VA vs. Watts

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markstg

Senior Member
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Big Easy
So you want to redefine the Joule?
A Joule per second is a Watt which power, not apparent power.
Take a reactive load like a capacitor.
The current it takes times the applied voltage gives it's VA.
Ignoring losses (a close approximation at power frequencies), there will be no power and thus no Watts. It will consume no Joules of energy in any one second period.
In short, in an AC circuit, Joules per second is not VA.
In short...Joules per second could be VA, could be VAR, could be Watt. It depends on which of the three electrical components you are defining the product of current through and the voltage across.

Electrical Engineers need to distinguish which type of Power we are talking about (Real, Reactive, Apparant), so we have come up with the VA, VAR, Watt units of measure. But we could also say Joules/sec inductive, Joules/sec capacitive, Joules/sec resistive.

From my Physics and Circuits Textbooks:

p = vi W

for resistor p=Ri**2 W
for inductor p=Li(di/dt) W
for capacitor p=Cv(dv/dt) W

Integration of the above equations results in the energy generated/absorbed by the device and its unit of measure is the Joule.


We could also call it Beer Liquid, Beer Foam....but that's another story.
 
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charlie b

Moderator
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Seattle, WA
So you want to redefine the Joule?
I did no such thing, nor did I so much as attempt to do so.

My reference is the McGraw-Hill Dictionary of Scientific and Technical Terms. The term ?Joule? is defined as the MKS unit of energy, with a note that it is equivalent to a Newton-Meter. The term ?Watt? is defined as the MKS unit of power, being ?equal to 1 Joule per second.? Thus, a ?Watt? is not the exact same thing as a ?Joule per second,? but it is numerically equal. That fact does not prevent us from using the phrase ?Joule per second? for other purposes to which that phrase can apply. Indeed, the phrase ?Joule per second? has no definition of its own. It is not, to be specific, defined as being a unit of ?real power.? It is a unit of measure that is derived from other units of measure.

All dogs are animals, but not all animals are dogs. All ?Watts? are ?Joules per second,? but not all ?Joules per second? are ?Watts.?
 

charlie b

Moderator
Staff member
Location
Seattle, WA
VA and Watts are not the same thing except for the specific case of unity power factor.
Here again, I never said any such thing.
Specifically, a “Joule per second” could be a “Volt-Ampere Reactive,” That's just wrong.
No it is not wrong. Real, Reactive, and Apparent Power all have the same fundamental units, and therefore they are given different names to allow us to more easily distinguish them.
 

Besoeker

Senior Member
Location
UK
Indeed, the phrase ?Joule per second? has no definition of its own. It is not, to be specific, defined as being a unit of ?real power.? It is a unit of measure that is derived from other units of measure.
Quite right.
A Joule is not a measure of power. It is a unit of energy. One Watt second.
There are no Watts in the reactive component.
 

charlie b

Moderator
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A Joule is not a measure of power. It is a unit of energy. One Watt second.
A Joule is certainly equal to one watt-second. But it is not defined as one watt-second. The definition of Joule does not include the notion of ?real power,? as compared to reactive or apparent power.
There are no Watts in the reactive component.
Once again, I never said there were. But there are ?Joules per second? in the reactive component. I believe that you need to get past the notion that ?Watt? and ?Joule per second? are absolutely synonymous. They are not.

Do you have a response to the observation that two of us have made, to the effect that W, VA, and VAR have the same fundamental units (kilogram meter squared per second cubed)?
 

markstg

Senior Member
Location
Big Easy

A Joule is certainly equal to one watt-second. But it is not defined as one watt-second. The definition of Joule does not include the notion of ?real power,? as compared to reactive or apparent power.

Once again, I never said there were. But there are ?Joules per second? in the reactive component. I believe that you need to get past the notion that ?Watt? and ?Joule per second? are absolutely synonymous. They are not.

Do you have a response to the observation that two of us have made, to the effect that W, VA, and VAR have the same fundamental units (kilogram meter squared per second cubed)?
Charlie, add me as a third.
 

DavisIMI

Member
No, please don't stop. This is the best post I think I've ever read.

Thanks Charlie, I too have been in this buisness for 23 yrs and even though I've known the difference between watts, va, var, and pf I couldn't ever put the triangle thing together,(just never could see the relationship). So you have done in 5 minutes what 20+ yrs and all these books I own couldn't. Thanks a million.
 

steve66

Senior Member
IMO, Charlie and Spongebob have it right.

All three have the same units - joules per second.

For watts, the "joules per second" have been coverted to heat, or something else, and they are forever lost to our electric circuit. Kind like the beer that gets drank.

But the VAR "joules per second" are just temporarly stored in an electric or magnetic field, to be returned to the circuit on the next cycle.

Steve
 

spdtrx

Member
No, please don't stop. This is the best post I think I've ever read.

Thanks Charlie, I too have been in this buisness for 23 yrs and even though I've known the difference between watts, va, var, and pf I couldn't ever put the triangle thing together,(just never could see the relationship). So you have done in 5 minutes what 20+ yrs and all these books I own couldn't. Thanks a million.
My kid ask me something about this the other day and I didn't know enough
about this to be able to explain it to him. Now I think i'm going to take him out for a couple of beers. Thanks Charle B.:wink:
 

Cold Fusion

Senior Member
Location
way north
charlie b said:
--- The units of measure for the three lines described above are fundamentally the same. They all relate to the rate of use of energy, and can all be expressed in terms of “joules of energy per second of time.” ---
drbond24 said:
---I think I would be correct in saying that the point here is that Watts, VA, and VAr all describe the same quantity of energy. They all describe different kinds of energy, of course, but not different amounts. ---
charlie b said:
--- Do you have a response to the observation that two of us have made, to the effect that W, VA, and VAR have the same fundamental units (kilogram meter squared per second cubed)?
Besoeker
The way the wise men have laid out their argument. they are right. However, I am with you, it is unpalatable. It would be like someone telling me that "torque" and "work" are the same. Here is the conversation:

Them: Torque and work are fundamentally the same. They can both be expressed in terms of "foot-pounds".

Me; So you are saying 550 ft-lbs (of torque) per second is 1 horsepower??:-?

Them: Well, no, I didn't say that,

Me: Okay, explain this again how they are fundamentally the same.
-------------------------------------

Vars, watts have the same units, they are not the same thing - they don't have the same direction. To only look at the units looses the vector information that tells us that "vars" aren't "power". Just saying, "vars is power", hurts my sliderule.

Maybe the wise men could say, "vars" are "(j)energy" (pronounced, "gen-er-gy"):)

cf
 
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charlie b

Moderator
Staff member
Location
Seattle, WA
Them: Torque and work are fundamentally the same. They can both be expressed in terms of "foot-pounds".
Here I have to throw a flag on the play. I am well aware that this statement is absolutely wrong. Sorry to be harsh, but the suggestion that I would say such a thing is way off base. The "foot" in foot-pounds of work is not the same as the "foot" in foot-pounds of torque. Any engineer would know that, as I am sure you know that. But for the sake of our members who are not engineers, let me prove that I know that.

Work can be expressed in foot-pounds, but the context is a pound of force that actually moves a object in a straight line for a distance of one foot. If an object is pushed, as hard as you like, and if the push does not cause the object to move, then no work is being done on the object.

Torque can also be expressed in foot-pounds, but the context is a pound of force acting at a distance of one foot from the thing it is attempting to influence. If the push is in the direction of the object, then torque is zero. Torque exists only if the push has a tendency to spin the object, whether or not it actually succeeds in making that object spin. If you use your torque wrench to attempt to tighten a nut, and if the length of the torque wrench handle is one foot, and if you apply one pound of force in the tightening direction, and if that is not enough to make the nut spin, then (1) You have applied 1 foot-pound of torque, and (2) You have performed exactly zero foot-pounds of work.

Vars, watts have the same units, they are not the same thing - they don't have the same direction. To only look at the units looses the vector information that tells us that "vars" aren't "power". Just saying, "vars is power", hurts my sliderule.
We know that VARS and Watts are not the same thing. We understand the vector relationships. We never said that “vars is power.” Look back at the posts and tell me that we have said any of these things.

All we have said is that the fundamental units of measure of real power, reactive power, and apparent power are all the same: Joules per second. What has been said in return is that the unit “Joule per second” is identical to “watt,” and that is not true. So how about, for the sake of putting this to bed, I submit the following:

? Real Power is measured in Watts, or equivalently in Joules per second.
? Reactive power is measured in VAR, which although is literally equivalent to saying “Joules per second – reactive,” is, in common practice, never actually called that.
? Apparent power is measured in VA, which although is literally equivalent to saying “Joules per second – apparent,” is, in common practice, never actually called that.
 
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gar

Senior Member
090611-2050 EST

The correct unit for torque is #-feet, #-in, NM, etc. and NOT foot-#. The difference in names was to distinguish between two different animals.

The torque wrench manufacturers that label their wrenches in foot-# are mislabeling their product. The difference in names for torque and work were emphasized to me way back in high school physics a long time ago. Even in various references that predate my high school physics by many years I can show the correct usage of #-ft for torque. To a large extent the torque wrench manufacturers can be blamed for this common labeling and usage problem.

I have some additional comments on VA. Separately the voltage and current should be the RMS values. Really the important part is that the current be the RMS value. It makes sense to also nominally measure the RMS voltage. That V and A of VA are RMS measurements does not mean that VA is RMS, and so one should not say RMS VA. One could say VA based on RMS values of voltage and current. One could use non-RMS values, but this would not be very useful. The normal assumption should be that VA is derived from the RMS measurement of V and of A.

Why is the RMS value of importance? This is because the RMS value of the current is related to the heating of a resistance (wire) and therefore the temperature rise.

If there is little distortion in the voltage waveform then a voltage measurement with a Simpson 260 or a Fluke non-RMS meter is still a very good estimate of the RMS voltage. These instruments perform a full-wave average measurement of the AC signal, and use a meter scale calibrated to provide the RMS value of a sine wave. Put a distorted waveform into these instruments and the reading may be quite different than the true RMS value. The ratio between RMS and full-wave average for a sine wave is 0.707/0.636. If you used a full wave bridge rectifier and a DC meter, then the DC reading needs to be multiplied by 1.112 to obtain the RMS value.

Why are devices like transformers rated in VA? This is because a larger percentage of the power dissipated in the transformer is from I^2*R power loss. So if a transformer can tolerate 100 A input current for its full rated load, then it does not matter what the phase relationship between the input voltage and the current is, the internal power dissipation in the transformer is the same. Thus, a purely capacitive load on this transformer that produces an input current of 100 A and a power factor of approximately 0 produces as much transformer heating as does a pure resistive load of the same input current but a power factor of 1.

To clarify: What does RMS mean? The RMS value of something (a variable) is obtained by taking the square of the instantaneous value of a variable, averaging this over a period of time, and taking the square-root of the average. In general this is done on a variable that has a periodic variation, is statistically stationary, and the calculation is performed over an integral number of cycles, or a very large number of cycles so that starting and ending points for the average duration do not contribute significant error.

Power factor is always defined as the ratio of POWER to VA no matter what are the waveform distortions. With distorted waveforms the cos of the angle of the voltage and current zero crossings may not equal the power factor.

An unloaded transformer will have a low power factor and this is a case where cos won't work well to define power factor because of the severe distortion of the excitation current. This same transformer at full load will have an input power factor very close to the power factor of the load on the transformer. The contribution to the input current of the magnetizing current to the transformer core may be in the 1% range.

A good discussion on power factor can be found at
http://en.wikipedia.org/wiki/Power_factor

.
 

Cold Fusion

Senior Member
Location
way north
Here I have to throw a flag on the play. I am well aware that this statement is absolutely wrong. Sorry to be harsh, but the suggestion that I would say such a thing is way off base. The "foot" in foot-pounds of work is not the same as the "foot" in foot-pounds of torque. Any engineer would know that, as I am sure you know that. But for the sake of our members who are not engineers, let me prove that I know that.---.
Charlie - you're making me laugh. Can't say I ever thought you would say that ft-lbs torque was the same as ft-lb work - any fool would know they aren't the same. You certainly didn't have to prove that you knew the difference.

I highly suspect the point of my post was the units can be the same and the physical phenomena significantly different.

However, just so we are clear, you're telling us that the "foot" in foot-pounds (torque) is different that the "foot" in foot-pounds (work). But the "joule" in joules/sec (watts) is the same as the "joules" in joules/sec (vars).

(laughing so hard I nearly can't stay on the cahir to type.)

edit: apparently can't type - that would be "chair" not "cahir"

cf
 
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