Power factor and VA vs Watts

[Smart $]

If it is a parallel RC circuit, the average value of the reactive component wouldn't be zero.

... And, did I hear right? The net power to a cap is greater than zero?? ...

I said no such thing.

Not exactly... you said it wouldn't be zero.

So I asked...

What would it be?

...and you have not answered.

 

[rattus]

I said no such thing.

Well then, tell what you mean by this,

"If it is a parallel RC circuit, the average value of the reactive component wouldn't be zero."

Literally it says C has a non-zero value, but maybe a word has been omitted or some other typo.

 

[jghrist]

Correct me if I'm wrong, but it seems that the key to viewing the real and reactive currents and power the way Smart portrays them is to convert any circuit to a parallel resistor and reactive component (either inductor or capacitor). Then the real and inductive elements see the same voltage. Any series R and X can be converted to an equivalent parallel R and X.

The current through the resistive element would be considered the "real" current. The current through the reactive element would be the "reactive" current.

The voltage at any instant times the current through the resistive element at that instant would be the "instantaneous" power through the resistive element, or "real" power at that instant. The voltage at any instant times the current through the reactive element at that instant would be the "instantaneous" power through the reactive element, or "reactive" power at that instant.

 

[Smart $]

Correct me if I'm wrong, but it seems that the key to viewing the real and reactive currents and power the way Smart portrays them is to convert any circuit to a parallel resistor and reactive component (either inductor or capacitor). Then the real and inductive elements see the same voltage. Any series R and X can be converted to an equivalent parallel R and X.

The current through the resistive element would be considered the "real" current. The current through the reactive element would be the "reactive" current.

The voltage at any instant times the current through the resistive element at that instant would be the "instantaneous" power through the resistive element, or "real" power at that instant. The voltage at any instant times the current through the reactive element at that instant would be the "instantaneous" power through the reactive element, or "reactive" power at that instant.

Correctomundo

I should add though that...

and reactive component (either inductor or capacitor)

...can be both, also.

 

[rattus]

Correctomundo

I should add though that...

...can be both, also.

Smart, could be more than one element in the circuit, but in the equivalent it would be only one reactive element assuming constant frequency.

JG, you could go one step further and specify the parallel equivalent as,

admittance, conductance, and susceptance.

 

[Smart $]

Smart, could be more than one element in the circuit, but in the equivalent it would be only one reactive element assuming constant frequency.

Understood.

I suppose I should have elaborated to this degree... :roll:

 

[Hameedulla-Ekhlas]

From my perspective, correct... though as the graph depicts a net-capacitive-only load, all values of the vi product would be "VAR", not just at 45?. If information were limited to the vi product at 45?, there would be no way to determine real and/or reactive portions (with respect to the load)... so the VA unit is appropriate in a real power vs. apparent power load analysis.

Smart$:
I could not untertand the above red line can you please make it alittle bit clear with explaination.

 

[Hameedulla-Ekhlas]

Smart, could be more than one element in the circuit, but in the equivalent it would be only one reactive element assuming constant frequency.

JG, you could go one step further and specify the parallel equivalent as,

admittance, conductance, and susceptance.

Bes's graph is ok for reactive and there is no problem. Here is one circuit parallel and separate both graph.

 

[Smart $]

Smart$:
I could not untertand the above red line can you please make it alittle bit clear with explaination.

Break the sentence down to phrases...

If information were limited to the vi product at 45?, there would be no way to determine real and/or reactive portions (with respect to the load)... so the VA unit is appropriate in a real power vs. apparent power load analysis.

We lack information to determine, in one way or another, the power factor. The vi product can only be ascertained to be volt-amperes... not watts, for we'd need to know the power factor is 1... and not watts and volt-ampere reactive combined, for we'd need to know the power factor and that it is not 1 or 0... and not volt-ampere reactive, for we'd need to know the power factor is 0.

 

[mivey]

But we can plot instantaneous values of real and reactive power, and in so doing we separate the real and reactive portions of the vi product which you say is impossible. Just like you plotted reactive power for the cap, we can plot real power on the same graph.

Absolutely correct. While W, var, & VA all have the same fundamental units (kg?m?/s?), we can separate the power into its real and reactive portions and plot the same.

 

[mivey]

You can't resolve into real and reactive components. It is just 1W at that instant in time.

You can if you have additional information that extends past that one instant in time. But given just one voltage and one current without the information that covers the rest of the cycle, you just have the one value of volts (kg?m?/s?/A) times current (A) equals v*i_kg?m?/s?

 

[weressl]

Your statement does not make sense. The argument for the trig functions must be in radians, not seconds. e.g., the units of "wt" are radians.

Theta is the lead or lag as the case may be between the voltage across and the current through a load. e.g., the general form for i(t) is,

i(t) = Im*sin(wt + theta)

Theta is described as a phase angle in,

[Kerchner and Corcoran, (Alternating-Current Circuits, 3rd edition, John Wiley & Sons, 1951)]

If you think otherwise, give us a solid reference.

In instantaneous measurement there is no time involved. As I previsouly noted, it is a theoretical measurement a practical impossibility.

The vectorial representation of U and I illustrates the angular difference between the two values. The vector always represents the full value of each quantities, regardless where they are in the cycle of rotation. Since the angle between U and I remains constant, the U and I - value equals sqrt(2) of maximum - are muliplied by cosine(phi) for the trig summary power vector value.

Besoekers explanation was much simpler and elegant, should have been easier to grasp than the above 'longhand', .

 

[Besoeker]

They are already defined.

As RMS values.
Now think about what RMS means.

 

[rattus]

I don't understand:

I don't understand:

In instantaneous measurement there is no time involved. As I previsouly noted, it is a theoretical measurement a practical impossibility.

I think you said there is no time involved in f(t) or f(wt)?? How can that be?

Please explain why it is an impossible measurement. It is done all the time with oscilloscopes and sampling instruments. And, you can compute the value at any time.

 

[Besoeker]

What would it be?

From the current and voltage given by Rattus, about 0.707VAr

 

[Besoeker]

You can if you have additional information that extends past that one instant in time.

Thank you mivey. You made my point more eloquently than I have.
If you need the passage of time to work out the the real and reactive components you can't plot instantaneous values.

 

[weressl]

I think you said there is no time involved in f(t) or f(wt)?? How can that be?

Please explain why it is an impossible measurement. It is done all the time with oscilloscopes and sampling instruments. And, you can compute the value at any time.

Any measurement involves time therefore it is not instantaneous. Instantaneous is a theoretical value and can not be measured.

Main Entry: in?stan?ta?neous
Pronunciation: \ˌin(t)-stən-ˈtā-nē-əs, -nyəs\
Function: adjective
Etymology: Medieval Latin instantaneus, from instant-, instans, noun
Date: 1651
1 : done, occurring, or acting without any perceptible duration of time <death was instantaneous>
2 : done without any delay being purposely introduced <took instantaneous corrective action >
3 : occurring or present at a particular instant <instantaneous velocity>

In other words, instantaeous measurement is an oxymoron.

 

[rattus]

Any measurement involves time therefore it is not instantaneous. Instantaneous is a theoretical value and can not be measured.

Main Entry: in?stan?ta?neous
Pronunciation: \ˌin(t)-stən-ˈtā-nē-əs, -nyəs\
Function: adjective
Etymology: Medieval Latin instantaneus, from instant-, instans, noun
Date: 1651
1 : done, occurring, or acting without any perceptible duration of time <death was instantaneous>
2 : done without any delay being purposely introduced <took instantaneous corrective action >
3 : occurring or present at a particular instant <instantaneous velocity>


In other words, instantaeous measurement is an oxymoron.

Laszlo, that is NIT PICKING; in fact that is the nittiest pick I have heard in a while. I can't see that it adds anything to the discussion. We speak of instantaneous measurements routinely while knowing full well that they cannot be measured in zero time.

Now we can compute p(t) at any instant, but are you going to remind me that the result is never exact because infinite precision is unattainable?

 

[rattus]

Thank you mivey. You made my point more eloquently than I have.
If you need the passage of time to work out the the real and reactive components you can't plot instantaneous values.

Bes, must be a man of few words because this is the first time I have heard him explain his position, and this is a no-brainer. BTW, I said essentially the same thing a few posts back.

He still hasn't commented on the validity of my plot.

 

[weressl]

Laszlo, that is NIT PICKING; in fact that is the nittiest pick I have heard in a while. I can't see that it adds anything to the discussion. We speak of instantaneous measurements routinely while knowing full well that they cannot be measured in zero time.

Now we can compute p(t) at any instant, but are you going to remind me that the result is never exact because infinite precision is unattainable?

Well, if you followed the discussion and arguments, you would see that my comment was pointing out exactly what you are saying except from a different angle.

There are; theoretical explanation of observed phenomena that can be formulated into a scientific 'law', representative mathematical formulas and tools to express a law, and observable measurements that give indirect readings of the observed parameters. When they are not separated and terms are intermingled errors will be made in trying to express or understand each and contradiction occurs. The 'nit' in this case can be refered to the contamination of clear ideas and principles and methods expressed in the laws, formulas and empirical mesurements/results. In that case picking those nits are entirely appropriate.