240v debate....

[weressl]

Anode and cathode on scope leads....think you lost the plot there.
If you want to measure the the two voltages to neutral on a centre tapped transformer you need a dual trace oscilloscope. You would have have a common point to which you would connect ONE of the grounded leads. And measure and display both with respect to that.


Neither is reversed.

Well, sort of. It is one continous winding, that is tapped at midpoint, installed over a single core. So the flux induces voltage that will cause the current to flow in the same physical direction in both segments of the coil. Grounding the center tap will establish the common reference point in relation to ground potential at the center tap. Ground potential is commonly referenced to and taken as 0 voltage. If one is to connect a dual trace scope that the base potential of each trace - the reference point - is both connected to the grounded center tap and the measuring probe one is connected to L1 and the other is to L2, then we will see the two potentials 180 degrees apart, each having close to equal peaks at 90 and 270 degrees and 0 crossing at 0 and 180 degrees.

Should one is to connect the probes of an ISOLATED input scope and connect it in the following fashion: L1 to base(1) and N to measuring probe(1), then N to base(2) and L2 to measuring probe(2), one would see two nearly overlapping traces for each phase that would peak and fall nearly identically. (I am using 'nearly' as to allow for manufacturing tolerances, possibly resulting in fractions of a volt differences between the two segments of the winding.) Of course if we have a third probe and we were to connect that base(3) to L1 and measuring probe(3) would receive L2, we sould see a trace that is IN PHASE with the above traces, but twice the amplitude.

 

[jwelectric]

All this talk about phase shift and time warp got me to thinking.
One can’t see ones reflection in the present time as the speed of light and the distance between the eye and the reflection takes time. Therefore when we shave we are looking at our self in the past. The more distance away we stand the longer into the past we can look.

Edited to ask

Any suggestions how far back I should stand?

On second thought don’t answer that.

 

[Little Bill]

All this talk about phase shift and time warp got me to thinking.
One can?t see ones reflection in the present time as the speed of light and the distance between the eye and the reflection takes time. Therefore when we shave we are looking at our self in the past. The more distance away we stand the longer into the past we can look.

Edited to ask

Any suggestions how far back I should stand?

On second thought don?t answer that.

I've heard you have a face made for radio, but I won't answer your question.

 

[Rick Christopherson]

I am asserting that a phase shift is a time shift.

Nobody is disputing that.

I am asserting that because we know that you cannot have a time shift in a single phase transformer (as David has now affirmed regarding my non-symmetry example) then you cannot have a phase shift in a single phase transformer.

Nobody is asserting that you can.

Thank you for clearing this up. I appreciate knowing that we are not in disagreement on these points. The purpose of my earlier posting today was to get clarification as to whether these topics were of contention. Iwire misunderstood the purpose of that posting, so I am very glad that you did not take it the same way that he did.

Personal? I think not. If you want to compare the two 120V sections you need to have a common point. There are just three terminals. You are correct in saying that you could use any one as a common point. If you use either end as a common point then you would not be comparing the two 120V sections of the winding. Don't you see that?

This answer, however, has really caught me off guard, and I did not realize that this was the basis for your contention. If I had realized this sooner, my approach to the topic would have been completely different. This topic should be a little easier to address.

Knowing that we are actually in agreement on the first two topics above, please bear with me as we address this last topic. To get a better handle on this topic, may I ask you for some further clarification as to why you believe this information to not be of a "personal preference" nature? I am not sure if Jim Dungar is saying the same thing as me, but I have great respect for his knowledge, and I think his posting is probably applicable to this series of questions. Could you look at his posting and see if that bears any relevance to how you view this "personal preference" question?

Like Jim, my viewpoint is more along the lines of KVL, and I prefer to see the voltages add sequentially around a loop or mesh. I also understand that some people prefer to view this differently, and that is why I had assumed this was a matter of personal preference.

 

[Besoeker]

Should one is to connect the probes of an ISOLATED input scope and connect it in the following fashion: L1 to base(1) and N to measuring probe(1), then N to base(2) and L2 to measuring probe(2), one would see two nearly overlapping traces for each phase that would peak and fall nearly identically. (I am using 'nearly' as to allow for manufacturing tolerances, possibly resulting in fractions of a volt differences between the two segments of the winding.) Of course if we have a third probe and we were to connect that base(3) to L1 and measuring probe(3) would receive L2, we sould see a trace that is IN PHASE with the above traces, but twice the amplitude.

You could. But that would measuring each in isolation, not with respect to neutral.

 

[Besoeker]

Thank you for clearing this up. I appreciate knowing that we are not in disagreement on these points.

The issue is that the assertions were irrelevant.

To get a better handle on this topic, may I ask you for some further clarification as to why you believe this information to not be of a "personal preference" nature?

To reiterate, if you want to compare the two 120V sections you need to have a common point. There are just three terminals. You are correct in saying that you could use any one as a common point. If you use either end as a common point then you would not be comparing the two 120V sections of the winding. You would be comparing a 120V half of the winding with the 240V of the entire winding. Simple innit?

 

[mivey]

Just because your scope is limited in its connections, (it forces you to use the neutral as a common) you see something that appears as a 'phase shift' or an 'inversion' of two waves.
Use a different scope arrangement so that you can measure V1n and Vn2.

When I was taught KVL, I would have written the loop formula as V12=V1n+Vn2, which IMO 'looks correct' even if there is no circuit connection from 'n' back to the center point of the source. My professor would have had a conniption, if I had written this simple loop as V12=V1n-V2n, even if it is mathematically equal.

Appearances aside, and mesh analysis preferences aside, the fact remains that a force in either "direction" is a valid EMF (not sure "direction" is the best choice of words but just go with it for now). With the force oscillating in direction, a force in the "other" direction does not just appear mathematically as a force, it is a real force.

Refer to my post #98 and the second graphic and let V be the voltage across the resistor. To deny that the force in the other direction is real is to say that the only way to get 2 x V is to re-wire one of the units and move the ground. That is simply not true.

To focus on how the voltages were created in the single-phase transformer, and to constrain how we use those voltages based on that source configuration, is using only the one-dimensional view of the voltages. In fact, a set of two dimensional voltages occupies the same space. That is what allows us to take advantage of the economics of only having to use a single source instead of having to use two sources like in my post #98.

The source configuration does not completely define the way we can use voltages we take from it. As an example, the circuit I showed in post #69 uses two single-phase transformers to illustrate how the real force in the "other" direction from the same winding is used to complete a set of voltages that most everybody would agree have a 120? displacement. We have the first voltage Va=V<0? that has the same displacement as its source. We have a second voltage Vb=V<240? and has the same displacement as its source. I think we are all in agreement that these two are real voltages.

Now we come to the third voltage: is it real or just some fancy math and not a real force in a different direction? It is a combination of the EMF in the direction opposite of the first voltage (i.e., we use the forces in opposite directions from the first winding) and the EMF in the direction opposite of the second voltage (i.e., we use the forces in opposite directions from the second winding) or Vc=-Va-Vb=V<120?. This is not just a mathmatical construct but a real EMF that will complete the set and help drive a real motor that requires three real EMFs with a 120? displacement.

 

[readydave8]

42

That makes sense, everything is clear now.

 

[mivey]

Thank you for clearing this up. I appreciate knowing that we are not in disagreement on these points. The purpose of my earlier posting today was to get clarification as to whether these topics were of contention. Iwire misunderstood the purpose of that posting, so I am very glad that you did not take it the same way that he did.

Where I think the breakdown might have been is that you are saying a phase shift and a phase displacement are the same thing. Refer to my post #98 and you will see that the waveforms from the two units have a phase displacement but are not shifted in time. To shift something is to say you took it and moved it around but we are saying that there was no need to move anything as the voltages in the other direction is a force that exists already and did not require that you shift anything to create it.

I think what is making you think of this as a shift is that you have predetermined that, for both EMFs, one direction is forward and the other is reverse. The fact that they were created by one source EMF does not mean that the voltages we take have to be in a specific configuration. The separation between the primary and secondary by a flux allows us to use the secondary voltages like we want.

This answer, however, has really caught me off guard, and I did not realize that this was the basis for your contention. If I had realized this sooner, my approach to the topic would have been completely different. This topic should be a little easier to address.

Knowing that we are actually in agreement on the first two topics above, please bear with me as we address this last topic. To get a better handle on this topic, may I ask you for some further clarification as to why you believe this information to not be of a "personal preference" nature? I am not sure if Jim Dungar is saying the same thing as me, but I have great respect for his knowledge, and I think his posting is probably applicable to this series of questions. Could you look at his posting and see if that bears any relevance to how you view this "personal preference" question?

Like Jim, my viewpoint is more along the lines of KVL, and I prefer to see the voltages add sequentially around a loop or mesh. I also understand that some people prefer to view this differently, and that is why I had assumed this was a matter of personal preference.

It would appear the biggest issue is that we do not agree on whether or not the source configuation completely defines every load configuration.

 

[weressl]

You could. But that would measuring each in isolation, not with respect to neutral.

Since the Neutral is one component of each trace, that would be common to both if each channel is scaled identically. Nu?:roll:

 

[Rick Christopherson]

To reiterate, if you want to compare the two 120V sections you need to have a common point. There are just three terminals. You are correct in saying that you could use any one as a common point. If you use either end as a common point then you would not be comparing the two 120V sections of the winding. You would be comparing a 120V half of the winding with the 240V of the entire winding. Simple innit?

Besoeker,
At the time this discussion started, I had no idea that you or anyone else were of the position that the method of observing the two voltage sources was not a matter of personal preference. I knew that many people preferred it, but I never realized that some people didn't know that this was "optional". If I had known this, I would have taken a different tack on the subject.

The image below is an excerpt from "Engineering Circuit Analysis", 3rd edition, Hayt and Kemmerly, pages 365 and 366. I have merged the two pages into a single page for your convenience.

I have highlighted two blocks of text with red outline, and pointed out key words with blue. In the first block, a single-phase system is defined such that V_an and V_nb are equal. The second block goes on to say that the two voltage sources must have the same phase angle. However, note the blue highlighting at the bottom. From another viewpoint, they may be thought of as being 180? out of phase.

The key point here is that the proper definition of the system is that they are in-phase, and it is the viewpoint that permits us to consider them being 180? out of phase.

 

[david luchini]

I have highlighted two blocks of text with red outline, and pointed out key words with blue. In the first block, a single-phase system is defined such that V_an and V_nb are equal. The second block goes on to say that the two voltage sources must have the same phase angle. However, note the blue highlighting at the bottom. From another viewpoint, they may be thought of as being 180? out of phase.

The key point here is that the proper definition of the system is that they are in-phase, and it is the viewpoint that permits us to consider them being 180? out of phase.

Rick,

Thanks for posting the text from Engineering Circuit Analysis. It confirms exactly what Besoeker, Mivey and I have been saying all along. I have highlighted some of your text in blue above because you have inserted a thought concept that is not written in the book.

The selected text says that the phasor voltages of Van and Vnb are equal. It further says that Van and Vnb being equal, have the same phase angle. It goes on to say the the voltages with the system viewed from the outer wires to the neutral, in other words Van and Vbn, are exactly 180? out of phase. This is exactly what we've been saying from the beginning.

I highlighted your text in blue above because you have introduced the words "thought of as being" where those words do not appear in the Engineering Circuit Analysis text at all. I don't believe the authors were using the word "viewpoint" as meaning an attitude of mind or a differing opinion. I believe the authors were using the word "viewpoint" as meaning a position of observation or a reference for measuring. From the first "viewpoint" the voltages are measured from A to N and from N to B. From the second "viewpoint" the voltages are measured from A to N and from B to N. Van and Vnb have the same phase angle and, Van and Vbn are 180? out of phase.

 

[jwelectric]

I have a silly question. If these two voltages are 180 degrees apart what happens when they meet in the middle. I have witnessed firsthand what happens when two 120 volts meets when they are only 120 degrees apart and it was very enlighten (yes pun was intended)

 

[Rick Christopherson]

David,
You are correct that I did add the explanatory words "thought of as being" but not for the intent of changing the author's meaning.

There is a reason why the author(s) have stated that the system is defined as having both sources in-phase. When you "Define" the two sources as being out of phase, then if you performed a full mesh analysis on a single-phase system with both 120V and 240V loads, eventually you would encounter the situation where your B-phase current opposes the B-phase voltage source. This is what I have alluded to in the past when I mention the "hidden minus sign". A full mesh analysis would suddenly reveal a minus sign that you hadn't predicted.

When I started to write my posting above, I was actually just looking through my old text book to see if it had a pre-made diagram that I could use to demonstrate this mesh analysis. Instead, I stumbled across the single-phase section in the book that already stated what I have been trying to point out throughout this discussion.

I have never argued that anyone didn't have the option of how they view the system. I have done the same method myself at times. However, it would appear that at least some of the people participating in this discussion do not realize that this is an optional method of viewing the system. When you define the system as being out of phase, you remove this option and make it absolute.

 

[pfalcon]

I wanted to run a pump at 240v vs 120v, for obvious reasons, voltage drop, wire size, etc. But... I get this a lot from people, 240v is more dangerous. So I'm looking for a better way to explain to people that its just as safe as a 120v circuit but with better benefits.

Limited to extremely low amperage 120Vac is only technically safer because you require a minimum current to supply enough power to trigger fibrillation. Strictly speaking this occurs at half the amperage for a 240Vac circuit. However, if we're using amperages that low we typically use 24VDC or less. Therefore for fibrillation it's practical to assume they're the same level of danger.

At higher levels of amperage we leave the fibrillation zone and move into the cooking or burn zone. Technically speaking the 240Vac will reach this zone first. But again the difference is very modest. And you're already above the fibrillation and muscle lock-on requirements. So again, in a practical sense the 120Vac is just as dangerous.

Double may seem significant to the lay person but electrically the difference between 120Vac and 240Vac safety is only of scholastic interest.

 

[jumper]

That makes sense, everything is clear now.

It is a Deep Thought. It usually takes about 7 1/2 million years to figure it out....

 

[Besoeker]

Rick,

Van and Vbn are 180? out of phase.

Yes and pretty much what I posted about 200 posts ago with graphs.

Blue is L1-N
Magenta is L2-N

The order of the subscripts matter.
Maybe that was missed at the time.

What also seems to be getting ignored is that if you have equal loads on each of the two 120V supplies delivered from a centre tapped 120-0-120 supply, the neutral current is zero. That can only happen because of the 180deg displacement between the two wrt neutral.

 

[Rick Christopherson]

What also seems to be getting ignored is that if you have equal loads on each of the two 120V supplies delivered from a centre tapped 120-0-120 supply, the neutral current is zero. That can only happen because of the 180deg displacement between the two wrt neutral.

Besoeker,
I am kind of confused by your response, and I want to make sure that I understand what you are saying here. Please confirm your posting.

Are you still suggesting that there is no other way of viewing the system besides having the 180? phase shift? Are you contradicting an engineering text book from an accredited University (and used by several accredited Universities)? Do you not know how to perform a mesh analysis on a system? Do you not understand the ramifications of where you are now heading in this discussion?

Weigh your words carefully, because this discussion is a lot more focused than it was previously. I strongly recommend that you dust off your own text books before answering.

 

[Besoeker]

Besoeker,

Are you still suggesting that there is no other way of viewing the system besides having the 180? phase shift?

I have not once suggested that there is a phase shift. As Mivey said, there is no shifting going on. You suggested that early on with your

A 180? phase difference is an 8ms time delay at 60Hz

from post #39.
The waveforms I presented instantaneous values of two waveforms. That is how they relate to each other at the same instant in time. No phase shifting and no time delay involved.
I should have thought that obvious but..... well....

Are you contradicting an engineering text book from an accredited University (and used by several accredited Universities)? Do you not know how to perform a mesh analysis on a system?

I'm not. Read the subscripts very carefully. The order matter and the book is correct in that respect.
I do have a little bit of of a concern with the use of + and - signs on an alternating current source.

I note also, that you have not come up with an alternative and credible explanation of why the neutral currents cancel that negates the obvious that they are displaced by 180deg.
Nor an answer to my post #109.

 

[readydave8]

It is a Deep Thought. It usually takes about 7 1/2 million years to figure it out....

yeah but after the first few million I usually forget the question. and why I'm carrying this towel around.