ECN Forum Forums Technical Discussion Electrical Theory and Applications 2 phase neutral current?

sparkee,

The diagram that you've just presented is _not_ a 120/208V system, but is instead a 120/240V _single_ phase system. If you redraw the diagram with 208V between legs rather than 240V, then the diagram will be wrong.

In the 120/208V system, you have to have at least two transformer cores, each on a separate phase, supplying a separate supply leg to the residence. The two transformer 'legs' carry magnetic flux 120 degrees out of phase, generating voltages that are 120 degrees out of phase. The vector difference of 120V at 0 degrees with 120V at 120 degrees gives the 208V.

In the diagram that you've presented, you only have a _single_ transformer core, so both coils must be in phase.

-Jon

Winnie
"The whole point of saying "2x1560VA being supplied by the transformer, yet only 2704W being delivered to the load" is an explicit example that phase angle differences are present in this system and must be accounted for using vector math."

This is absoultly not true. The reason the is 2704 watts deleiverd is because of the vector addition at the transformer terminals. The transformer only delivers 2704 watts.

"Clearly, 13A is flowing through leg A transformer coil, and 13A is flowing through the leg B transformer coil. Thus the heating experienced by the _transformer_ is what one would expect from having 1560VA per phase supplied by that transformer. But the power being delivered to the load is only 2704W, and the _difference_ between 3120VA and 2704W is explicitly caused by phase angle differences."

Absoultly not true. There is no 3120 watts.
The transformer delivers 2704 watts.
This is my last post on this subject.
Winnie you have just enough information to get it all wrong. Except the fact that you "MIGHT" be wrong and get with an engineer in your area. He has the math backgroung to explain how this works. Just check. No need to continue the rest of your life being wrong.

[This message has been edited by Bob (edited 03-12-2006).]

Bob,

While not making me an expert, does my BSEE, my PE, and my job as a Power Systems Engineer qualify me to discuss/debate vector additions?

Stop trying to always use the neutral as a reference point. Only look at the circuit analysis using the 208V line to line voltage. The phase angle difference between A-N and B-N voltages is accounted for by using the A-B voltage of 208V.

JBD
You are correct. I could not agree with you more. I don't think you will find any of my statements that would disagree with you.
However the initial post had 2 loads connected phase to neutral and the question was related to the neutral current.
My disagreement is with this statement by Winnie.
" But for 13A to flow through the load, 13A has to flow through both phase A and phase B. This means that 1560VA is being supplied by each phase. If 3120VA is being supplied, but only 2704W delivered to the load, then there has to be a power factor somewhere "

[This message has been edited by Bob (edited 03-12-2006).]

Just a confirmation, with the 120/240 single phase system like below if there is a load of for example 1700w on Line B and a load of 1500W on line A total current on the neutral = 1700 - 1500/120 = 4.1A?




Does anybody know of a diagram of the 120/280 system? This thread is kinda hard to read without the quotes in bold

Is it like this without the extra phase?



[This message has been edited by Sparkee (edited 03-12-2006).]

Bob,

I strongly suggest that you review what I wrote. If you don't know the difference between a volt-ampere and a watt, then I we will have a very difficult time communicating.

My core statement is that in the given example, the current flow through the phase A coil is _not_ in phase with the voltage induced in the phase A coil, and that the current flow through the phase B coil is _not_ in phase with the voltage induced in the phase B coil.

If we use the phase A coil (the voltage measured from the A terminal to neutral) as our reference zero phase, then the B terminal to neutral voltage is at 120 degrees, and the current flowing _out_ of the phase A terminal is at -30 degrees and the current flowing _out_ of the phase B terminal is at 150 degrees.

If you agree to this statement, then we are on the same page regarding the mathematics of what is happening, and are simply misunderstanding each others words.

-Jon

[This message has been edited by winnie (edited 03-12-2006).]

sparklee,

Your first diagram is a single phase 120/240V single phase system. If you had a load of 1500W on line A and 1700W on line B, then you would have 1500/120= 12.5A on line A, 1700/120 = 14.17A on line B, and (1700 - 1500)/120 = 1.67A on the neutral.

Your second diagram is a three phase delta to wye transformer. If you just use X1, X0, and X2, and ignore X3 to supply your loads then it is exactly the system that I believe that you were talking about in your first post. In this case, if you had 1500W connected from X1 to X0, and 1700W connected from X2 to X0, you would still have 12.5A flowing through X1, 14.17A flowing through X2, but now you would have 13.5A on the neutral, as calculated by Bob and JBD at the top of the thread. Note that Bob and JBD used different calculation methods, and got answers that were not exactly the same, but this was because of differences in rounding; if you follow either method and don't round off, you will get the exact same result.

-Jon

Thanks Winnie and to everyone else that helped!

I think I have it now

Sparkee;

The inbalanced current of the 1Ø 3 Wire system will be 1.6667 Amps on the Center Tapped Neutral.
The "Common Load" of 1500 VA - or 12.5 Amps @ 120 VAC, will flow between the "Ends" of the Secondary Winding.

(oops, already answered! Sorry!)

Here is a crude schematic of a 4 wire Wye:



I thought for sure this schematic was re-drawn and replaced on the server, but I guess not!
Place this task on my "To-Do List" at entry # 37,525,089

Anyhow, this thread is kind of going everywhere, yet gaining absolutely nothing! (like walking to the store, then walking back using the same route, yeilds no gain in distance!).

Here's a few simple questions to ponder:

If I take ØA, ØC and the tapped Conductor from the Star Point (the common grounded conductor in most cases, AKA "The Neutral"),

* Could this be viewed as a Polyphase Setup?

* Could this be thought of as a 3Ø setup?

* Can I connect this to the Primary of a 3Ø Transformer, and achieve a 3Ø 3 or 4 wire output? (depending on the secondary configuration)

* If I connect a 50,000 Watt Pure Resistive Load (P.F. = 1.0) between Phase A and Phase B of a 480Y/277V 3Ø 4 Wire system, will the load current be:
<OL TYPE=A>

[*] 104.167 Amps,

[*] 180.51 Amps, or

[*] 60.169 Amps.
</OL>

* If a Spaceship lands in your back yard, how many cubic yards of Sand will it take to fill your Dog's House? - True or False?

I joke a bit about Electro-Techno stuff, mainly because where else can someone add up 1 + 1 + 1 and get 1.732!!!
(or 3 + 4 = 5!!!).

I think for the sake of others reading this thread - who are not as advanced in these areas of theory as others, that we stay on track with the data being discussed - and whenever possible, show references!

If you cannot upload a drawing or text, then contact me via e-mail, to make arrangements and such, and I will do the honors of uploading and embedding images to these threads.

With drawings and schematics, please complie them into .GIFs or even .JPGs first. I don't have as much "Free Home Time" as before, so converting images by me is a rare event!

Please do not take this message as any level of dicketry!
I just wanted to make a point of reminding everyone about the vast audience viewing this forum area - and that many viewers are trying to better themselves in the theory area.
Help these people by making things a little easier to follow and understand whenever possible.

< soapbox mode = off >

Scott35