Is power a phasor or vector or neither?

[ggunn]

That is the point, to get to the TRVTH, whatever it is.

I disagree. I think you have decided what the truth is and your obsession (your word) is with trying to get everyone to agree with you, to say that you are right and everyone who disagrees with you is wrong. I fear you are doomed to be forever frustrated in that quest. There is no absolute truth in this case. Your interpretation of it is just that - your interpretation.

Consider this: Is light made up of waves or particles? There are mathematical models of both ways of looking at it which to about the same degree explain the observable facts. Which one is the truth? Both, or neither. They are just models and exist only in our minds. Light simply is what it is. Mathematics models reality, not the other way round.

What is the point of substituting V for U in "truth", anyway? Is that supposed to signify something?

 

[rattus]

I disagree. I think you have decided what the truth is and your obsession (your word) is with trying to get everyone to agree with you, to say that you are right and everyone who disagrees with you is wrong. I fear you are doomed to be forever frustrated in that quest. There is no absolute truth in this case. Your interpretation of it is just that - your interpretation.

Consider this: Is light made up of waves or particles? There are mathematical models of both ways of looking at it which to about the same degree explain the observable facts. Which one is the truth? Both, or neither. They are just models and exist only in our minds. Light simply is what it is. Mathematics models reality, not the other way round.

What is the point of substituting V for U in "truth", anyway? Is that supposed to signify something?

Off topic.

 

[rattus]

I couldn't say it any better than that.

If you have ever seen technical textbooks geared to a 2 year Associates degree in electronics, you may find that those students are frequently given a very in depth course in AC analysis without phasors.

AC waveforms and impedences are represented by vectors, and all the usual circuit laws apply the same as if phasors were being used. No phasors necessary.

Obviously, many things can be represented by phasors, vectors, or even real numbers (RMS values). It all depends on what you need to calculate, or what you want to model.

Steve, how do you calculate the L-L voltages and angles in a 3-phase wye using vector math?

 

[ggunn]

Off topic.

No, it's not. All this furor is about the mathematical model and the terminology surrounding it. No one is disagreeing over what the voltages are between any pair of the three terminals of a center tapped transformer or what the waveforms look like. It is what it is. Build it, wire it up, and it does what it does. You can argue and bicker all you want over whether it is more "correct" to call it single phase, two phase, split phase, etc. ad nauseum but it makes not a whit of difference to what those voltages are, what the waveforms look like, or how you wire a house to use them.

And again I ask, what is the significance of "TRVTH"? It's apparently not a typo, since you have used it several times. Isn't there an absolutely "correct" way to spell "truth"?

 

[jim dungar]

This thread is beginning to read a lot like one from back in 2008 on a different forum.
http://forum.allaboutcircuits.com/showthread.php?t=15741

 

[LMAO]

I say neither because the expression for real instantaneous power involves a dc offset modulated by a sinusoid of frequency 2wt.

Phasors cannot represent the dc offset. And what about the phase angle, how would it apply to a wave of T/2?

Some think that the power triangle proves that power is a vector, but here we simply use complex numbers to represent real and imaginary power. Also, the power triangle always falls in the first quadrant.

BTW, we have been around this block before.

Phasor is a two-dimensional vector (a spacial vector). Power has a magnitude only, with no direction. So Power is an scalar quantity.

 

[pfalcon]

Just like squares are rectangles. Phasors are vectors but not all vectors are phasors. Vectors can be used to model anything done with phasors but often end up with some complex math. Phasors inherently work for rotating values such as sinusoidals but vectors can be used for non-rotating values.

 

[Rick Christopherson]

Power has a magnitude only, with no direction. So Power is an scalar quantity.

No. Power is not magnitude only. That's just the most common way we think about it.

p = iv = V_m I_m cos(ωt + ϑ) cos(ωt) = 1/2 V_m I_m cos(ϑ) + 1/2 V_m I_m cos(2ωt + ϑ)

Depending on the value of ϑ, the instantaneous power can be either positive or negative. For a non-reactive load, the power is always positive. For a reactive load, the power is periodically negative because the load stores energy and releases it later.

As for whether power can be represented as a phasor, my math is way too rusty to say for certain, but I believe that it can. However, we don't represent it as a phasor because doing so has no value for us. The answer won't be found in a book on electrical systems. It's more of a raw mathematics question. I'm sure I could find it in my Vector Analysis book, but the topic isn't really important enough to me to go dig for it.

 

[Rick Christopherson]

Just like squares are rectangles. Phasors are vectors but not all vectors are phasors. Vectors can be used to model anything done with phasors but often end up with some complex math. Phasors inherently work for rotating values such as sinusoidals but vectors can be used for non-rotating values.

This is absolutely correct. I believe that Rattus has forgotten about this over the years. The electrical field/education system, did not invent phasors. They are simply a mathematical tool that we employ. In some of the older educational systems, phasors may have only been taught as part of the electrical education, but in more thorough educational systems, these are also taught in the mainstream mathematics courses too. It's not just something for EE's. It's universal math.

 

[LMAO]

No. Power is not magnitude only. That's just the most common way we think about it.

p = iv = V_m I_m cos(ωt + ϑ) cos(ωt) = 1/2 V_m I_m cos(ϑ) + 1/2 V_m I_m cos(2ωt + ϑ)

Depending on the value of ϑ, the instantaneous power can be either positive or negative. For a non-reactive load, the power is always positive. For a reactive load, the power is periodically negative because the load stores energy and releases it later.

As for whether power can be represented as a phasor, my math is way too rusty to say for certain, but I believe that it can. However, we don't represent it as a phasor because doing so has no value for us. The answer won't be found in a book on electrical systems. It's more of a raw mathematics question. I'm sure I could find it in my Vector Analysis book, but the topic isn't really important enough to me to go dig for it.

No. Power is a SCALAR quantity; it is not a vector/phasor. Your equation is describing the "instantaneous" value of power as a function of time. Just because there is a "ϑ" in your equation, does not make power a phasor.
A phasor should have two dimensions; power has only one. Now, you CAN argue that power (VA, not Watts) can have "real" and "imaginary" (reactive) dimensions but in general, power is real and is represented by a ? magnitude.
Generally: P = F ? V where P is the power, F is force, V is the speed and ? represents cross multiplication. V and F are both vectors and as a result and by definition, P is a real number, not a vector.
However, electrical engineers define another dimension for power to represent reactive power. S(VA) = P(W) + jQ(VAR), where S is a vector (phasor), P is the real power and Q is the reactive power (both P and Q are real numbers and not vectors).

 

[Rick Christopherson]

No. Power is a SCALAR quantity; it is not a vector/phasor. Your equation is describing the "instantaneous" value of power as a function of time. Just because there is a "ϑ" in your equation, does not make power a phasor.
A phasor should have two dimensions; power has only one. Now, you CAN argue that power (VA, not Watts) can have "real" and "imaginary" (reactive) dimensions but in general, power is real and is represented by a ? magnitude.
Generally: P = F ? V where P is the power, F is force, V is the speed and ? represents cross multiplication. V and F are both vectors and as a result and by definition, P is a real number, not a vector.
However, electrical engineers define another dimension for power to represent reactive power. S(VA) = P(W) + jQ(VAR), where S is a vector (phasor), P is the real power and Q is the reactive power (both P and Q are real numbers and not vectors).

This is a topic that is not very important to me personally, so I am not interested in arguing it or belaboring it, but I think you're going to want to re-read what you wrote and revise your answer accordingly.

As you already stated above, power is 2-dimensional, and therefore may be represented as a vector. My math is too rusty to defend it, but I am pretty sure that power may also be mathematically represented as a phasor, because it is a rotating vector, but with an offset from the primary axis. The only thing in question is whether this offset precludes it from being called a phasor. And my math is way too rusty to answer that question.

 

[LMAO]

This is a topic that is not very important to me personally, so I am not interested in arguing it or belaboring it, but I think you're going to want to re-read what you wrote and revise your answer accordingly.

As you already stated above, power is 2-dimensional, and therefore may be represented as a vector. My math is too rusty to defend it, but I am pretty sure that power may also be mathematically represented as a phasor, because it is a rotating vector, but with an offset from the primary axis. The only thing in question is whether this offset precludes it from being called a phasor. And my math is way too rusty to answer that question.

unfortunately, you have to go back and review your math or you will never fully grasp this.

General definition of power:
P=Force x velocity, power measured in Joules/s or watts, real number

Electrical engineering definitions:
P: real power, measures in watts (W), real number
Q: reactive power, measured in vars, real number
S: complex power, measured in volt-ampere (VA), complex number, aka phasor

S = P + jQ

j=-1^0.5

 

[rattus]

unfortunately, you have to go back and review your math or you will never fully grasp this.

General definition of power:
P=Force x velocity, power measured in Joules/s or watts, real number

Electrical engineering definitions:
P: real power, measures in watts (W), real number
Q: reactive power, measured in vars, real number
S: complex power, measured in volt-ampere (VA), complex number, aka phasor

S = P + jQ

j=-1^0.5

Beg to differ. In my book, apparent power is defined simply as V*I without a phase angle.

Complex notation is merely employed as a convenience in describing the relationship between P and Q and S.

The angle is simply the power factor angle

 

[iceworm]

I think several new books are needed.

charlie b defined "power" pretty well, so I will concentrate on "Complex Power".

Complex power is defined as EI(conjugate) Both E and I(conjugate) are phasors. (Or vectors - makes no difference to me. I have yet to be confused by the difference.)

Yes, that is the accepted definition - there is no other, this is not an opinion, it is not debateable.

If needed, IEEE 100 is my quickest reference - anyone of a dozen texts are others.

ice

 

[iceworm]

... And, many engineers don't fully understand phasors and the difference between phasors and vectors. Frankly, when I joined this forum I didn't fully understand them either, but I dug into my old texts and doped them out. ...

I must be one of them. Because I am having a hard time understanding why any of this and other similar posts matters. Some have a physical direction, some don't. Some have a rotational component, some don't. Some have a specific definition, some are defined in the context, some are defined for the specific model - which is about the same as the context.

I'm not willing to discuss them, because I don't have an opinion, or an obcession., barely even care Perhaps you could give us a series of posts (consider them lectures) that expalin the differences that you now clearly understand. And I would be perfectly happy to listen and learn.

The reason I ask, is because so far, the reason I ask is this is the only reason I have heard:

...There are differences. For example, if you treat the voltages in a split phase stem as force vectors, the opposing forces cancel yielding 0V between L1 and L2. But if you treat them as phasors, you subtract one from the other to obtain 240Vrms.

And that to me is a "single phase power - who cares". Before you told me I was ignorant, I didn't think I had any difficulty with Fortesque, Wagner and Evans, or Stevenson (symetrical components). So, I would definitely look forward to a series of posts. Make it the short form. Assume a reasonable understanding of complex math. Call them RatClif Notes. No derisive remark intended. I think it would be a good name. A legacy to rival - even eclipse, "Charlies Rule".

... It is my obsession to spread the TRVTH about phasors from time to time-- ...

I respectfully request to lay off the "TRVTH" and the rest of the derivitives. Some precieved it as a derogatory term when it came up a few years ago.

ice

 

[rattus]

Complex Power:

Complex Power:

I must be one of them. Because I am having a hard time understanding why any of this and other similar posts matters. Some have a physical direction, some don't. Some have a rotational component, some don't. Some have a specific definition, some are defined in the context, some are defined for the specific model - which is about the same as the context.

I'm not willing to discuss them, because I don't have an opinion, or an obcession., barely even care Perhaps you could give us a series of posts (consider them lectures) that expalin the differences that you now clearly understand. And I would be perfectly happy to listen and learn.

The reason I ask, is because so far, the reason I ask is this is the only reason I have heard:



And that to me is a "single phase power - who cares". Before you told me I was ignorant, I didn't think I had any difficulty with Fortesque, Wagner and Evans, or Stevenson (symetrical components). So, I would definitely look forward to a series of posts. Make it the short form. Assume a reasonable understanding of complex math. Call them RatClif Notes. No derisive remark intended. I think it would be a good name. A legacy to rival - even eclipse, "Charlies Rule".


I respectfully request to lay off the "TRVTH" and the rest of the derivitives. Some precieved it as a derogatory term when it came up a few years ago.

ice

ice, 'complex power' is nothing more than VI treated as a complex number It is part of the power triangle. Its components are:

Preal = VI*cos(theta)

Preactive = VI*sin(theta)

A phasor is a complex number,

But, a complex number is not necessarily a phasor.

 

[LMAO]

ice, 'complex power' is nothing more than VI treated as a complex number It is part of the power triangle. Its components are:

Preal = VI*cos(theta)

Preactive = VI*sin(theta)

A phasor is a complex number,

But, a complex number is not necessarily a phasor.

from MathWorld: Phasor is the representation, "beloved of engineers" and physicists, of a complex number in terms of a complex exponential
in other words, every complex number can be represented in "Phasor" form, i.e., Phasor ≡ Complex number ≡ 2-D vector

P: real power, real number
Q: reactive power, real number
S: apparent power, complex number or phasor

S = P +jQ

http://mathworld.wolfram.com/Phasor.html

I am always right.

 

[iceworm]

I must be one of them. Because I am having a hard time understanding why any of this and other similar posts matters. ...

ice, 'complex power' is nothing more than VI treated as a complex number It is part of the power triangle. Its components are:

Preal = VI*cos(theta)

Preactive = VI*sin(theta)

A phasor is a complex number,

But, a complex number is not necessarily a phasor.

Hummm ... first I'll assume you meant to answer a post other than the one you quoted. In fact I'll assume you meant to quote the one on complex power.

I certainly appreciate your definition of complex power. Being as I was the one to point out that complex power is precisely defined and give the definition, I suspect I clearly understand complex power. Although my understanding is likely biased toward what it takes to actually get the power to a specific load.

However, I am undenyably one of the great mass of unwashed Phillistines that do not have the complete "phasor" understanding. Consequently, "a complex number is not necessarily a phasor" - this one leaves me in the cold. I don't know what that has to do with complex power.

Snippets and one-liners don't appreciably help. Still waiting for the mini-lectures (the RatClif notes)

ice

 

[iwire]

I am always right.

But what is right?

Where is the reference point?

If I am negative left does that make me right?






(I am outa here)

 

[rattus]

from MathWorld: Phasor is the representation, "beloved of engineers" and physicists, of a complex number in terms of a complex exponential
in other words, every complex number can be represented in "Phasor" form, i.e., Phasor ≡ Complex number ≡ 2-D vector

P: real power, real number
Q: reactive power, real number
S: apparent power, complex number or phasor

S = P +jQ

http://mathworld.wolfram.com/Phasor.html

I am always right.

Pa = V*I

There is no phase angle associated with Pa. We merely treat it as complex to break it into its real and imaginary components. And how would you use a phase angle if there were one? My texts do not call it a phasor or a vector. They merely treat the power triangle as a right triangle.

We are of course speaking of static phasors as used in AC analysis. There are voltage phasors, current phasors, and impedance phasors. That's all.

Certainly phasors may be applied to other fields as well, but that is of no concern to us.