mivey
Senior Member
I've updated the picture in photobucket but it may take a while for their cache to clear.
240v debate....
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Thank you for your kind words. As I said, it reflects my day job.
It was an attempt to educate. A failed attempt it would appear.
As I posted in #49, the words of Jeremiah 5:21 (KJV) come to mind.
Rick Christopherson
Senior Member
When someone has remained within a single sect of the industry for 40 years, there may sometimes be a propensity to forget that there are many other sects that make up the industry as a whole. Evidence to this is forgetting that transformer theory spans many more areas than just power transmission.
It is not important that transformers can and do exist for pulsed inputs. The quiescence of the input signal between 180? and 360? will reduce the overall core flux in the transformer and you won't automatically reach saturation as long as you are not already operating near saturation. That is not germane to the topic. The point of my previous exercise was simply to demonstrate that when you break the symmetry of the waveform about pi, your rationale for time-shifting at 180? fails.
What hasn't been lost on me is that neither of you two have attempted to address the principle of non-symmetry itself. You have resorted to handwaving and distraction to avoid this topic.
Even if you disagree with my simple method of showing non-symmetry about pi, there are many other real-world conditions that will result in this broken symmetry (including simple noise). So nevertheless, the original question stands. When you have a waveform that is asymmetrical about pi, how can you claim v(t) = -v(t + 180?)? If your center-tap transformer does not allow you to make this assertion with a non-symmetrical wave, then it also does not happen with a symmetrical waveform. It is equivalent but is not equal.
It is not important that transformers can and do exist for pulsed inputs. The quiescence of the input signal between 180? and 360? will reduce the overall core flux in the transformer and you won't automatically reach saturation as long as you are not already operating near saturation. That is not germane to the topic. The point of my previous exercise was simply to demonstrate that when you break the symmetry of the waveform about pi, your rationale for time-shifting at 180? fails.
What hasn't been lost on me is that neither of you two have attempted to address the principle of non-symmetry itself. You have resorted to handwaving and distraction to avoid this topic.
Even if you disagree with my simple method of showing non-symmetry about pi, there are many other real-world conditions that will result in this broken symmetry (including simple noise). So nevertheless, the original question stands. When you have a waveform that is asymmetrical about pi, how can you claim v(t) = -v(t + 180?)? If your center-tap transformer does not allow you to make this assertion with a non-symmetrical wave, then it also does not happen with a symmetrical waveform. It is equivalent but is not equal.
On the whole, I don't deal with the power transmission field although power electronics is used there too for HVDC transmission.
But a 120-0-120V transformer or indeed any other voltage centre-tapped winding is a pretty simple arrangement and one that we frequently use for generating firing pulses for line commutated SCR circuits. For a B6C, a W3C or a hexaphase power circuit, you need a pulse every 60 deg. Six pulse, you see. Hexaphase. The pulses are generated from the three-phase power using an arrangement of three centre-tapped transformers to produce the necessary six phases to achieve this.
Here is a simple B6C:
Phases here were Red, Yellow and Blue. But consider them, L1, L2 and L3 if you wish.
Looking at the diagram, devices R+ and R- have to get firing pulses 180 deg apart. Same with yellow and blue.
How do we do that? Here's a detail from the actual circuit.
Arrangement look familiar? Yet, it's based on a design I did possibly thirty yeas ago. The concept still works. Denigrate if you want but it is in operation on a lot of plants .
Indeed they do.
Here's one that we designed.
Note the three windings.
Input and output as phased the same.The dots indicate that. The third winding, fed by R4is the reset winding used to reverse the flux in the core. Without that, the circuit wouldn't work.
Economics dictate that that power frequency transformers operate as close as they can to saturation.
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Rick Christopherson
Senior Member
That's a fine posting with a lot of information. What is still missing is any discussion on the non-symmetry ramifications of your 180? phase shift. When your input waveform is not perfectly symmetrical about pi, it becomes very obvious that inverting the signal is not the same as phase-shifting it 180?. That is the topic at-hand. Can we stick to that topic?
Rick Christopherson
Senior Member
I didn't want to cloud the current topic with something off-topic, but for those following, what you are saying below is that I wasn't actually wrong in my original assertion......just not "economical". But let's just leave it at that and stick to the current topic.
Evidently not fine from your POV. Although it is actual real world stuff. How many such circuits have you designed that are currently in operation in numerous applications doing a real world job?
Perhaps you would be good enough to reciprocate by posting examples of your work.
What non symmetry?
Look back at post #22. The two 120V waveforms are perfectly symmetrical about the neutral reference. Your point?
Finegood. Go for it.
Explain why my two firing pulses for SCR1 andSCR2 in the drawing in post #86 need to be 180 deg apart. And yhey are BTW.
The obvious explanation for anyone who has done any work in this field is that the two 120V-N voltages are 180deg apart. Do you have an alternative (and credible) explanation?
Rick Christopherson
Senior Member
No, that's because you have been avoiding the topic asked. You assert that there is a phase shift in the transformer. Looking at a non-symmetrical wave will dispel this. I think you know that, and that is why you keep avoiding the topic.
With a non-symmetrical input, are you still claiming that the two phases are 180? apart?
Rick Christopherson
Senior Member
Yes. Yes I do, and that is the topic we have been trying to cover for a few days now. It is not a phase shift, and examining a non-symmetrical waveform will reveal this. It is nothing more than a magnitude reversal. A minus sign.
This argument began when you stated that there was a phase shift. And that it was an absolute phase shift, not just a perception or tool that you "choose" to utilize. You defined the system as though it contained the phase shift.
Your arguments up to this point have been circular. Your argument has been that because you can get a correct answer in your analysis, then the assumption must have been true. To refute this, I am giving you a situation where your analysis does not provide a correct answer, and therefore proves that the assumption was false.
You can't "prove something to be true" with 100 examples, but you can "prove it false" with just one single failure. That's what this is about.
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Ummm....The first mention of "phase shift" occurs here (post #25):
Rick Christopherson
Senior Member
You're arguing for the sake of arguing. Read a little more carefully. It was stated in post #3, refuted in post #21, then Beoseker stated it as an absolute in post #22, and we were off to the races, so to speak. Was this really the most important contribution you could make to this discussion?
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Rick,
I could ask you if your above comment was the most important contribution you could make.
Please try to tone it down a bit with the condescending comments.
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Talk about the pot calling the kettle black.
The words "phase shift" do not appear anywhere in Post #3 or Post #22, and Post #3 was not written by Besoeker. Your statement that Besoeker "started the argument" by stating that there was a phase shift is factually incorrect. Perhaps you should read a little more carefully yourself. The "argument" appears to start in Post #25, IMO.
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Let's try this thought experiment.
If you have a 120/208V transformer secondary and want to figure the voltage across a load from terminals A-to-B. From Kirchoff's Voltage Law we know that the sum of the potentials around a closed circuit is 0. So Vl+Vbn-Van=0, or Vl=Van-Vbn. In order for Vl to have a magnitude of 208V, we know that the voltage angles of Van and Vbn must have 120deg between them. Lets call Van=120<0 and Vbn=120<-120:
Vl= Van-Vbn = (120<0)-(120<-120)
...................= (120+j0)-(-60-j103.9)
...................= (180+j103.9)
...................= 208<30
Now consider a 120/240V transformer secondary and want to figure the voltage across a load from terminals A-to-B. From Kirchoff's Voltage Law we know that the sum of the potentials around a closed circuit is 0. So Vl+Vbn-Van=0, or Vl=Van-Vbn. In order for Vl to have a magnitude of 240V, we know that the voltage angles of Van and Vbn must have 180deg between them. Lets call Van=120<30 and Vbn=120<-150:
Vl= Van-Vbn = (120<30)-(120<-150)
...................= (103.9+j60)-(-103.9-j60)
...................= (207.8+j120)
...................= 240<30
This is exactly the point that Besoeker made in Posts #22 & 24. But you chose to insert "phase shift" into the discussion in Post #25 to provide a counter argument against an argument that hadn't even been made. You projected the concept of "phase shift" into Besoeker's comments so you could argue against it. That is the epitome "arguing for the sake of arguing."
weressl
Esteemed Member
Yeah, condescension - at elast this is what I've been accused of - is my domain and I resent intrusion:lol:
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Rick Christopherson
Senior Member
First off David, 180? is a phase shift.
Secondly, the same question remains posed to you that I posed with Beoseker. If this 180? view (i.e. perspective) was correct, then you should get the same results analyzing a non-symmetrical waveform as you are able to achieve with your symmetrical waveform.
That is the question that was asked a long time back and still remains unaddressed. It is a tool that you may use, but several of you members here are applying it as though it was the definition of the system. As a tool, it works 90% of the time. A definition must work 100% of the time. When it finally becomes obvious that your tool fails for a non-symmetrical waveform about pi (180?), then it should be obvious that it is only a tool, and not the proper definition of the system.
I'll repeat what I said earlier so you don't misunderstand. I am NOT telling you that you cannot use your 180? tool. What I am stating is that it is not correct to claim that the system "must have" the 180? shift. When you do that, not only are you in error, but you force everyone to use your tool regardless whether they want to or not.
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Well, I must admit I am well and truly gobsmacked. I think I may have whiplash.
I'm not insisting that anyone "must" do anything. Mr. Kirchoff produced the "must" in this case. Take it up with him.
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