Just to clarify.

[bcorbin]

I'll add my two cents here regarding the reference issue. I don't think reference to the neutral "point" makes much sense. It takes two points to have a voltage difference, and any other voltage, in phasor notation or otherwise, would be referenced to that voltage in both magnitude and direction.

 

[mivey]

How does this:

It takes two points to have a voltage difference, and any other voltage, in phasor notation or otherwise, would be referenced to that voltage in both magnitude and direction.

support this:

I don't think reference to the neutral "point" makes much sense.

Or did I miss the point? Granted, I did just get back to town at 5 AM this morning and am due for some sleep.

 

[mivey]

Jim Dungar's homework assignment

Jim Dungar's homework assignment

Ok Jim, here is the result of the time domain analysis and what would happen in the real world:

Any problems so far?

 

[rattus]

No sir:

No sir:

Then you need to reverse the + and - on the lower source symbol.

The +/- signs are like the polarity dots on a transformer, Not easy to change, and this seems to be a common misconception. If we did that, the two voltages would be IN phase, and there would be zero volts between them.

 

[Rick Christopherson]

If we did that, the two voltages would be IN phase, and there would be zero volts between them.

Could you elaborate on this a little more?

 

[jim dungar]

Ok Jim, here is the result of the time domain analysis and what would happen in the real world:


Any problems so far?

More complicated than I would have done, but in the correct direction. Now work on the current flows through your blueberry and each of the voltage sources (for extra credit do it with and without a connection to the n point). This would bring you current in our discussion. If your BB is purely resistive, then common practice would have the currents in phase with their driving voltage.

 

[rattus]

Could you elaborate on this a little more?

Just like having one of two flashlight batterys in backward. The voltages cancel, and the little light no longer shines.

Just let the + sign represent the polarity mark on the secondary. You would have to climb the pole and swap leads on half the secondary to change it. Hard to do while hanging on with hooks and a belt. Maybe a little easier with a bucket truck.

 

[rattus]

Don't get it:

Don't get it:

I'll add my two cents here regarding the reference issue. I don't think reference to the neutral "point" makes much sense. It takes two points to have a voltage difference, and any other voltage, in phasor notation or otherwise, would be referenced to that voltage in both magnitude and direction.

If you reference a voltage to the neutral, it is simply one of the points you mention. Since the voltage at "n" is zero, we don't need to subtract.

And, strictly speaking, the phase angle indicates time, not a direction.

 

[Rick Christopherson]

The voltages cancel, and the little light no longer shines.

The reason why I was noting your comment is because when the two series sources are out of phase, they cancel, and when in phase, they add. So I wasn't sure if I had just missed some previous condition regarding this statement.

 

[rattus]

1. The current must always leave the source.

No.

2. The current cannot always leave the source.

Yes.

3. The currents in source 1 is identical to the current in source 2.

Yes, except one source is a source, and the other is a sink.

4. If the current is not in phase with your chosen voltage, fix the math by changing the direction of that current only for the part of the circuit giving you problems rather than changing the way the voltage is referenced.

Not really, it is a matter of sink or source. I negate the VI product, not the voltage.

Now, I can't find any references to support this last statement, so this is a theory on my part, but it makes sense.

 

[rattus]

The reason why I was noting your comment is because when the two series sources are out of phase, they cancel, and when in phase, they add. So I wasn't sure if I had just missed some previous condition regarding this statement.

Not sure what you are saying here.

 

[jghrist]

The +/- signs are like the polarity dots on a transformer, Not easy to change, and this seems to be a common misconception. If we did that, the two voltages would be IN phase, and there would be zero volts between them.

Well, if you can't change the +/- signs, then the phase of each voltage must be 0? or else the voltages with 180? and 0? angles will cancel.

I can't believe how complex the explanations of such a simple circuit are getting. The only points in contention seem to be how to define the direction of a voltage. It's all a matter of convention and semantics.

 

[rattus]

Well, if you can't change the +/- signs, then the phase of each voltage must be 0? or else the voltages with 180? and 0? angles will cancel.

I can't believe how complex the explanations of such a simple circuit are getting. The only points in contention seem to be how to define the direction of a voltage. It's all a matter of convention and semantics.

I can't believe it either.

We are not swapping leads on one of the sources. We are recognizing the fact that V1n and V2n are inverses of each other which is equivalent to a 180 degree phase difference between V1n and V2n.

 

[Rick Christopherson]

Not sure what you are saying here.

Going back to the original text book scan before I modified it to fit your example, these two sources are in phase, and they add. Above, you said that two sources that were in-phase would cancel.

Note that the (+) (-) signs are the same as the battery example I gave in the other thread. These two sources are in-phase, and their total voltage adds.

 

[rattus]

Not at all Larry.

Not at all Larry.

You say that as if it's a universal requirement.

Larry, this comment applies to the problem at hand, let's not escalate.

The question is, "Are V1n and V2n 180 degrees out of phase?" It is not a matter of convention or personal preference. It is simply a matter of TRVTH!

 

[rattus]

Yes and no.

Yes and no.

Going back to the original text book scan before I modified it to fit your example, these two sources are in phase, and they add. Above, you said that two sources that were in-phase would cancel.

Note that the (+) (-) signs are the same as the battery example I gave in the other thread. These two sources are in-phase, and their total voltage adds.

True,

Van = Vnb = 115V @ 0.

You are using different references for the two sources. Use "n" for the common reference.

 

[mivey]

More complicated than I would have done...

I really did not do that either. I was really going a little overboard to make the point that we can define our reference points where we want and the analysis still works. I just defined my voltage from a reference point, drew my loop currents, wrote out my loop equations, then crunched the matrix. Voila! It was not a problem. I'm going to stop the whole blueberry thing since I don't see anyone disagreeing that you can make the analysis from any reference point. I was tired of writing all that mess anyway.

That being said, I'll concede that for the particular one resistor case and the 3 resistor case you cited, an easy solution would be to not use the neutral because you can simplify the analysis.

That also being said, how many times do we get the ideal circuit where this works? Our real world has RLC circuits that are full of angles and signs and unbalanced loads, etc. In those cases, pick the reference point, write the loop equations and crunch the numbers.

With the real RLC loads we have that are a blend of single and three phase loads, I don't see how one can say the corner of a delta is a better reference than the middle of the delta. I don't see that the analysis would be simplified by picking the corner.

[edit: maybe I'll change my mind when I get caught up]

 

[mivey]

Matrix on a simple circuit? No bites?
ok, I'll "NOT!" myself

Here is the reason I concede the simple circuit is easier with a corner reference:

b reference
step 1: Vb = 0
step 2: Va = 240
step 3: Vn = Vab/2 = 240/2
step 4: Ia = In = Vab/R = 240/100

n reference
step 1: Vn = 0
step 2: Va = 120
step 3: Vb = 120@180
step 4: Ia = (Va -Vb)/R = (120-120@180)/100
step 5: Ib = -Ia = 2.4@180

[edit: I forgot to mention this was the simple, 1 resistor 100 ohm circuit]

 

[mivey]

Jim,

Now that I've conceded the corner point for a simple circuit, can the same be said for finding the internal voltages and currents for something complex, say like this:

Just at first thought, I wouldn't think the corner is any easier than the neutral. It's not obvious to me, but what do you say?

 

[winnie]

(discussing the meaning in the diagram way back in post #9)

The +/- signs are like the polarity dots on a transformer, Not easy to change, and this seems to be a common misconception. If we did that, the two voltages would be IN phase, and there would be zero volts between them.

In which case, the drawing is not internally consistent.

The numbers next to the source symbols indicate voltage and phase angle. This is the voltage and phase angle between the two terminals of the source itself.

The + and - on the source symbol indicates the sense of the voltage measurement. Since you can always swap leads to get the - voltage (or a 180 degree phase difference), you _must_ define the sense of the measurement or it is meaningless.

The + and - symbols in the diagram indicate that the two voltage measurements are Van and Vnb.

But when looking at the diagram as a whole, it is clear that the voltage measurements are Van and Vbn.

Thus the diagram is not self consistent.

-Jon