listed them previously. I left none out. None have 240 volts.
|> |> OK, hope the above helps.
|> | Nope. I'm still wondering why you see your "star" as any different
|> | than what we see on 90% of the poles in the country.
|> 208Y/120 is _very_ different. 208Y/120 has THREE line wires coming out at
|> 120 degree phase angle equal intervals. The "6 star" (maybe we can call
|> it 240*/208/120) has SIX line wires coming out at 60 degree phase angle
|> equal intervals.
|> If I provide you with exactly 3 transformers which are wired up with one
|> winding for the primary voltage and one winding for the secondary at 120
|> volts, it can be wired up by connecting the primaries in whatever they
|> need for the type of service (delta or wye) and connecting the secondaries
|> in a wye configuration. There, you have 208Y/120 just like 90% of the
|> three phase poles in the country (I'll just accept your stats of 90% as I
|> do not know the actual figures).
| Look at 100 random poles. You'll see 90 of them wired as 3-phase
| "wye" (really doesn't matter if they are delta). The secondaries
| are CTed, so you have the six points of your "star". Nothing new
You really think that?
Sure, a 120/240 volt split phase transformer used normally for single
phase service could be used for 208Y/120 by connecting only ONE SIDE.
And it might be done in a few places. But it would be very rare since
it is a waste of half the transformer capacity, and thus a higher cost
than needed. They do make transformers that can deliver their entire
capacity on just 2 lugs at 120 volts. There is no center tap. Or if
there is, the center of 120 is 60 so it would not be used.
The majority of 208Y/120 services derived using single phase pole pigs
is done with 3 cans that have 120 volt ONLY secondaries.
|> I have never seen, and never heard of, any "6 star" or 240*/208/120 setup
|> anywhere. I have never seen any utility tariff (I've looked through a few
|> dozen over the past few years) that offers such a service.
| You've seen it, just haven't called it that. There is no point to
| it, so it isn't named.
So you really do have 208Y/120 mixed up with "6 star" or whatever someone
else might refer to it as.
|> I HAVE seen a couple three phase setups where a 120/240 volt pole pig was
|> used, and only HALF of it was wired up to get 120 volts. I HAVE seen one
|> manufacturer detail that they do make cans with the 120 volt windings in
|> parallel internally, and still have 3 lugs with one of them not connected.
|> So these are not necessarily a case of wasting half the capacity. The 3rd
|> lug may simply be there are part of the process of manufacturing only one
|> set of empty cans instead of two different sets.
| Irrelevant manufacturing detail.
It's very relevant. It decides if the transformer has only 120 volts or
if it has 120/240 volts. It decides in the case of 2 separate 120 volt
windings whether they can be paralleled or not (and they need to be in
parallel for the 208Y/120 service). The only transformer design that
allows an external choice of configuring 120 volt parallel or 120/240 volt
series center tapped is one with 4 lugs. I have seen such a transformer.
But virtually all the rest of 3 lugs or 2 lugs.
| Phil Howard KA9WGN (ka9wgn.ham.org) / Do not send to the address below |
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