O.T. Three Phase by Single Phase Transformers

[jrwb4gbm]

Try to envision going to the power generating station and somehow disconnect the windings that generate phase B and phase C. Even with just phase A winding connected, you still must have two wires to transport the power to you. The interaction of the other phases, while good to think about, doesn't really affect the outcome of the A phase from the power station to the transformer windings.

 

[Gil47]

Here is a post from a few years ago that might make interesting reading for you
At post 40 I had a graph where I tried to add to the confusion

http://www.plctalk.net/qanda/showthread.php?t=41408

 

[keithkyll]

And my confusion at post 42.
A-B, B-C, and C-A are ALL single phases. They are 120 degrees apart from each other. When you use just 2 legs, you are using ONE of the 3 phases, not 2.
The confusion is because you only see 3 wires, but really there are 6 wires coming out of the 3 phase generator. Start and Finish for each of the 3 windings. It's clear if you think of them tied together in a Delta.
It's confusing when they are connected as Wye. Each leg does look like it's 120 degrees apart, but, as one leg is going up, the other is going down. The net voltage is a true sine wave, but the voltage is reduced. For 120/208, It's 120 * the square root of 3 (1.732).
Forget about 180 degrees. There is no 180 degree signal in 3 phase.
The 240 at your house is NOT 2 phases, 180 degrees apart. It's single phase. There are 2 windings coming from a single iron core. The Finish lead of the first winding is connected to the Start of the second winding (and grounded), but both of them are still coming from ONE phase, also from a 3 phase line. The 3 lines you see on the pole from the power company.

 

[iant]

Gents:

When the primary of a 480v 3ph by 240v 1ph transformer is connected across two legs of a three-phase source, is it not true that the 240v secondary is actually only 120 degrees in phase separation, not 180 degrees like we find in household 240v power?

I've Googled this and find no definitive answer (that I'm capable of understanding, anyway!).

Thanks,

Bill

This is only a single phase input to single phase output
transformer
You are assuming that 480V is three phase - it is not - it is a single phase voltage.
as a result the output is only 180 deg out of phase to its input.

the other phase angles only come in to the calculations when you are using poly phase transformers
these are polyphase input and polyphase output type on;y.
there is no such beast as polyphase to single phase.
you will hear of DELTA/DELTA DELTA/STAR STAR/DELTA
or DELTA/V (open Delta)

 

[iant]

bucking and boosting of transformers is a totally different ballgame

 

[leitmotif]

Iant
Oh Lawzy let us not get into buck and boost. We got troubles enough with 3 single phases connected together to derive this magic 3 phase. After 40 years I finally got the confusion of 3 phase in my head under control when I sat down and drew it all out and did some calcs and graph in Excel.

Teaching my students that this 3 phase is derived from 3 single phases and there is NO 3 phase load they are all single phase seems to work. The magic comes about when you connect them with them magic delta wye connections.

Dan Bentler

 

[Helliana]

I actual worked in a plant that had Wye->Wye type transformers. To make things worse, the neutral ground was cut when the parking lot was made. They couldn't figure out why none of their instrumentation worked.

Has anyone else ever seen a wye-wye connection? I know is generally a bad idea because there is no path for 3rd harmonics, but just wondering if anyone else has seen the connection being used.

 

[iant]

I actual worked in a plant that had Wye->Wye type transformers. To make things worse, the neutral ground was cut when the parking lot was made. They couldn't figure out why none of their instrumentation worked.

Has anyone else ever seen a wye-wye connection? I know is generally a bad idea because there is no path for 3rd harmonics, but just wondering if anyone else has seen the connection being used.

wye-wye or STAR-STAR as is reffered to here is normally only used for isolation setup as 1:1 ratio
using it for actual transformation the detla - star is easier

- for your refference
the only course I ever got an A+ was transformers -
generaly just passed other subjects LOL
I am better practically

 

[iant]

Iant
Oh Lawzy let us not get into buck and boost. We got troubles enough with 3 single phases connected together to derive this magic 3 phase. After 40 years I finally got the confusion of 3 phase in my head under control when I sat down and drew it all out and did some calcs and graph in Excel.

Teaching my students that this 3 phase is derived from 3 single phases and there is NO 3 phase load they are all single phase seems to work. The magic comes about when you connect them with them magic delta wye connections.

Dan Bentler

it took me ages to master VEN diagrams for calculation of current in the neutral - But you get that when you are learning.

 

[Lancie1]

Has anyone else ever seen a wye-wye connection? I know is generally a bad idea because there is no path for 3rd harmonics, but just wondering if anyone else has seen the connection being used.

Yes, and we considered wye-wye to be safe because it does have a hard ground point on the secondary side (as does Delta-Wye). I never heard the thing about 3rd harmonics - at least I don't remember it which is not the same thing.... Delta-Delta and Wye-Delta are the transformers you want to avoid in a plant setting where normal people could be exposed to ungrounded systems. When I worked for a large government utility company, we used the Delta (Star) secondaries on major substation transformers located behind locked fences, because the only people exposed would be highly trained high-voltage linemen. The advantage to the utility was that one grounded phase would not cause a breaker trip, allowing time to clear the fault.

 

[iant]

Hi Lancie1.
You are right - third harmonics are load dependant not supply as is the case with Transformers.
I have a High Voltage License - up to 220KV

Third harmonics relate directly to the current in the neutral.
As a ballanced or unballanced load with high PF correction can produce a high I
at the end of the day this has nothing to do with the transformer other that excess load.
so cable sizes etc are issues.

 

[iant]

Further to my last post
the Wye - Wye transformer would also pass on the third harmonic as it is frequencey based