VA vs. Watts

[Cold Fusion]

--- That V and A of VA are RMS measurements does not mean that VA is RMS, and so one should not say RMS VA. ---.

You're right. one should never say "rms power" (unless you are a salesman dealing with audio amps)

Laast time I looked, rms volts X rms amps gives Average power

(no I didn't forget the pf)

cf

 

[charlie b]

However, just so we are clear . . . the "joule" in joules/sec (watts) is the same as the "joules" in joules/sec (vars).

Yes. Quite true.

If I had it in mind to measure the rate at which energy is being exchanged between the magnetic field of a transformer's secondary windings and the magnetic field of a motor being powered by that transformer (the physical phenomenon we know as "reactive power"), the answer would be expressed in units of energy divided by time. So I would shop around for an energy measuring instrument and a time measuring instrument. I see no reason that I couldn't buy an energy meter that is calibrated in units of Joules, as opposed perhaps to units of BTUs. I see no reason that I couldn't buy an time meter that is calibrated in units of seconds, as opposed perhaps to units of hours.

 

[mparkstexas]

Wow, 42 post and counting...

And reading all the responses made my head hurt...

 

[Besoeker]

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

I do.
The dimensional analysis is not disputed.
One Volt times one Amp is one Watt. Provided they occur at the same time.
It works for instantaneous and steady state values. For time varying or periodic waveforms the instaneous values of current and voltage give Watts.
For an alternating circuit, displace the current and voltage by a quarter of a cycle and you still have VA but no W.
They are not the same units.

 

[Cold Fusion]

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.

Originally Posted by Cold Fusion

However, just so we are clear . . . the "joule" in joules/sec (watts) is the same as the "joules" in joules/sec (vars).

Yes. Quite true. ...

I have to blow a whistle - two minutes in the penalty box for intentional selective quoting with intent to mis-direct. Sorry to be harsh, but doing such a thing is way off base.

The "joule" in watts is not the same as the "joule" in VA - they don't have the same direction.

This is very similar to the "foot' in "foot-pounds (torque) is not the same as the "foot" in "foot-pounds (work) - they don't have the same direction.

As I said earlier, that was the my point. The units can look the same and the phenomena completely different.

Now, if you tell me that the "joules" don't have a direction (ie. not vector quantities)- you're going to make me laugh again.

cf

 

[Besoeker]

The "joule" in watts is not the same as the "joule" in VA - they don't have the same direction.

They are not the same.
A Joule is one Watt/second.
One VA is just that. One VA. One VA could be one W/s or zero W/s

 

[Besoeker]

They are not the same.
A Joule is one Watt second.
One VA is just that. One VA. One VA could be one W or zero W

Is what I meant.

 

[cadpoint]

Wow, 42 post and counting...

And reading all the responses made my head hurt...

Welcome to the Forum!

I love it when the Heavy Hitters go at it!

I also Love it when the Heavy Hitter "EC's" Post Business threads!!!

 

[steve066]

The "joule" in watts is not the same as the "joule" in VA - they don't have the same direction.

So if I walk 5 feet east, those units are different than if I walk 5 feet north?

As I said earlier, that was the my point. The units can look the same and the phenomena completely different.

I guess so, if you consider walking east a completely different phenomena than walking north.

But for Watts and VAR's, one can argue the phenomena is exactly the same and can be explained in either case as a transfer of energy per time. For real power the transfer is energy converted from electricity to some other form, like heat. For reactive power, it is energy stored in an electric or magnetic field. In either case, Joules per second are the correct units.

Besides, the units have to be the same for both power and VAR's. Otherwise, we wouldn't be able to add them together. Dimensional analysis requires that to add two quantities (even vectorally), they must have the same units. If VAR's and Watts had different units, it would be like adding 3 pounds to 4 feet and getting 5 seconds. Just doesn't make any sense at all.

 

[steve066]

Cold Fusion:

I tired to delete the north -east comment. I have to admit, that really oversimplify's the matter, and isn't really a fair comment. I realize that when you say direction, you are refering to the relative phases of the voltage and current, and its just not fair to compare that to walking two different directions.

But I do stand by the rest of my previous post, and I hope you see the point. I hope there aren't any hard feelings.

Steve

 

[LarryFine]

So if I walk 5 feet east, those units are different than if I walk 5 feet north?

I guess so, if you consider walking east a completely different phenomena than walking north.

What if your pedometer only measures northbound walking?

 

[rattus]

A joule is a joule is a joule:

A joule is a joule is a joule:

The "joule" in watts is not the same as the "joule" in VA - they don't have the same direction.

Now, if you tell me that the "joules" don't have a direction (ie. not vector quantities)- you're going to make me laugh again.

cf

Must argue with the whistle blower.

The key to this discussion I think is that one VA "appears" to be one joule/sec, but it is not. For example, if the phase angle is 30 deg. and the apparent power is 1 VA, 0.866 joules/sec are transferred to the load while 0.5 joules/sec flow back and forth in the reactive portion of the load.

Energy is scalar; it may carry a sign, but you cannot describe energy with a complex number, therefore it cannot be quantified with a vector. Energy cannot be quantified as a phasor either. Only RMS voltages, currents, and impedances can be described as phsors.

 

[Cold Fusion]

Must argue with the whistle blower.

The key to this discussion I think is that one VA "appears" to be one joule/sec, but it is not. For example, if the phase angle is 30 deg. and the apparent power is 1 VA, 0.866 joules/sec are transferred to the load while 0.5 joules/sec flow back and forth in the reactive portion of the load.

Energy is scalar; it may carry a sign, but you cannot describe energy with a complex number, therefore it cannot be quantified with a vector. Energy cannot be quantified as a phasor either. Only RMS voltages, currents, and impedances can be described as phsors.

hummm - all interesting ideas - have to think about them.

Watts transfers energy. VA can transfer energy from zero to all, VARS doesn't transfer energy at all. All have units of joules/sec.

Energy (joules) is scalar (rattus).
The direction of seconds certainly doesn't change in our known universe (cf)

So what is the difference in the three?

The definition? Sometimes that is all it is - just the way it is defined.

good point - could be - still thinking about this one.

cf

 

[charlie b]

Now, if you tell me that the "joules" don't have a direction (ie. not vector quantities)- you're going to make me laugh again.

I would offer you the chance to get that laugh, but Rattus beat me to it. For the record, however, let me just say that energy is not a vector quantity.

Happy Monday!

 

[iwire]

This has been a fascinating thread, I only wish I understood more of it but I think I learning a bit anyway.

 

[techntrek]

+ 1 for iwire. As much as I thought I knew about this subject, there is much still to be learned about the Force. May the Force be with us all (just not through us...).

 

[charlie b]

They are not the same. A Joule is one Watt/second.

That is not the definition of either item. Neither is defined as being "identical to" the other. The two are equivalent, but not identical.

If someone owed you $200, and was ready to pay you, and called to say he had ten $20 bills in his pocket, and wanted to make sure you were home to accept delivery, but said he was going to first stop by the bank and trade in the bills for pennies, you would consider the two forms of payment to be identical? Equivalent in value, perhaps, but not "identical." I am guessing that you would prefer to be paid with the $20 bills, instead of being paid with 2,000 pennies.

You can inform me that one dollar is the same as 100 pennies, but you can't prevent me from saying that four quarters is equivalent to a dollar as well.

 

[drbond24]

If someone owed me $200 and gave me 2,000 pennies, my first concern would be where the other 18,000 pennies are. Then I'd get upset that they paid me in pennies.

 

[charlie b]

If someone owed me $200 and gave me 2,000 pennies, my first concern would be where the other 18,000 pennies are.

My mistake. Typo. Should have checked the math. :roll:

 

[charlie b]

I think we are essentially in the middle of the following argument:
? Person One says, ?I submit that the grass is green.?
? Person Two replies, ?No, you are wrong. The sky is blue.?

Let me try to resolve the issue in the following manner:
1. If someone wanted to buy carpet for the living room, and if the sales agent asked them for the area of the room, could they give an answer in units of acres? Well, yes they could. But that is not the usual unit of measure used in this application.
2. If someone asked you how far it is to the nearest grocery store, could you give an answer in units of inches? Well, yes you could. But that is not the usual unit of measure used in this application.
3. If you were considering buying a hot tub, and if you asked the sales agent how much water is needed to fill the tub, could the agent give an answer in units of milliliters? Well, yes he could. But that is not the usual unit of measure used in this application.
4. If someone asked me how fast energy is being exchanged between two inductive machines, could I give an answer in units of Joules per second? Well, yes I could. But that is not the usual unit of measure used in this application.

Regarding item 4, if I were to choose to give the answer in that unit of measure, it would be incumbent upon me, for the sake of avoiding future confusion, to mention that this number of Joules per second is being exchanged at a phase angle of 90 degrees from the number Joules per second that is being supplied to resistive loads. It would certainly be simpler if I gave the answer in units of VAR. That way, I would have less explaining to do.