3-phase

[Kidd6488]

I understand that in 3-phase feeds, each phase is 120° out of phase with the next. But what I don't understand is the Delta, and Wye, and also, how you get 120V for all your standard loads, and why you get 208 1-phase instead of 240...

Help is appreciated,

~Jonathon Reinhart

[Rewired]

Ahh, this is going to be a bit tricky to explain but I think I can pull it off. I am a little rusty in how currents add in the windings for delta and wye, its been a long time since I took it in school, and its complicated as hell, but I will be back there soon!
Anyways in "Delta", the three windings are wired in a triangle (Delta)type formation A-B' B-C' and C-A' where for example "A" is one end of the "A" winding and "A'" is the other. your 3-phase power is supplied (or delivered in the case of a load" to these three points. generally if a service is supplied from a delta system its NOT grounded and is only 3-wire.

For a WYE system the windings are wired in just that formation, with A'-B'-C' all joined in the "center" and A, B, and C supply your load (or is connected to supply if its a load).
IF this wye connection happens to be from a transformer or generator say, that "center" connection becomes your neutral and is most likely grounded, to provide a "grounded" supply. so off this type of connection you have 3 hots and a neutral and you can get different voltages.

Now, if I haven't lost you so far, you were asking why you get "208" SINGLE phase instead of 240..
In a single phase system, say out of your transformer on the pole supplying your house, it has two windings with a center tap connection between the two, each winding is configured to provide 120 volts, but they are 180 degrees apart at all times, so each winding will provide 120 volts individually but when you put them both "together", you get 240 volts..
With a 3-phase system that supplies 120 / 208 V, each winding is supplying 120 volts but they are 120 degrees apart [u]not a full 180 </u>degrees. Because of that phase difference, only 208 V appears across any 2 legs of a 3-phase 120/208 V system.

In some commercial applications and generally everywhere in industry,
each winding may provide an "odd" voltage such as 277V (mostly in U.S.A and 347V (here in Canada). These odd voltages can and ARE used to supply large lighting loads, but you get the tradional 480V (U.S.A) or 600V (Canada) to operate large machinery out of the same supply.

I hope I havent confused you, please let me know if I have and I will see if I can clarify it for you!

A.D

[Kidd6488]

I understood most of that. But don't they use standard 240V center-tapped "pole-pigs" (as us tesla-coilers like to call them... hot-streamer.com) for the transformers in 3-phase systems? can't they then use one wired normally for 120/240VAC and use it wired to the other ones for 3-phase.

~Jonathon Reinhart

[Rewired]

Yes, I see how you picture hooking them up, and I asked someone about that once before and they said it would throw the whole thing out of balance, or you could get wierd voltages or something like that, I am not sure though, it was a long time ago that I asked.
I do know that they used to provide a "240V 3-phase DELTA" service, BUT they could also bring the "center tap" in from ONE of the 3-transformers on the pole so it would act like a regular single phase "120/240V" service, BUT having the center tap grounded, because its a neutral and it has to be, the remaining phase will show 208V to ground. This is known as the "high leg" and it can't be used to power anything connected to it and the center tap!

Hope I didnt confuse ya!

A.D

[HayZee518]

sometimes what the utilities use isn't a full three phase transformer but two transformers connected as an auto-transformer. The input leg for one phase forms the output in the line - the other two go through the transformers to increase or decrease voltage, additive or subtractive windings. But should a transformer open you'll have full line voltage on that lead Ouch!!

[Spark Chaser]

Sometimes a Delta system will have 1 leg go to ground and every thing still runs ok - However - now you have 2 legs that will read ie: 480 to ground and 1 leg will read 0 volts to ground - You really have to be careful on these systems. If for example - You were touching the 0 leg and for some reason the grounded leg went high (such as a plating operation where the motor didn't run all the time when it moves parts from tank to tank) - You could get the leg voltage through you to ground.

[LazyPup]

while we are on the subject of primary supply voltage i would like to share something my master taught me while in the apprenticeship.

We were in a large industrial primary control room and Roy says look around and tell me what is common to everything in this room?

I said it is just an assortment of line starters.

He says yea, but look close, each one has two levers on it.

I said "hell no Roy,Your only allowed one control lever on a pull box"

Roy says..."Wrong boy,, there is Lever A..and Lever B....and I am telling you right now,,,,if you don';t understand lever A..you better damn well leeeever Beeee!!"

[HayZee518]

Lazy - I don't understand the levers you speak of. However I do understand what you are inmplying "leave her B" No touch!

[Kidd6488]

So If I understand, Voltage from:

C is high leg, and is 208 to gnd.

ABC are three-phase connections

A to B is 240

A or B to gnd is 120

E D or F to gnd is 240

What else did I miss?

~Jonathon Reinhart

[LazyPup]

hayzee,,the old timers pronounced the word lever as leeevers..thus leever A and leeever B.

[Kidd6488]

I don't know if any of this leeever B stuff is directed towards me, but don't worry, I dont plan on doing any 3-phase wiring, I was just curious, as usual...

~Jonathon Reinhart

[Rewired]

Jonathon:

That last drawing you have with the "Y" winding, the voltages could be anything really, depending on what it was set up for, like
"120/208" "240/416".. "277/480" "347/600" even "16,000/27,600". It all depends on what transformers you use.

The "centertapped delta" you have drawn is correct though, a system like that is always going to be those voltages, there is no real other use for a centertapped delta service other than to provide some 120V power to some light loads.

Sometimes you could have a delta service where it would be ungrounded, and any voltage is possible, again depending on what transformers are used.

[Kidd6488]

Ok, yes, I was assuming they were 120/240 V center-tapped transformers. You are right, they could be anything. I was more interested in the ratios of voltages. Thanks

~Jonathon Reinhart