Steve Nosko wrote:
. . .
I was, however, a bit puzzeled, Roy, why you went to the trouble of
calculating the the RMS value of a power earlier. I suspect it was just to
show that the value is indeed different (didn't check your math).

Yes, that was the only reason. As I mentioned, I'd never before had the
occasion to calculate the RMS value of a power waveform -- it's simply
not useful for anything. Except to illustrate that it's different from
the very useful value of average power.

I will differ with Roy on one issue. The RMS value of voltage and current
have, for many years, also been referred to as the "effective" values. This
was to relate it to the DC heating effect (of resistance) we are all
familiar with. It is, indeed just as "effective" as the same DC value, in
producing power. This is another terminology issue I suspect some of you
may wish to squabbling about, but is is not a 'what is correct technically'
issue. It is cleat that probably all of you understand the math, but this
is simply an nomenclature issue.

I don't have any disagreement with this. I just have to keep cautioning
people not to extrapolate it to power. That is, just because the RMS
value of voltage or current is an "effective" or DC equivalent value,
don't think it implies that the RMS value of power must be its
"effective" or DC equivalent value. It's not.

. . .

I am not not trying to prolong the pain (or this thread), it is just that I
was born with a bone in my head that makes it hard for me to give up
explaining some basic concept like this. (yep, it can be a curse) . . .

Egad, another person with the same genetic defect! Welcome!

Roy Lewallen, W7EL

"Roy Lewallen" wrote in message
...
Steve Nosko wrote:
. . .

I am not not trying to prolong the pain (or this thread), it is just
that I
was born with a bone in my head that makes it hard for me to give up
explaining some basic concept like this. (yep, it can be a curse) . . .

Egad, another person with the same genetic defect! Welcome!
Roy Lewallen, W7EL

So it's genetic!...

Yea. And I do this in front of a class or 5-15 green students. It SURE is
rewarding when someone says :"Oooh! NOW I get it!"

73,
--
Steve N, K,9;d, c. i My email has no u's.

My friend, N7TCY wrote this bit regarding RMSpower a few years ago.
We work with inverters so there was some call for it.

http://www.eskimo.com/~bgudgel/cgi-bin/rms/rms.html


boB
K7IQ








On Thu, 21 Oct 2004 10:49:39 -0500, "Steve Nosko"
wrote:


"Roy Lewallen" wrote in message
...
Steve Nosko wrote:
. . .

I am not not trying to prolong the pain (or this thread), it is just
that I
was born with a bone in my head that makes it hard for me to give up
explaining some basic concept like this. (yep, it can be a curse) . . .

Egad, another person with the same genetic defect! Welcome!
Roy Lewallen, W7EL

So it's genetic!...

Yea. And I do this in front of a class or 5-15 green students. It SURE is
rewarding when someone says :"Oooh! NOW I get it!"

73,

boB_K7IQ wrote:
My friend, N7TCY wrote this bit regarding RMSpower a few years ago.
We work with inverters so there was some call for it.

http://www.eskimo.com/~bgudgel/cgi-bin/rms/rms.html

That looks like a good analysis. I've just put the finishing touches on
a non-mathematical analysis which I hope will be helpful for folks who
can't or don't want to wade through the math. It uses simple square
waves to illustrate the concepts. The link is

http://eznec.com/Amateur/'RMS Power'.pdf

Comments and corrections are welcome, either posted here or emailed to me.

Roy Lewallen, W7EL

Hm, my browser doesn't like the name I gave the file. So I've uploaded a
second copy with a different name. You can also get it as

http://eznec.com/Amateur/RMS_Power.pdf

Roy Lewallen, W7EL

Roy Lewallen wrote:


That looks like a good analysis. I've just put the finishing touches on
a non-mathematical analysis which I hope will be helpful for folks who
can't or don't want to wade through the math. It uses simple square
waves to illustrate the concepts. The link is

http://eznec.com/Amateur/'RMS Power'.pdf

Comments and corrections are welcome, either posted here or emailed to me.

Roy Lewallen, W7EL

Both the Rosenbaum and Lewallen papers look very good and well written.

I would, however, Roy, make a more explicit statement that while RMS power
can be calculated, is has no practical value for the normal considerations
of power...and add a note indicating something like that:

"The phrase "RMS Power" has been used in some circles not as an exact use of
the term RMS, but rather as an informal "standard" that actually means
"average power under specific text conditions". (I refer to an earlier post
telling of the single channel, steady state audio power amp measurement)
This use of the term "RMS" was originally initiated to call attention to the
specific test. Unfortunately, this use has caused some confusion in the use
of this terminology and is some of the motivation for the paper."

73, Steve, K9DCI

--
Steve N, K,9;d, c. i My email has no u's.




"Roy Lewallen" wrote in message
...
Hm, my browser doesn't like the name I gave the file. So I've uploaded a
second copy with a different name. You can also get it as

http://eznec.com/Amateur/RMS_Power.pdf

Roy Lewallen, W7EL

Roy Lewallen wrote:


That looks like a good analysis. I've just put the finishing touches on
a non-mathematical analysis which I hope will be helpful for folks who
can't or don't want to wade through the math. It uses simple square
waves to illustrate the concepts. The link is

http://eznec.com/Amateur/'RMS Power'.pdf

Comments and corrections are welcome, either posted here or emailed to
me.

Roy Lewallen, W7EL

Thanks for the suggestions, Steve. I've updated the file to incorporate
them. The new file is at http://eznec.com/Amateur/RMS_Power.pdf.

During recent trips to Powell's Technical Bookstore and the library, I
looked through a number of books about electrical circuits which are
oriented toward hobbyists and technicians (that is, ones lacking the
math of a college level circuits text). I'm glad to say I didn't find
any which were plainly wrong about average and RMS power (like the Joe
Carr book quoted earlier here). But what nearly all of them do is to
introduce RMS voltage and current pretty early on in the text, and
explain that the RMS values of voltage and current are important because
they represent equivalent heating values (which is correct). From then
on, they simply use E and I with the assumption that they represent RMS
values of voltage and current. At some point, they introduce the
equation P = E * I or, in the more advanced ones, E * I * cos(phase
angle), and maybe at that point mention that P is the equivalent heating
power (which is also correct). What I didn't see in any of them was the
fact that the product of the RMS values of E and I is the *average*, and
*not* the RMS value of P. It's easy to understand, then, why a lot of
people, like a number of the folks who posted comments and questions
here, naturally (and incorrectly) assume that the product of Erms and
Irms is RMS power. The books simply don't contain the information you'd
need in order to discover that Erms * Irms = Pavg. Hopefully the paper
posted by Rosenbaum and the one I did will help fill the void.

Roy Lewallen, W7EL

Steve Nosko wrote:
Both the Rosenbaum and Lewallen papers look very good and well written.

I would, however, Roy, make a more explicit statement that while RMS power
can be calculated, is has no practical value for the normal considerations
of power...and add a note indicating something like that:

"The phrase "RMS Power" has been used in some circles not as an exact use of
the term RMS, but rather as an informal "standard" that actually means
"average power under specific text conditions". (I refer to an earlier post
telling of the single channel, steady state audio power amp measurement)
This use of the term "RMS" was originally initiated to call attention to the
specific test. Unfortunately, this use has caused some confusion in the use
of this terminology and is some of the motivation for the paper."

73, Steve, K9DCI