VA vs. Watts

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Cold Fusion

Senior Member
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Quote:
Originally Posted by Cold Fusion
VA is the units for apparent power.

And is also the units for complex power

Quote:
Originally Posted by Cold Fusion
I promise to not use the term "Apparent Power" when describing a complex quanity.

You will find some reference texts that do exactly that, even if it does not strictly adhere to what I would call standard conventions (and is the convention used in most of the texts I have). ---.

Yes, I've seen that. Really makes my sliderule hurt. I've never seen how one could use VA as units for complex power. I've always thought the units should be Xwatts + Yvars. Complex power is a vector quanity. Using a scalar term, just gnarls me:-?

---Also, I guess it doesn't help when we mix the uses of variables and units that have the same letters, like W, V, A, VA, etc which have been used as variables and units.---
As rick pointed out, I'm guilty of idomatically using the units for the functions. And, except for this thread, I'll likely continue to do so - where the context is clear.

cf
 

rattus

Senior Member
Because power is not sinusoidal:

Because power is not sinusoidal:

If it works for voltage, current, & impedances, why not for complex power as well since it is the product of two phasors?

The voltage phasor:
|E|e^jα

The current phasor:
|I|e^jβ

The conjugate of the current phasor:
|I|e^-jβ

Complex power is also called phasor power and is given by:
EI* = |E||I|e^j(α-β) = |E||I|e^j(Θ) = |E||I|(CosΘ+jSinΘ)

and the complex value is given by P+jQ

mivey, try this defintion:


"pha⋅sor
  /ˈfeɪzər/ Show Spelled Pronunciation [fey-zer] Show IPA
?noun Physics.
a vector that represents a sinusoidally varying quantity, as a current or voltage, by means of a line rotating about a point in a plane, the magnitude of the quantity being proportional to the length of the line and the phase of the quantity being equal to the angle between the line and a reference line.
Origin:
1940?45; phase + (vect)or
Dictionary.com Unabridged
Based on the Random House Dictionary, ? Random House, Inc. 2009."

It may look like a phasor; it may look like a vector; but it doesn't quack like a phasor, and it doesn't waddle like a vector. Therefore it is neither a phasor nor a vector. It is merely the power triangle plotted in the complex plane.
 

Cold Fusion

Senior Member
Location
way north
---It may look like a phasor; it may look like a vector; but it doesn't quack like a phasor, and it doesn't waddle like a vector. Therefore it is neither a phasor nor a vector. It is merely the power triangle plotted in the complex plane.
Oh yeah - quacks and waddles.

--- It is merely the power triangle plotted in the complex plane.
hummm --- sounds like any other vector /phasor type quanity.

As I said in post 70: Like it or not, integrate this:

cf
 

Rick Christopherson

Senior Member
I'm not going to take time to write it out so you will be able to nit pick this one to past death...
Translated into English:
I'm not going to take time to write it out because I can't really defend what I have to say other than to keep repeating it until no one really cares any more...
You like to argue, but you really don't present any information. If you can't support your position, just move on to something that you understand well enough to support.
 

mivey

Senior Member
...a vector that represents a sinusoidally varying quantity...
There is still a piece of the puzzle I'm missing.

What is not sinusoidally varying about the power in a phase?:
p = EI[cos(2ωt + θ) + cos θ]
where θ is the angle displacement between current and voltage
 

charlie b

Moderator
Staff member
Location
Lockport, IL
Occupation
Retired Electrical Engineer
I submit that power, as a function of time, in a single phase system, is sinusoidal. Actually, I already said so. Please refer to the definition I quoted in post #87(page 5).
 

mivey

Senior Member
Backing up from charlie b's post, I see that it is the cos θ term that is causing rattus heartburn. It would appear that the sin wave must be centered on the x-axis for rattus to consider it to be sinusoidal.

Is that the gist rattus?
 

Cold Fusion

Senior Member
Location
way north
This will be a three part response to keep the length down.

--- but you really don't present any information. If you can't support your position, just move on to something that you understand well enough to support.
Rick -
Yes I understand this really well. I also know that you, charlie, mivey, besoeker and even rattus understand the concepts. I never thought any of you were really wrong and I thought that most of your (that would be "all y'all's) statements were very good. I just took exception to some of the phrasing used to describe the concepts. They weren't really wrong, just didn't taste good.

Just so we are clear on my response, I'll lay out your post 119 statement, and following responses:

Post 119 Chris' statement:
Besoeker, the sentences I highlighted from your post #44 are not correct. You have confused the difference between units of measure and their usage. As has already been stated (or maybe not) VA = j/s = Watts. They are all the same units of measure.

Post 120 CF's question:
Did you forget "vars" in this list? Or did you leave them out on purpose?

Post 131 This is your questionable statement. I don't follow your logic that VARS are "special". I don't see them as any more special than watts.

Here's what you said:
cris in post 131 said:
I am sorry, I could have sworn I posted a response to this last night, but maybe I am just getting senile. VARs is an unusual case because we have taken a basic unit of measure and added a secondary condition to it. The unit of measure is the same, which is VA or Watts or j/s, but we have added a condition to this indicating that the units are limited to being "reactive".

This is comparable to using units of Feet-Right or Feet-Left when dealing with distance. The core units are still Feet, but we have added the exterior condition of Right or Left. If you stepped 5 feet to your left, but were instructed to use the dimension of Ft-R, then you would have to say that you moved (-5) Ft-R. The core units are still feet, but you have moved a portion of the function into the units.

Edited to add, that this does not apply to VA, Watt, or j/s. This is ONLY the special case of VAR, and it is the "R" in VAR that compels this.

I have read the posts and I fully understand the topic. Your VARs example is the only situation that comes close to fully understanding the situation. But note that VARs is the ONLY unit throughout this discussion that imposes this additional condition.

Next I'll discuss your post 131 statement doesn't make sense.

cf
 

Cold Fusion

Senior Member
Location
way north
Your post 131 statement could equally apply to Watts. I?ll rewrite it so you can see it. I?ll leave out the part about Feet right/Feet left. The metaphore (simile? Never could remember the difference) doesn?t really help much.

CF?s version of RC 131 as applied to Watts: (changes are in red)

.. said:
Wattsis an unusual case because we have taken a basic unit of measure and added a secondary condition to it. The unit of measure is the same, which is VA or VARS or j/s, but we have added a condition to this indicating that the units are limited to being "real".

(deleted piece on ft-R/ft-L)

Edited to add, that this does not apply to VA, VAR, or j/s. This is ONLY the special case ofwatt, and it is the "real part" in Watt that compels this.

Vars are no more special than watts. Both are an integral part of complex power

cf
 

Cold Fusion

Senior Member
Location
way north
Now lets look at the last part on your post 131 statement:
rc in 131 said:
---I have read the posts and I fully understand the topic. Your VARs example is the only situation that comes close to fully understanding the situation. But note that VARs is the ONLY unit throughout this discussion that imposes this additional condition.
Yes, I knew you understand the concepts ? that?s never been the issue.

By my ?var example?, I?ll assume you mean post 74. Well, it says that the motor has to draw Vars or it can?t produce any torque and the shaft won?t spin. No torque, no shaft spin ? no watts. Got to have them both ? neither is more special. Now if you wish to have a special unit, pick VA. That one is special ? it?s pointless.

cf
 

Cold Fusion

Senior Member
Location
way north
rc in 164 said:
Translated into English:
Originally Posted by Cold Fusion
I'm not going to take time to write it out because I can't really defend what I have to say other than to keep repeating it until no one really cares any more...

Two things about this one:
I would really appreciate it if you would not put your words under my name ? that?s disgusting

??no one really cares any more"

I think your pretty close on this one. The topic has been well flogged

cf
 

charlie b

Moderator
Staff member
Location
Lockport, IL
Occupation
Retired Electrical Engineer
It would appear that the sin wave must be centered on the x-axis for rattus to consider it to be sinusoidal. Is that the gist rattus?
I suspect not. I think his issue is with a DC offset (i.e., the wave must be centered on the y-axis). The plot of single phase power has one of these. It is mostly above the y-axis, and dips below it (briefly) twice for every cycle of the 60 hz votlage (or current) curve.

To be honest, the definition I quoted does leave room for that interpretation. It speaks of the form of "sin(u)," and then speaks of "analygous" periodic behavior. One could argue that a wave might have behavior that is periodic, but that is not analygous to that of sin(u), if it is not symmetrical with respect to the y-axis.
 

charlie b

Moderator
Staff member
Location
Lockport, IL
Occupation
Retired Electrical Engineer
We are still here, I believe:
  • Person One says, ?I submit that the grass is green.?
  • Person Two replies, ?No, you are wrong. The sky is blue.?
Let me also point out that the OP has posted only one reply, after the original post. So we have gone way, way off topic.


This is how we are going to close this discussion out.
  • I am temporarily closing this thread.
  • It will be reopened Friday morning (at or around whatever time I get up, Pacific Daylight time).
  • This gives everyone two days to come up with whatever else you want to say.
  • Any interested party may post one, and only one, additional comment.
  • If you post a second comment thereafter, I will delete it without reading it.
  • I suggest that that one opportunity be used to give a summary (please keep it short) of the things you think are most important to convey, out of all the things you have posted so far.
  • If you think you have said something that is true and correct, and that others have not agreed with, then you might reiterate that (or those) point(s).
  • The thread will be closed for good no later than first thing Monday.
  • If you need additional time to put together you final post, then send me a PM to explain.
If anyone disagrees with my actions, feel free to PM me, or to PM the Chief Moderator, or to hit the "Report Post" button.
 

charlie b

Moderator
Staff member
Location
Lockport, IL
Occupation
Retired Electrical Engineer
I apologize for the delay in getting this reopened. My (weak) excuse is that I had family visiting from out of town over the weekend.

I will keep this open until there are no more comments added, or until the close of business Thursday. Please keep to the "only one more post" rule. This thread needs to be put out of our misery.
 

rattus

Senior Member
Nail in the coffin:

Nail in the coffin:

Although some refer loosely to any complex number as a vector, this is incorrect.

Now, back to phasors and complex power: This from a lecture by a prof at the University of Illinois. Check out slide 9 of this Power Point presentation.

courses.ece.illinois.edu/ece476/Documents/ECE4762007_Lect2.ppt
 

Rick Christopherson

Senior Member
I will keep this open until there are no more comments added, or until the close of business Thursday. Please keep to the "only one more post" rule. This thread needs to be put out of our misery.
I don't understand why active threads need to be closed in the first place, especially when they have educational content. I can understand it if the discussion is off-topic, like "happy father's day", but an electrical discussion is not off-topic regardless of the original question that fostered it.

There is a lot that can be learned from these discussions. Not just the obvious learning by reading what others write, but in many cases even the writer learns something as they research their answers. These complex discussions are a great mental exercise to keep people thinking and reviewing their knowledge. If one doesn't continually challenge their knowledge, it will atrophy.

If a thread has runs its course, there is no need to lock it because there is nothing more to add. If a thread has not run its course, then locking it prematurely stops the active communication.
 

mivey

Senior Member
I don't understand why active threads need to be closed in the first place, especially when they have educational content. I can understand it if the discussion is off-topic, like "happy father's day", but an electrical discussion is not off-topic regardless of the original question that fostered it.

There is a lot that can be learned from these discussions. Not just the obvious learning by reading what others write, but in many cases even the writer learns something as they research their answers. These complex discussions are a great mental exercise to keep people thinking and reviewing their knowledge. If one doesn't continually challenge their knowledge, it will atrophy.

If a thread has runs its course, there is no need to lock it because there is nothing more to add. If a thread has not run its course, then locking it prematurely stops the active communication.
Have you been reading my PMs?:grin: Your post matches my sentiments as well.

I have seen many times that the over-played "sky is blue/grass is green" phrase gets painted over an entire thread even when it does not apply to the whole thread. If you (not you Rick, but the ones with a missing glass slipper) get bored with a thread, then quit reading it. Don't try to impress folks with your ability to point out that a thread, in your opinion, has run its course.

There are many threads, in my opinion, that are boring from the very first post. I realize the community at large finds the finer points of technical discussions boring and would judge them as being played out long before those still posting would reach their limits.

"Oh, but those points have already been made in earlier posts" some will say. Tell me in a real life discussion when points are not re-stated. Often times people have to hear something more than once before it begins to sink in. IRL, when both parties of a peaceful disagreement have run their race, they either reach common ground or agree to disagree. IMO, as long as the discussion remains civil, let the debate play out.

If emotions get out of control, either correct the offending parties or let the debate move to a different thread so people can cool off. I find it sad that a thread often gets closed because of a few offending parties. Why not put the offending parties in the penalty box (24 hour ban?) instead of punishing everybody in the thread?

As for charlie's proposed "solution", I find it to be weak at best. It invites the interested parties to make one final post to drive their points home, clarify issues, etc. The problem is, any great final post would be wasted in this thread. If it generates any value, there is no opportunity for further discussion in the thread.

I prefer an open discourse on topics, not posting a single bill with no opportunity to discuss further the ideas and questions the post may generate.

If this thread is to be closed, and some feel there are issues important enough to continue being discussed, let them start a thread dealing with that particular topic (like rattus did: http://forums.mikeholt.com/showthread.php?t=115308). At least there would be an opportunity for further discussion.

The mods seem worried that the discussion does not stick precisely to the question posed by the OP but I feel the discussion is related. It's not a completely unrelated topic like what someone is having for supper. Aren't we here to stimulate our brains? I can see starting a new thread for a completely different topic, but do we really need to generate a new thread every time a new idea pops up? I wonder how difficult it would make discussions when they keep branching into new threads. I would think many of the threads would not be followed, and it would actually quell some of the discussions.

I know the mods have to try to maintain some semblance of order around here. I just don't always agree with or follow the thought process (not that I have to). Nor do I think the rules are always applied the same or with even-handedness. But, overall, I do think the mods are doing a decent job given the circumstances.

I would like to see: 1) consideration given for a penalty box for offenders instead of thread-closing 2) quit thinking that on-topic must mean to stick precisely with an OPs post and that related thoughts (we are here to stimulate thought) are OK. 3)This one is selfish, and I realize I am more of a prude than most, but: Try to quell the influx of vulgarity. I see it becoming more and more of a norm in our society, but it does not have to be common to us unless we allow it. Why travel a low road when there is opportunity to take a high road?

Add:
While I'm here:
Although some refer loosely to any complex number as a vector, this is incorrect.

Now, back to phasors and complex power: This from a lecture by a prof at the University of Illinois. Check out slide 9 of this Power Point presentation.

courses.ece.illinois.edu/ece476/Documents/ECE4762007_Lect2.ppt
One prof does not a nail make.:grin: While I agree, you must also agree that your statement that an impedance is a phasor is also wrong. If you can bend the rule for an impedance, others can bend the rule for power. Power is not a pure sine wave because of the DC offset, but the shape is sinusoidal.
 
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Besoeker

Senior Member
Location
UK
I submit that power, as a function of time, in a single phase system, is sinusoidal.
Not always sinusoidal, and possibly not generally so for domestic supplies.
Not everything takes sinusoidal current. Computers, televisions, chargers etc. Pretty much anything electronic.
If one of the components isn't sinusoidal, the product won't be.
Here's something I measured earlier.
A few lights, one of the televisions, a couple of computers, printers, monitors.....usual stuff for us.

Domesticpower.jpg

Sinusoidal?
 

rattus

Senior Member
One prof does not a nail make.:grin: While I agree, you must also agree that your statement that an impedance is a phasor is also wrong. If you can bend the rule for an impedance, others can bend the rule for power. Power is not a pure sine wave because of the DC offset, but the shape is sinusoidal.

Agreed, according to the definition, impedance is a complex constant, not a phasor, and the expression for power contains sinusoidal terms but is neither a vector nor a phasor. Nuf sed!
 

markstg

Senior Member
Location
Big Easy
Not always sinusoidal, and possibly not generally so for domestic supplies.
Not everything takes sinusoidal current. Computers, televisions, chargers etc. Pretty much anything electronic.
If one of the components isn't sinusoidal, the product won't be.
Here's something I measured earlier.
A few lights, one of the televisions, a couple of computers, printers, monitors.....usual stuff for us.

Domesticpower.jpg

Sinusoidal?

I think it is unfair to introduce Nonlinear loads into the discussion, as all of this has revolved around the development/definition of the electrical entities we use in our everyday work for linear circuit elements. I say we get through this with linear circuit elements first before moving on to the Nonlinear devices.



The issue here (or at least I think this is the issue) is whether the power function p(t)=Pcos(2wt-@) + K is a cosinusoidal function or not.

I'm trying to be rigourous here so here is what I say:

v(t)=Vcos(wt) and i(t)=Icos(wt-@) are trigonometric functions (because they are periodic) consisting of a cosine function term. The period is T. It's waveform is sinusoidal and is centered around the t axis at P=0 with a period T.

p(t) is a trigonometric function (because it is periodic) consisting of a constant term K, and a cosine function term. The period of p(t) is T/2. It's waveform is sinusoidal and is centered around the t axis at P=K with a period
T/2.
 
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