May I ask a question about the single vs two phase stuff

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LarryFine said:
{The diagram most recently in post 617}
Yes, this one. To me, the two sine waves are incorrect. They should be oriented "in sync" to match the + and - symbols. When someone says that they're accurate as seen by a 'scope from the neutral, I say "That's because you're only relocating one probe, not both."
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I agree, that diagram is not correct, because the sine waves as drawn do not match the measurement symbols.

Here is an idea for the + and - as used to indicate the sense of the _measurement_; they should be drawn near the voltage value (at the center of the 'dimension lines') not near the lines of the schematic indicating electrical connection. When near the electrical connection lines the + and - indicate the polarity of the wires which is incorrect for AC. Near the value they would better indicate that they are related to the _measurement_.

-Jon
 
winnie said:
I agree, that diagram is not correct, because the sine waves as drawn do not match the measurement symbols.

Here is an idea for the + and - as used to indicate the sense of the _measurement_; they should be drawn near the voltage value (at the center of the 'dimension lines') not near the lines of the schematic indicating electrical connection. When near the electrical connection lines the + and - indicate the polarity of the wires which is incorrect for AC. Near the value they would better indicate that they are related to the _measurement_.

-Jon
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Jeez, you wanna go fix a perfectly confusing and misleading diagram. The nerve!
 
Drawings are what they are. Many have errors. I have seen many typos in text books. A lot of textbooks are written from the professor's notes and include original boo-boos from the notes and some are errors introduced by grad students, publishers, or editors.

The concepts presented are the information. Understand those and the typos take care of themselves.

Lectures are the same. When listening, I try not to correct mis-statements unless it hinders the understanding of the topic presented.

I try to correct concept errors. Sometimes the lecturer gets it, sometimes they don't. I will try to clarify errors at a sidebar or via email if it is too complex for a brief talk. Probably 97-98% of the people want to learn, especially those that take the time and effort to teach others via lectures.
 
Besoeker said:
OK. Explain why in simple terms for this ignorant Brit.
To clarify my question would it matter which you called the start and finish as long are you are consistent?
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If, for example, you state clearly that in all cases you consider the winding to be made by winding clockwise around the iron core, then you have defined a manufacturing process with a beginning and an end. The start and end identification is fully determined.
You can agree to call the start the end and the end the start, and as long as you are consistent you will be OK.

If you ignore the process aspect, you can simply say that if you start at A and move clockwise around the coil to get to B, then A is the start. If you go counterclockwise to get to B, then B is the start.

You could also agree to specify counterclockwise instead of clockwise, as long as you are consistent.

Sent from my XT1585 using Tapatalk
 
mivey said:
Drawings are what they are. Many have errors. I have seen many typos in text books. A lot of textbooks are written from the professor's notes and include original boo-boos from the notes and some are errors introduced by grad students, publishers, or editors.

The concepts presented are the information. Understand those and the typos take care of themselves.

Lectures are the same. When listening, I try not to correct mis-statements unless it hinders the understanding of the topic presented.

I try to correct concept errors. Sometimes the lecturer gets it, sometimes they don't. I will try to clarify errors at a sidebar or via email if it is too complex for a brief talk. Probably 97-98% of the people want to learn, especially those that take the time and effort to teach others via lectures.
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Yep.:thumbsup:

Wanna know how much was wrong in my calculus book?:cry:

Thank the big guy my professor was smarter. Bloody undergrads!
 
GoldDigger said:
If, for example, you state clearly that in all cases you consider the winding to be made by winding clockwise around the iron core, then you have defined a manufacturing process with a beginning and an end. The start and end identification is fully determined.
You can agree to call the start the end and the end the start, and as long as you are consistent you will be OK.

If you ignore the process aspect, you can simply say that if you start at A and move clockwise around the coil to get to B, then A is the start. If you go counterclockwise to get to B, then B is the start.

You could also agree to specify counterclockwise instead of clockwise, as long as you are consistent.

Sent from my XT1585 using Tapatalk
Click to expand...

I am pretty sure I know what you mean, but schematics and such are easier for me to read.

A pic and defined references would help if you could please.
Yes, I know a bit of PITA.
 
jaggedben said:
I think this is wrong. What do you mean by 'if you shift the blue or green'? That doesn't make sense. They are a property of the source.

The 'special case' that is the 120/240 system involves the ability to theoretically reverse the scope probes (i.e. invert polarity) on one 'phase' and have it look identical to the other. That won't be the case for any 'phase shift' other than 180deg.
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Ok, I think I understand what you are saying. The first attachment 120-0-120 I swapped the reference between measurements & the waveforms overlapped each other.
Doing the same thing on the 2-Phase did not produce an overlap.
 

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LarryFine said:
Look at the two + and - symbols at the sources. I call that impossible, or at least incorrect.
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I agree the lower voltage source is upsidedown in Post 616. Better upsize that neutral !

Spice uses +/- to indicate instantaneous polarity of the voltage sources as shown below.
 

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LarryFine said:
At a frozen-in-time (1/2 cycle) moment, L1 is positive and N is negative, and sat the same time, N is positive and L2 is negative. Overall, L1 is positive and L2 is negative. Just like two batteries in series.

1/2 cycle later, everything is reversed, which is okay, because it's all reversing in sync, or in phase. Just like two batteries in series that are both being reversed in sync, or in phase. The voltages are additive.
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If and only if you choose YOUR reference of an endpoint. And it is only "just like two batteries" if you want to use the markings and terminal caps someone else chose and if you think a static system can equate to a dynamic system.

AC is not just like a battery stack. Imagine a battery with generic caps on each end and no +/- markings with the inside continuously cycling. You can use it as a positive supply or negative supply at an instant but no polarity at one instant is ASSUMED to be a "correct" direction indicator like you keep thinking.

When you keep thinking of a battery, you keep ASSUMING there is a certain polarity and positive direction implied and there is not. Someone has marked + and - on the batteries but you could not mark the terminals if the inside was constantly changing.

The AC wave can begin life as a positive or a negative signal. There is NOTHING that says it has to be one or the other. It is the user's choice to pick a reference direction.

If you keep standing on the equator and facing east or west, you will think left or right must always be directed north or south. If that is all you know, you will never be able to understand that you can stand on the North Pole and both left and right can be directed south.

It is a fundamental concept that most engineers understand about physics, or should have understood at one time. It is one of the very first things taught in basic electrical physics. A voltage reference is a choice and picking one over the other is not against any physical law but is actually how the physical world works.

But if it makes it any easier for you to think about, you can use a battery as a positive supply or a negative supply. I can't begin to tell you how many times I've used a "positive" voltage regulator as a negative supply in a circuit.

A rise in one direction is a fall in the opposite direction. You get to choose the reference direction and either way is valid.

It really is that simple.
 
SG-1 said:
Post569 was in response to Jaggedbens comment.


I modeled the system in spice. Voltage source one was set to 120V @ zero degrees. Voltage source two was set to 120V @ 90 degrees.
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Oooh that was all wrong. Voltage two should have been at 270 degrees. Your polarity did not match the polarity I assume to be correct.

jk
 
Mivey, I definitely know the difference between AC and DC. I only use the battery example because, being wound on a common core and driven by the same single primary waveform, the two halves of the secondary are always varying in sync.

I consider two series-connected 120v secondaries and a single center-tapped 240v secondary to be identical here. At any moment, the "positive" end of one half of the secondary is connected to the "negative" end of the other at, or as, the neutral.

I agree that, let's say, L1 is positive-going while L2 is negative-going (and vice versa), and from the neutral's point of view, certainly appear out of phase, but as two series-connected sources that form a single source, in order to add, must be in phase.

As for looking at it from an end-point instead of mid-point, look at a 240v secondary (without a center tap) as a source for a line-to-line-only load. Would you describe the entire secondary as being in phase, or would you say half of it is out of phase with the other half?
 
LarryFine said:
Mivey, I definitely know the difference between AC and DC. I only use the battery example because, being wound on a common core and driven by the same single primary waveform, the two halves of the secondary are always varying in sync.

I consider two series-connected 120v secondaries and a single center-tapped 240v secondary to be identical here. At any moment, the "positive" end of one half of the secondary is connected to the "negative" end of the other at, or as, the neutral.

I agree that, let's say, L1 is positive-going while L2 is negative-going (and vice versa), and from the neutral's point of view, certainly appear out of phase, but as two series-connected sources that form a single source, in order to add, must be in phase.
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Well there's the rub then. In synch does not mean the same as in phase. The three phases in a 3-phase system are in synch, but we will all agree they are not in phase.

Definitions would appear to be our difference.

LarryFine said:
As for looking at it from an end-point instead of mid-point, look at a 240v secondary (without a center tap) as a source for a line-to-line-only load. Would you describe the entire secondary as being in phase, or would you say half of it is out of phase with the other half?
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Two in-phase waveforms can combine to make a larger waveform. Two out-of-phase waveforms can combine to make a larger waveform. The larger waveform does not define the smaller waveforms.

A winding is not a waveform. Each little piece of the winding has its own life. Even take a small coil part. When the flux impresses a voltage across it, it is kind of like a dessicant swelling. It "swells up" with voltage so to speak and is direction agnostic. There is no defined direction until we define one and use it.

The "swelling" in an individual coil is not a waveform we are utilizing. We make use of the waveforms we take from the transformer.

The transformer is a source for the waveforms, not a waveform itself. I can produce the waveforms from a single transformer, two transformers, or two 180d displaced generator windings. What is under discussion is the system of waveforms available. We define what we take away from the source but the source is not a waveform.

The end-to-end voltage is a waveform we define when we take it from the source. The end-to-mid voltages are waveforms we can take away separately but are not the same waveform as the end-to-end waveform. Nor does the end-to-end waveform define the end-to-mid waveforms.

We can combine the smaller waveforms and get the same larger waveform but we define how we take away the waveforms from the source.
 
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