How does a typical residential utility transformer work?
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Huh?
Huh?
Eric, where do you get that line of reasoning?
First of all, the term "phasor" is preferred over "vector" and has been for 50 years or so, and I repeat that phasors can be fixed or rotating.
Each rotating phasor traces out instantaneious values on the real and imaginary axes. Each fixed phasor provides the RMS value and fixed phase angle. There are three voltage phasors in a 3-ph wye--six if you count the line to line voltages.
In short, a phasor is a complex number describing a sinusoidal voltage or current--single phase or whatever. You are dead wrong when you say there are no phasors in single phase systems. For example,
V1n = 120V @ 0 is a fixed phasor
v1(t) = 170V x exp(-jwt) is a rotating phasor
V2n = 120V @ 180 is a fixed phasor
You seem to be confusing phase sequence with rotating vectors--not the same animal. Phasors describe the individual waveforms.
You did I believe admit that we could represent the line voltages as I have done above. No?
Huh?
eric stromberg said:Rattus,
Correct. There are, in fact, phasors in three phase systems. A "Phasor" can be thought of as a "Vector rotating with a constant frequency." There are no "Phasors" in single phase systems, there are only "Vectors."
And, by the way, i haven't admitted any kind of defeat. :smile: :smile:
Keep it coming, we'll get through it. I've often heard Mike Holt say "I've never learned anything from someone who agrees with me all the time."
Eric, where do you get that line of reasoning?
First of all, the term "phasor" is preferred over "vector" and has been for 50 years or so, and I repeat that phasors can be fixed or rotating.
Each rotating phasor traces out instantaneious values on the real and imaginary axes. Each fixed phasor provides the RMS value and fixed phase angle. There are three voltage phasors in a 3-ph wye--six if you count the line to line voltages.
In short, a phasor is a complex number describing a sinusoidal voltage or current--single phase or whatever. You are dead wrong when you say there are no phasors in single phase systems. For example,
V1n = 120V @ 0 is a fixed phasor
v1(t) = 170V x exp(-jwt) is a rotating phasor
V2n = 120V @ 180 is a fixed phasor
You seem to be confusing phase sequence with rotating vectors--not the same animal. Phasors describe the individual waveforms.
You did I believe admit that we could represent the line voltages as I have done above. No?
eric stromberg
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rattus said:
Rattus,
If you overlay a picture of a single phase transformer winding, (240 Volts, neutral in the center, one leg pointing left and one leg pointing right, over the top of a three phase phasor diagram, you will see similarities. Trouble is, the coordinate systems are completely different. The three phase phasor diagram that you see is in the complex number domain. The x-axis represents magnitude and the y-axis represents time. Complex numbers are a convenient way of plotting time as a distance. The single phase diagram is in a coordinate system that has no reference to time.
In an earlier post you mentioned that this discussion had nothing to do with things rotating. In fact, it does. A three phase system has an inherently rotating magnetic field, due to the physical difference in the way the windings are placed in the generator. A single phase system has no inherently rotating magnetic field. Therefore, it has no need for a phasor representation because no time differences need to be taken into account.
I will scratch out a drawing and show you what is really going on inside a transformer. The "neutral" point of a single phase winding is simply a point halfway up the winding. The magnetic flux that travels through the core of a transformer causes there to be a certain volts/turn on the secondary winding. Each turn in the secondary rises and falls together. It is just like a series of batteries in a flashlight. Let's say there are 3 Volts per turn. A 240 Volt transformer would have 80 turns. As the magnetic flux passes through the secondary winding, each turn rises to 3 volts together. Sum them all up and you have 240 Volts. The center point is just that, a center point. If you 'pin' this point down to zero, you have what looks like a see-saw. But each turn of the winding is rising and falling together; perfectly in phase with one another.
Our issue here is simply with the word 'Phase.'
No need really:
No need really:
Eric, you can spare me the tutorial on transformers. I have several semesters of AC Machinery and have served my time in the power lab.
"Phase" can mean several things to us. We can speak of phases A, B, and C in a 3-ph system; we can speak of phase lead or lag in reactive circuits, and we can speak of the phase angle of a sinusoidal voltage or current, and that is what this discussion is all about. Forget all this talk of 3-ph systems and rotating magnetic fields--not germane to this discussion.
This is a simple concept: By defining V1n and V2n as the voltages on nodes L1 and L2 relative to the neutral, we see that the phase angles of V1n and V2n differ by 180 degrees. That is fundamental!
Again, any sinusoidal voltage or current can be described by a phasor. In fact we almost have to. Look it up! Don't tell me that I can't use phasors in a single phase system.
In a 3-ph system, you would have 3 rotating arrows. In the 1-phase system, you would have two--one for V1 and one for V2.
No need really:
Eric, you can spare me the tutorial on transformers. I have several semesters of AC Machinery and have served my time in the power lab.
"Phase" can mean several things to us. We can speak of phases A, B, and C in a 3-ph system; we can speak of phase lead or lag in reactive circuits, and we can speak of the phase angle of a sinusoidal voltage or current, and that is what this discussion is all about. Forget all this talk of 3-ph systems and rotating magnetic fields--not germane to this discussion.
This is a simple concept: By defining V1n and V2n as the voltages on nodes L1 and L2 relative to the neutral, we see that the phase angles of V1n and V2n differ by 180 degrees. That is fundamental!
Again, any sinusoidal voltage or current can be described by a phasor. In fact we almost have to. Look it up! Don't tell me that I can't use phasors in a single phase system.
In a 3-ph system, you would have 3 rotating arrows. In the 1-phase system, you would have two--one for V1 and one for V2.
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The Full Wave Rectifier:
The Full Wave Rectifier:
Consider the full wave rectifier which comprises a center-tapped transformer, two diodes, and a filter. This circuit depends on the fact that the voltages on the two legs of the transformer are 180 degrees out of phase. Then the diodes conduct alternately in each half cycle as opposed to a half-wave rectifer which conducts only in one half cycle.
This is the same principle as in the power transformer with a CT.
The Full Wave Rectifier:
Consider the full wave rectifier which comprises a center-tapped transformer, two diodes, and a filter. This circuit depends on the fact that the voltages on the two legs of the transformer are 180 degrees out of phase. Then the diodes conduct alternately in each half cycle as opposed to a half-wave rectifer which conducts only in one half cycle.
This is the same principle as in the power transformer with a CT.
coulter said:
A phasor does not necessarily rotate.
Whatever you call it, a fixed phasor provides a magnitude and phase angle and does not vary with time. You use fixed phasors for example in describing the magnitude and phase angle of 3-ph voltages.
"Vector" is an obsolete term in AC analysis although many still use it.
Rotating phasors are functions of time and provide instantaneous values.
eric stromberg
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rattus said:
Great example. But those two voltages are part of the same sine wave. That is like saying that a single sinusoid is out of phase with itself. This reminds me of "peak-to-peak" voltage. The peak voltage of a 120 Volt sinusoid is around 170V. The peak to peak, therefore, is 340V. But will you ever be able to measure 340V? No, because the peaks happen at different times.
A single sinusoid has a positive and a negative cycle. You can't have one without the other.
So tell me, how exactly can a single winding be 'out of phase' with itself?
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They are not really out of phase by 180 degrees...you are just measuring for the the center of the wave to each end....there is only one sine wave and it cannot be out of phase with itself.
Don
Don
eric stromberg said:
The source does not matter. If I presented these three terminals to you and told you they were from two function generators you would agree that the the two signals were out of phase, woudn't you? Same Same!
Now would you agree that V12 is 180 degrees out from V21? This is so fundamental it hurts! Depends on how you look at it! That is all this is about.
eric stromberg
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rattus said:
Indeed! I can connect my power quality analyzer to a single phase service and see two 'opposing' sine waves. If i put the black lead of my meter in the middle of a battery line-up i will read positive voltage on one end and negative voltage on the other; agreed.
A long time ago, my son and i were standing on a sidewalk. I looked up and saw a plane. The plane was at such an angle that it appeared to have no wings at all. I told my son to look closely at the plane in the sky. I told him: "one day you will be able to say 'But i have seen a plane, planes have no wings. I saw one flying in the sky and it had no wings.' You will be able to say this with complete accuracy and truthfulness." Trouble is, planes DO have wings.
It is all in how you look at it. :smile: :wink:
I think if I hooked one scope common to line # 1 and hot to neutral and
and another scope common to neutral and hot to line #2 you would see two
sine waves in sync positive at the same instance and to neg. at the same instance.
Like you said Rattus basic stuff.
and another scope common to neutral and hot to line #2 you would see two
sine waves in sync positive at the same instance and to neg. at the same instance.
Like you said Rattus basic stuff.
eric stromberg
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ronaldrc said:
You would also melt your leads and blow up your scope (or, at least, blow fuses). The commons of each lead are connected to the chassis of the scope which is, in turn, connected to the grounding pin on the Scope's power cord.
I think i'm becoming out of phase with myself.
Roses are Red,
Violets are Blue,
I'm schizophrenic,
and i am too! :smile:
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eric stromberg
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- Texas
ronaldrc said:
Sorry,
I went back and corrected my spelling.
And...... I'm spent!
Rattus me you Don and most on hear are on the same page.
Like I said most just confused phase and polarity, so please
don't start giving me a hard time.
You could reverse one of my scopes polarity and get your result
but it would not be correct.
Dag gon it and you know it.
And no Eric it would not blow anything. Not even leads or eyelets
or whatever.
Like I said most just confused phase and polarity, so please
don't start giving me a hard time.
You could reverse one of my scopes polarity and get your result
but it would not be correct.
Dag gon it and you know it.
And no Eric it would not blow anything. Not even leads or eyelets
or whatever.
eric stromberg
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- Texas
ronaldrc said:
Hmm, we must be talking about two different kinds of Scopes. I'm talking about the old traditional Oscilloscope that has BNC connectors on the front for the leads. The probe leads are coaxial and the braid is brought out to an alligator clip at the probe. The braid is connected to the chassis of the scope. If you were to use a dual-trace scope and were to connect the braids to two different points (with different voltages) you would short out these two points through the scope leads.
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