single vs. 3 phase

[coulter]

He would probably put them in the jugular of the one who asked him to do that.

You got me laughing again. Me?? Calm, collected me - of course not (giggle, snork)

carl

 

[coulter]

Yes, they do. ... There is no other place to put the black lead. ...

Sure there is. As one measures around the loop, you put the the red lead in front of the black lead - all the way around the loop until the red lead is back to the start. Kirchof (sp?) wasn't it?

Yes, they do. Both have neutrals, so why not measure the voltages in the same manner?...

Because the way one wires them is not necessarily the way one would analyze them.

... I don't see anything crippling about it. ...

I think we all knew that.:grin:

... It is just steady state analysis, and as I said you can analyze any way you wish. ...

Thank you

... Just think of a black box with 3 test jacks--red, black, and red. Now you are asked to describe the AC voltages on the red jacks relative to the black one. Where would you put your test leads?

Mivey's answer was pretty good. (Mivey - Bad Dog)

Well let's see: Three leads, and one voltmeter, no load, no external impedance.
No analysis available: You just get three voltages - no source impedance data available. Put the leads on any polarity you want - three times, don't repeat. As has been said, "trivial". Making one of the leads "black" is no more magical than calling one "N"

carl

 

[Don Randall]

Here ya go Rattus!

Either way of measuring is correct. In the first photo, the L1 to N voltage and the L2 to N voltage ARE indeed 180 out of phase. You cannot say that using the neutral as the reference is a "trick" to make the voltages out of phase. There is more than one way to look at any problem in physics, depending on the reference frame one chooses.

I'm wondering what reference frame you chose to use on the second, in phase, trace. Don

 

[crossman]

I'm wondering what reference frame you chose to use on the second, in phase, trace.

Glad you asked! Well, notice I said reference frame. This doesn't imply having a single point reference. The frame is as follows:

Channel 1 "red" lead on L1, Channel 1 "black" lead on neutral. Channel 2 "red" lead on N, Channel 2 "black" lead on L2. I guess it could be said that swapping the leads around like this is "a bit of a trick" to make the two voltages appear to be in phase.

(The following sentence is a test to see if Rattus is actually reading this): Hey rattus, you are welcome concerning the "thank you" for me having done the experiment.

C'mon guyz! It works either way. Both ways are legitimate. There is nothing wrong with either method. One method may make certain calculations a touch easier, but the other way has advantages too. Rattus' comparison to the 3-phase wye seems to be a legit comparison to me.

Just so I can ramble a bit.... using earth as a reference, I pretty much have zero velocity right now as I sit on my butt here typing these words. But with the sun as a reference, I am traveling with a velocity of about 33,300 mph in a circular path. Using the background radiation of the universe as reference, I am careening through space at some craZy velocity and odd wacky huge swirling direction which is actually pretty much a straight line but with several widely sized spirals cummulatively taking place. Which is correct? All of them.

Thank you for your time. And really, don't all you engineers and physics type dudes agree that either way of measuring is correct?

What if I make L1 the reference and put the black lead there for both channels? Then I take the red lead of channel 1 to N and the red lead of channel 2 to L2? I get a 120v RMS sine wave in phase with a 240v RMS sine wave. No big deal. The math still works if you do it correctly.

Now, in that last example, what about the "ADD" function on Don's o-scope? It now adds up to 360 RMS?

 

[Don Randall]

What if I make L1 the reference and put the black lead there for both channels? Then I take the red lead of channel 1 to N and the red lead of channel 2 to L2? I get a 120v RMS sine wave in phase with a 240v RMS sine wave. No big deal. The math still works if you do it correctly.

Now, in that last example, what about the "ADD" function on Don's o-scope? It now adds up to 360 RMS?

To me, it indicates that you have chosen another invalid reference point to compare the phases of two identical coils,(electrically the same as 2 coils.)

 

[crossman]

What would be the criteria for determining whether a chosen reference frame is valid or invalid?

 

[coulter]

What would be the criteria for determining whether a chosen reference frame is valid or invalid?

If the predictions made from the math model precipitated from the chosen reference agrees with empherical data - then the reference/model is valid.

Say that three time real fast.

carl

 

[Don Randall]

What would be the criteria for determining whether a chosen reference frame is valid or invalid?

Actually, I accidentally uploaded that line and wasn't finished with my reply. (I bumped my keyboard.)
I was going to say that the scope results prove that the setup is invalid. There are rules to follow to determine the phase relationship of two identical ac coils (electrical equivalent of one center tapped coil), and I believe you need to follow them go get valid results with a scope. In this case the test equipment doesn't agree with your view. (What is the purpose of taking that view? You have to manipulate the math to get it to work.) That's my view, (which can change with what I would consider valid evidence). Of course my disagreement isn't that you can take any view you like. It is that some people flatly state that L1 and L2 are 180? out of phase with each other without any qualifiers such as "using the center tap as a reference point". Don
The center-tap does not change the relationship between L1 and L2.

 

[Smart $]

...some people flatly state that L1 and L2 are 180? out of phase with each other without any qualifiers such as "using the center tap as a reference point".

But yet those people are likely to say L1 (A) and L2 (B) are 120? out of phase discussing an ungrounded delta secondary w/o any center taps. In this case, you cannot even prove a 120? relationship by entering L3 (C) into the picture with basic measurements. One has to resort to a math/physics model to prove it, because the system neutral point is a virtual one, without any real world connection point.

But the real question here is, how many of the "center-tap not a valid reference point" group would accept the 120? relationship statement without question?

 

[crossman]

If the predictions made from the math model precipitated from the chosen reference agrees with empherical data - then the reference/model is valid.

Okay. Sounds good.

So how many and which of the following reference frames would be valid?

1. Neutral used as reference, L1 and L2 measured in respect to neutral

2. L1 used as reference, N and L2 measured in respect to L1

3. L1 measured with respect to N, N measured in respect to L2 (this would be, say, black lead on L1 and red lead on N, then lift both leads, placing black on N and red on L2

Do any of these fail to represent the empirical data if we are stringent and proper in the use of the math?

 

[coulter]

...So how many and which of the following reference frames would be valid? ...

I don't know - sort of depends on what you expect from the model.

...Do any of these fail to represent the empirical data if we are stringent and proper in the use of the math?

crossman - good buddy, we have discussed this. Single phase is trivial. Nobody bothers to do any single phase analysis. If the expectation to to get the meter to read as you expect, then any will work.

carl

 

[crossman]

Sorry Carl... I guess I still have it in my head that some people in this thread are saying that using the neutral as the reference is wrong.

Wait... some people ARE saying that!!

 

[crossman]

The center-tap does not change the relationship between L1 and L2.

L1 to L2 is a single voltage measurement and cannot be said to be in phase or out of phase.

 

[crossman]

I was going to say that the scope results prove that the setup is invalid.

Forgive my ignorance of typical o-scopes, but I have a question: with a two channel scope, don't the two channels usually have a common lead? There is a one lead that is used for both channels, then a CH1 lead and a CH2 lead? And in some o-scopes, the common lead is bonded to the o-scope chassis?

My LabVolt O-scope has 3 isolated channels, in other words, each channel has its own two leads and no interconnections between them. This isn't typical of scopes, is it?

 

[Don Randall]

L1 to L2 is a single voltage measurement and cannot be said to be in phase or out of phase.

I don't know about that. I would say, for instance, in the 2 lead output of a typical transformer, (bell, heater control, etc.), that every part of the that output circuit is in phase with every other part. (Of course I would say the same about a center-tapped transformer, with which you seem to disagree.) I'll draw you a simple diagram later to illustrate my thoughts. I believe phase is determined by the timing of the ac pulses, and the direction of the current flow for those pulses, (controlled by the voltage). If you wound half of the secondary opposite to the other half, then I would consider the two conductors, L1 and L2 to be out of phase with each other. If you put in a center-tap, it wouldn't change the phase relationship of those two conductors. Don

 

[mivey]

Now, in that last example, what about the "ADD" function on Don's o-scope? It now adds up to 360 RMS?

It does on mine.

To me, it indicates that you have chosen another invalid reference point to compare the phases of two identical coils,(electrically the same as 2 coils.)

No. It indicates you have the wrong settings on your oscope, unless you really intend to add the two signals.

If the predictions made from the math model precipitated from the chosen reference agrees with empherical data - then the reference/model is valid.

Say that three time real fast.

carl

thatthatthat

 

[Don Randall]

Forgive my ignorance of typical o-scopes, but I have a question: with a two channel scope, don't the two channels usually have a common lead? There is a one lead that is used for both channels, then a CH1 lead and a CH2 lead? And in some o-scopes, the common lead is bonded to the o-scope chassis?

My LabVolt O-scope has 3 isolated channels, in other words, each channel has its own two leads and no interconnections between them. This isn't typical of scopes, is it?

The key word is usually. They are also available with independently floating isolated inputs. Don

 

[coulter]

Sorry Carl... I guess I still have it in my head that some people in this thread are saying that using the neutral as the reference is wrong.

Wait... some people ARE saying that!!

Well, I think to use the back to back arrows to depict 1ph 120/240, in an educational setting, is needlessly limiting.

Also, in the odd, near useless event, if one were to do an analysis of a 1ph 120/240, I find the back to back arrows model with the phase angle issues, and currents that reverse as they pass the magical "N" point to be, well, silly. Can you do it? Sure. There are three wires, and all the vectors are in-line. What ever goes out of one has to go into the other two - doesn't get much simpler. Power factor rarely matters, and this "split-phase system" concept does not exist (s-p motor - yes; s-p system - no)

I find this BTBA model to fall apart when one extends to 3ph D. For 3phD, one connects the phasors nose to tail around the loop. Pretty simple. Now, pull a common from one of the coils and label the tap with the magic "N" word. Now all of a sudden we need to flip half of the 240 phasor around. ???

Yesterday I would have called that concept wrong. Today; it's silly, limiting, and it bothers the secret engineering moral code.

Hey, I've got a quarter - I'll call a chaplin when it gets bad.:roll:

carl

 

[Don Randall]

It does on mine.

Hey Mivey, What did your scope show when you used the add function on the two 180? out of phase traces? Don

 

[mivey]

Hey Mivey, What did your scope show when you used the add function on the two 180? out of phase traces? Don

A flat line.