| email@example.com says...
|> |> |> 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.
| Three transformers can't cost more than a three-phase transformer,
| or they'd do it for the 90% of the poles in existence now.
I don't have any prices for pole transformers. I do have prices for dry
type transformers from Eaton/Cutler-Hammer. I picked the following models
480 -> 120/240 Al 115C 75KVA T20P11F75EE $9,170 (3x = $27,510)
480D -> 208Y/120 Al 115C 225KVA V48M28F22EE $16,810.00
Three of those single phase 75 KVA = one three phase 225 KVA. You can wire
the singles to 120 volts and connect them as wye to be equivalent to the
three phase one. But you'd pay $10,700 extra for the three single phase.
The URL of the transformer catalog is:
The URL of the price list is:
Now if pole transformers had this same issue, I'd expect to see more of
them as three phase in one can. There must be some reason they don't do
that. OTOH, three phase in one can is common in many places in the world.
|> And there are no "6 star" transformers made (yet).
| I wonder why? Could it be because there isn't a reason?
There is a reason to have it. There might be a reason not to. That latter
reason could be ignorance among engineers. More likely it is that customers
doing the buying (building owners and their construction contractors) do not
care to make the voltage match. They shave all costs everywhere they can
and let the resident deal with the losses. They know the vast majority of
them will know even less than engineers. You could be one of those residents
and it appears you would not care if your water heater takes 33% longer to
return to stable temperature.
|> 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.
| Ok, there was confusion on that point. Metering with a current
| transformer would seem to have calibration problems. They really
| need to meter power, not current.
I see you don't know how metering with a CT is done. They do meter power.
A voltage reference is also connected.
|> What is done in on 90% of the poles? The "6 star" suggest is certainly not.
| It *CERTAINLY*
is. Look at the voltages on the top of those pigs.
| It is *EXACTLY*
what you describe as "six star". We generally look
| at it as three Edison 240V circuits, but it is *exactly*
| describe (six 120V circuits - each 60degrees from the next.
Not only do they not exist this way, it is also stupid engineering to do
so when the service is 208Y/120 (which is the most common type of three
phase service in the USA, leading 480Y/277 and 240D). The reason it is
stupid is if you have a pair of 120 volt windings in series and only use
120 volts of it (which is exactly how 208Y/120 is done), you have wasted
half the capacity of the transformer. These things cost thousands of
dollars, and this would be a few thousand dollars of waste for each and
every one used as you describe. No engineer would ever do that except
as an emergency temporary means to restore power when a 120 volt can is
|> |> |> Then you need to wrap these cores with 3 primary windings and 3
|> |> |> 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").
| Look again (open your eyes this time). You're not seeing the forest
| for the trees. It *IS*
on top of 90% of the poles in the country.
| The other 10% are single phase only, generally found only in rural
90% of poles have a three phase "6 star" and 10% have single phase?
You are on some really nasty drugs, dude!
|> |> |> 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
|> |> 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.
| Screws aren't expensive. The wires are there. It exists, though
| perhaps a bit different that you describe mechanically.
You can't just add screws on a terminal board in a transformer. You
have to put in a larger terminal board to have room to keep the spaced
apart. It's doable, but is also a big change since you're messing with
the wires coming off the windings. They put in terminal boards so the
installing electrician does not have to mess beyond that.
|> |> 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.
Not really at all.
|> 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
|> another issue).
| Huh? What are you blabbering about?
You have no idea where the term WYE even comes from?
|> |> |> 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.
| Get it through your head, IT'S THE SAME DAMNED THING. When you
| split the three phase ("wye" or delta) into three 240V Edison
| circuits you've just made your "six star". That's done on 90% of
| the poles in the country. As I said above, the other 10% are rural
| single-phase only circuits.
I know how to make the "6 star".
If you think it is being done somewhere, take a picture. I want to see
one with the center lug of all three transformers connected together and
the other 6 lugs all connected with 6 wires coming off (that is what a
"6 star" is).
|> 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.
| Good grief Phil, the CT transformer gives you 120V from the A-phase
| and it's mirror, 120V A'. Three of those gives you your "six-star".
|> So how do you get 240 volts from a 208Y/120 wired transformer bank?
|> Answer: you don't.
| The CT transformers on every damned pole in town. Man, you're
I know how it is built. Our disagreement is you think it is already
in use and I know damned well it is not.
|> |> 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?
| Does it matter? The secondaries are the same phase as the
|> Your statement "CT secondaries from A-N, B-N, and C-N" makes no sense.
| Think for a moment.
Your statement provides only TWO label letters per phase (for example
"A-N" for the first one). For the "6 star" you need THREE label letters
including N in the middle as the center tap.
|> 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.
| DAMN, you're thick!
Your statements aren't even consistent. You describe transformers that
have 3 lugs and then you describe 2 lugs?
The "6 star" has 3 (THREE) lugs PER PHASE (when built with 3 separate single
phase transformers). You tie the center (CT) lug for all three together and
you give it just ONE label "N". You ground that one, too. Then you still
have SIX (that's a 6) lugs. You need SIX (6) labels for them. Three is not
enough. An example of six labels is: A,B,C,D,E,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.
| You have three phases, each supplying a 240V Edison circuit.
| Connect the dotted lines...
I know how to connect them. Maybe you do, too. But where you are in
error is thinking they are already connected up this way. They are not.
They could be. All you need to do is put three of these split phase
pole pigs on one pole and wire things up the right way. You'll have a
neutral (grounded) and 6 (SIX) line wires.
|> |> |> 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
|> | here.
|> You really think that?
| I *KNOW*
You have a camera? Take some pictures! If you find one such setup, that
is very interesting. If you find 90, that is utterly strange.
|> 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.
| You are master of the irrelevant, Phil.
Saving thousands of dollars in transformer pricing is VERY VERY relevant.
|> The majority of 208Y/120 services derived using single phase pole pigs
|> is done with 3 cans that have 120 volt ONLY secondaries.
| A-HA! He does get it! Now look at the voltages on those
| secondaries; your "six-star".
Those "120 volt ONLY secondaries" are NOT Edison split phase 120/240 as
is done for single phase service.
|> |> 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.
| Phil, I'm not the one who is "mixed up" here.
You certainly have a 120 volt transformer mixed up with as 120/240 volt
|> |> 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.
| Totally irrelevant drivel.
Irrelevant to someone that seems to think all those 208Y/120 three phase
transformer banks are somehow coming up with more than just three line
| Phil Howard KA9WGN (ka9wgn.ham.org) / Do not send to the address below |
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