mivey
Senior Member
Maybe they do things differently across the big pond.
VA vs. Watts
- Thread starter mparkstexas
- Start date
- Status
They...we...whatever.:wink:
Sure, we do some things differently. And, in my view, generally more simply in our particular field.
We use Watts for power in whatever form. No conversion from HP to kW. Conductor sizes are all mm^2. No conversion from 1/0, #12, circ mils. Just one unit.
But fundamental electrical units are fundamental units.
Internationally.
Rick Christopherson
Senior Member
No, this statement is wrong. Yes, I do understand what you are trying to say, but nevertheless, it is wrong.
The reason it is wrong is because you are treating the units as though they were the function. Moreover, you are mixing units and function across the equal sign.
The proper statement is:
Real Power + Reactive Power = Complex Power.
No. VA is a unit of measure. Apparent Power has the units of VA, but it is incorrect to say that VA is Apparent Power. In a sense, you are putting the cart before the horse.
Similarly, Real Power is given the units of Watts, but it is not true that Watts is Real Power. The reason for this is because Watts, as a unit of measure, is used in many other ways besides electrical systems, such as thermal power and optical power.
So as I said before, the units are identical. It is the equation that uses the specific units, that defines the differences.
Oh, Mivey's posting came in while I was writing this, and he has stated this in a more effective manner than I did.
Cold Fusion
Senior Member
- Location
- way north
quote=Rick Christopherson;1068377 No, this statement is wrong. Yes, I do understand what you are trying to say, but nevertheless, it is wrong.
The reason it is wrong is because you are treating the units as though they were the function. Moreover, you are mixing units and function across the equal sign.
Yes Rick, what you say is true. For me to use "watts" and "vars" as idomatic phrases for, "the real and reactive parts of the complex power" is truly sinful.
The proper statement is:
Real Power + Reactive Power = Complex Power.
No. VA is a unit of measure. Apparent Power has the units of VA, but it is incorrect to say that VA is Apparent Power. In a sense, you are putting the cart before the horse.
Yes Rick, what you say is true. For me to use the unit "VA" as idomatic phrase for the function "apparent power" is again truly sinful.
Similarly, Real Power is given the units of Watts, but it is not true that Watts is Real Power. The reason for this is because Watts, as a unit of measure, is used in many other ways besides electrical systems, such as thermal power and optical power.
No that would not be true - for the same reasons as you stated above. "Watts" are units. Real power is a function. This seems uncharacteristic of you - after chastising me twice.
So as I said before, the units are identical. It is the equation that uses the specific units, that defines the differences.
As I recall from reading the thread posts, this has already been said several times - clear enough that even I got it.
cf
The reason it is wrong is because you are treating the units as though they were the function. Moreover, you are mixing units and function across the equal sign.
Yes Rick, what you say is true. For me to use "watts" and "vars" as idomatic phrases for, "the real and reactive parts of the complex power" is truly sinful.
The proper statement is:
Real Power + Reactive Power = Complex Power.
No. VA is a unit of measure. Apparent Power has the units of VA, but it is incorrect to say that VA is Apparent Power. In a sense, you are putting the cart before the horse.
Yes Rick, what you say is true. For me to use the unit "VA" as idomatic phrase for the function "apparent power" is again truly sinful.
Similarly, Real Power is given the units of Watts, but it is not true that Watts is Real Power. The reason for this is because Watts, as a unit of measure, is used in many other ways besides electrical systems, such as thermal power and optical power.
No that would not be true - for the same reasons as you stated above. "Watts" are units. Real power is a function. This seems uncharacteristic of you - after chastising me twice.
So as I said before, the units are identical. It is the equation that uses the specific units, that defines the differences.
As I recall from reading the thread posts, this has already been said several times - clear enough that even I got it.
cf
mivey
Senior Member
Indeed they do:
From Techniques of circuit analysis, Carter, 1972, Cambridge University Press: "...VI is not a power and must not be measured in watts..."
But may not all be on the same page:
From Power System Analysis, Nagsarkar, 2007, Oxford University Press: "...it is observed that complex power S, apparent power |S|, real power P, and reactive power Q are products of voltage and current. The measurement unit of all power quantities is watt. However, to avoid confusion..."
And some from the colonies:
From Power system Analysis, Bergen, 1986, Prentice-Hall:
"...the units in each case are watts...[referring to S, |S|, P, Q]"
From Elements of Power System Analysis, Stevenson, 1975, McGraw-Hill"
"...the power absorbed by a load at any instant is the product of the instantaneous voltage drop across the load in volts and the instantaneous current into the load in amperes..."
"...this component [Q]...is called the instantaneous reactive power and expresses the flow of energy..."
Remember that the designated unit for S = P + jQ is VA
Is that another way of you trying to say that no energy is flowing to the capacitor or inductor? I think you will find that is does flow. Internationally.
Last edited:
Cold Fusion
Senior Member
- Location
- way north
ammended from Cold Fusion;1068330
Note: Definition of the idomatic phrase, "have/has a direction": The function/unit shown has vector properties, It can be described in a variety of ways (A +jB), (i,j), magnitude/phase angle.
Watts have a direction
Vars have a direction
Real power + reactive power = Complex power (not apparent power)
Complex power has a direction
VA is the units for apparent power.
Apparent power = magnitude [complex power]
Apparent power does not have direction
Repeat after me:
P = EI* = Complex power. Complex power is a vector. Unless we are sizing wire, (which includes generators, transformers, switchgear) apparent power is pointless (not a vector) I promise to not use the term "Apparent Power" when describing a complex quanity.
cf
Note: Definition of the idomatic phrase, "have/has a direction": The function/unit shown has vector properties, It can be described in a variety of ways (A +jB), (i,j), magnitude/phase angle.
Watts have a direction
Vars have a direction
Real power + reactive power = Complex power (not apparent power)
Complex power has a direction
VA is the units for apparent power.
Apparent power = magnitude [complex power]
Apparent power does not have direction
Repeat after me:
P = EI* = Complex power. Complex power is a vector. Unless we are sizing wire, (which includes generators, transformers, switchgear) apparent power is pointless (not a vector) I promise to not use the term "Apparent Power" when describing a complex quanity.
cf
Last edited:
Cold Fusion
Senior Member
- Location
- way north
By the way, how about explaining your reasoning why vars are a special case and watts are not.
cf
Rick Christopherson
Senior Member
You are taking a logical discussion of a topic pretty personally. No one has chastised you, but you have chosen to take it that way. I wasn't correcting your statements simply to be mean, but because it went to the core of the discussion we are engaged in.
I already did explain this, but actually, Larry's suggested analogy does a better job.
Mikey also explained it quite well too. The differences are merely convention, but you are treating them as absolute. The convention is that the units on the right side of the equation are dictated by the function on the left side of the equation. You have reversed this and are stating that the function on the left side of the equation is dictated by the units on the right side of the equation. That might sound trivial at first glance, but it is very significant.
mivey
Senior Member
And is also the units for complex power
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).
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 for some of the other stuff:
1) VA and watts are not the same
2) I agree with charlie b's #4 that they have the same fundamental units
3) I disagree with charlie b's #116 where he states "Power is not the rate of doing work" (in doing so I agree with part of Besoeker's #115 and all of #118). Power is a measurement of the rate of work or energy flow. Doing work and using energy are not identical, but their rates can be equal (i.e. one unit of energy transferred for one unit of work done).
Yes. In and out. Mean value is zero. Mean work is zero. Mean power is zero.
That's why you need the term VA to describe it.
mivey
Senior Member
I agree the real power (or mean power) is zero at steady-state as shown in your graphic. I also agree that using the VA term will help show the energy flowing in & out of the load in equal quantity, creating a net zero flow of energy over the interval.
mivey
Senior Member
More fun with loads
More fun with loads
Ponder this:
An unenergized system having a motor load with local power factor correction to unity. The rest of the load is resistive. Assume ideal loads, etc.
At startup, there is a block of energy sent from the source to the motor/capacitor couple to charge the coupled energy storage system. At steady-state running conditions, there is no reactive power being transferred from source to load.
At shutdown, will the block of energy be returned to the source and would you then associate the block of energy sent with reactive power (a net zero transfer)?
What if at shutdown the energy was not returned (say the capacitor held a charge), would you then associate the block of energy sent with real power (not a net zero transfer)?
Do you see an issue with using a one-cycle definition vs a multi-cycle definition?
What would you say if the storage system was not a capacitor or inductor? Does it have to return energy every cycle? If it did, would you call it a reactive load?
More fun with loads
Ponder this:
An unenergized system having a motor load with local power factor correction to unity. The rest of the load is resistive. Assume ideal loads, etc.
At startup, there is a block of energy sent from the source to the motor/capacitor couple to charge the coupled energy storage system. At steady-state running conditions, there is no reactive power being transferred from source to load.
At shutdown, will the block of energy be returned to the source and would you then associate the block of energy sent with reactive power (a net zero transfer)?
What if at shutdown the energy was not returned (say the capacitor held a charge), would you then associate the block of energy sent with real power (not a net zero transfer)?
Do you see an issue with using a one-cycle definition vs a multi-cycle definition?
What would you say if the storage system was not a capacitor or inductor? Does it have to return energy every cycle? If it did, would you call it a reactive load?
The Power Triangle:
The Power Triangle:
So called "complex power" is nothing more than the power triangle expressed with complex numbers. We can do that to any right triangle. It is only a means of representation and does not make the scalar quantities into vectors--phasors either. The angle involved is the power factor angle. The so called "directions" are always zero, ninety degrees, and the PF angle.
Electromagnetic fields are properly described with vectors because the angles denote a direction in space--not so with complex power.
Impedances and sinusoidal voltages and currents are properly described with phasors, the angles denote time, not direction--not so with complex power.
The Power Triangle:
So called "complex power" is nothing more than the power triangle expressed with complex numbers. We can do that to any right triangle. It is only a means of representation and does not make the scalar quantities into vectors--phasors either. The angle involved is the power factor angle. The so called "directions" are always zero, ninety degrees, and the PF angle.
Electromagnetic fields are properly described with vectors because the angles denote a direction in space--not so with complex power.
Impedances and sinusoidal voltages and currents are properly described with phasors, the angles denote time, not direction--not so with complex power.
mivey
Senior Member
why not?
why not?
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
why not?
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
Cold Fusion
Senior Member
- Location
- way north
Cold Fusion
Senior Member
- Location
- way north
We have already been through this.
e is a vector
i is a vector
Complex Power = S = e x i* (and that is a vector quanity)
energy (work) = Integral (S) (vector quanity)
as I said earlier, I don't care if you use "vector', "phasor", or "first ranked tensor" that's the way the math works.
None of these quanities are scalar.
cf
Cold Fusion
Senior Member
- Location
- way north
Rick-
No you didn't. You only thought you did cause you didn't read what I asked.
Your treatment of VARS as special could equally apply to Watts - just change "VARS" to "Watts" and "reactive" to "real" and it pretty well fits. I'm not going to take time to write it out so you will be able to nit pick this one to past death - but the concept is still true.
cf
No you didn't. You only thought you did cause you didn't read what I asked.
Your treatment of VARS as special could equally apply to Watts - just change "VARS" to "Watts" and "reactive" to "real" and it pretty well fits. I'm not going to take time to write it out so you will be able to nit pick this one to past death - but the concept is still true.
cf
Cold Fusion
Senior Member
- Location
- way north
Rick -
hummmm - do I treat this as an absolute? Can't see any reason not to.. Fits the model well, easy to understand, and except for you it doesn't cause any grief.
Could I deal with another oreintation, like someone says "complex power = EI" (not EI*)? Sure I could fight my way through it, after all, it is just a definition - change the definition, the pictures move around, some signs change, but the model still works.
Other than this conversation, sticking with the above convention works really well.
cf
Lets see - Watts are left to right; VARS are up and down, complex power goes tail of watts to head of vars.
hummmm - do I treat this as an absolute? Can't see any reason not to.. Fits the model well, easy to understand, and except for you it doesn't cause any grief.
Could I deal with another oreintation, like someone says "complex power = EI" (not EI*)? Sure I could fight my way through it, after all, it is just a definition - change the definition, the pictures move around, some signs change, but the model still works.
Other than this conversation, sticking with the above convention works really well.
cf
Cold Fusion
Senior Member
- Location
- way north
Rick
Yes, this sounds really trivial. It does not contribute to the understanding of the model - which was the original point of the thread.
cf
Yes, this sounds really trivial. It does not contribute to the understanding of the model - which was the original point of the thread.
cf
Cold Fusion
Senior Member
- Location
- way north
Quote:
Originally Posted by Cold Fusion
- after chastising me twice.
Hummmm .... let's see, you pointed out twice where I was wrong in my idomatic application of unit terms, and then in the next two sentences used a very similar idomatic application.
The point of my post was that your "logical correction" was pretty poorly applied. As for it going to the "core of the discussion" - I'm missing that part.
cf
Originally Posted by Cold Fusion
- after chastising me twice.
Hummmm .... let's see, you pointed out twice where I was wrong in my idomatic application of unit terms, and then in the next two sentences used a very similar idomatic application.
The point of my post was that your "logical correction" was pretty poorly applied. As for it going to the "core of the discussion" - I'm missing that part.
cf
- Status