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RE: Average, RMS and Power Factor made easy!



Original poster: "David Dean by way of Terry Fritz <twftesla-at-uswest-dot-net>" <deano-at-corridor-dot-net>

Hi,

Back in the early 1970s there was a law passed, or the threat of a law being
passed, or something that happened to cause the "industry" to adopt a common
standard for the rating of power in audio equipment. Prior to that you could
find adds by one manufacturer stating output power in X # watts "music
power", another rated theirs in peak power, and the good ones in RMS. The
standard that was adopted was "RMS". What that means in this context is that
if mfg. A makes a stereo rated 30 watts RMS per channel, and mfg. B makes a
stereo with the same rating, that the consumer can make an informed decision
as to which one to buy based on price, features, and perhaps other
specifications, and not be fooled into thinking a unit rated "100 Watts IPP"
means the same thing as one rated "100W music power", or one rated "100
watts RMS". The rating is usually stated as X # watts RMS into X # ohms at
some % THD.

If you take a non-inductive resistor (dummy load) of X ohms (and of course
of a sufficient power rating) and hook it in place of the speaker, feed a
pure sine wave into the amplifier while observing the voltage across the
resistor with an oscilloscope, and slowly increase the amplitude of the
signal, you will observe a point at which the peaks on the waveform will
start to "clip". If the amplifier goes into clipping, that is when it starts
sounding bad. (harmonic distortion) If you take the peak voltage as seen on
the scope without clipping and multiply by sqrt 2, you get the maximum RMS
voltage into the load. Apply ohms law and the power formula and you have the
power rating of the amplifier. That is basically how the power rating is
determined. One could calculate based on the peak value of the waveform,
that would give the archaic term "peak power", or calculate the power
developed in the load when the entire contents of the filter caps in the
power supply would be dumped into the load in an instant "instantaneous peak
power" which is how some manufacturers called a 5 watt amp (if rated by the
RMS method) 100 watts. The RMS in the rating means that the calculation is
done using the RMS value of the waveform as opposed to the peak value or
some other method. That method was chosen because the result gives a "real"
power value which actually means something under the special test
conditions. That is the term referred to as average power as discussed in
this thread. This condition is only met under the conditions of the test and
has no bearing on the actual power developed in the load under normal
listening conditions. (the only near exception to that statement would be in
the case of a siren or alarm of some kind, and the are rarely pure sine
waves) If one were to put the dummy load into a container with a known mass
of water and then measure the temperature rise over time like in a
calorimeter, one could then calculate the average power, and with a music or
spoken word input at a normal listening level,  would find the value to be
much lower than the power rating. Sort of like the horsepower rating in a
car engine. Take two cars of the exact same make and model, one with a 90HP
4 cyd. and the other with the optional 140HP H.O. V-6, and of course the V-6
will feel a bit more spunky to the driver. Rarely, if ever, will either
engine deliver its rated horsepower under actual driving conditions, and in
the case it does, it will only be a peak, of short duration.

In the case of Tesla coils, we want to know the real power going in, and the
real power coming out, so as to be able to determine the system efficiency,
minimize losses by determining where they are actually occurring, and other
stuff like that. To tell the truth, I can't even remember what this tread
was about or how it got started in the first place. I just wanted to jump in
here and say that the watts RMS power rating as found in audio equipment is
a comparison standard, not electrical engineering terminology.

later
deano




> -----Original Message-----
> From: Tesla list [mailto:tesla-at-pupman-dot-com]
> Sent: Wednesday, January 10, 2001 9:04 AM
> To: tesla-at-pupman-dot-com
> Subject: Re: Average, RMS and Power Factor made easy!
>
>
> Original poster: "Terry Fritz" <twftesla-at-uswest-dot-net>
>
> Hi Al,
>
> 	I have heard of average power and peak power for various
> things, but the
> root mean square of "power" has no real use.
>
> Cheers,
>
> 	Terry
>
> At 04:31 AM 1/11/2001 -0500, you wrote:
> >Hi members.  But does not R.M.S. power have significance when used as a
> >means of rating various electrical components such as loudspeakers and
> >power amplifiers?  And would not these same ideas apply to a Tesla  coil?
> > After all, a loudspeaker is a coil within a magnetic field that becomes
> >excited when a voltage is applied to it, just like a Tesla coil.  And we
> >all know how terrible our sound equipment begins to sound once we have
> >exceeded the R.M.S. value of the rated components.      AL.
> >
> >On Mon, 08 Jan 2001 22:53:14 -0700 "Tesla list" <tesla-at-pupman-dot-com>
> >writes:
Snip
>
>