Hello PIC.ers,

Alan said:

>> RMS power is not a meaningful concept.  RMS voltage and current do
>> make sense, but RMS power, while mathematically possible to
>> compute, has little relevance to real systems.  RMS voltage and
>> current are useful precisely because they can determine the
>> *average* power into a resistor.
>
>RMS power *IS* a meaningful concept.  It is used to determine the
>D.C. power *EQUIVALENT* to the changing waveform power (AC+DC)
>dissipated in a load. (ex. the power dissipated by a resistor with

I have to agree with Olin on this one. RMS Power is not a useful
measurement, and as such does not make sense. It is NOT used to determine
the DC power equivalent. That is determined by taking the product of RMS
voltage and RMS current, while also taking into account any phase angle
shift between the two.

The product of RMS voltage and RMS current is NOT RMS Power. The term RMS
power may be bandied around to signify a power value arrived at by this
product, but that is not really the correct term for what is being measured.


>>>>>>>>>>>>>
Then Olin said:


> I'm beginning to get the feeling that we have a semantic seperation
> on this one.  I would view average power as the average voltage
> squared over the resistance.  I.e.

This statement is vague.  If you mean

  P = Ave(V**2) / R

then you are right.  However, P is definitely not Ave(V)**2 / R, which is


<etc, etc. snips>

>>>>>>>>>>

then Spehro said:


>Replace "sum of the powers" with "integral of the power" and I agree.

Specifically, the definite integral over an integer multiple of periods.

A notational nit.. *instantaneous* voltages, currents etc. are normally
represented by lower case letters, whereas average, peak or RMS or DC
values are conventionally represented by upper case letters.

And, yes, I agree with Olin that "RMS power" is meaningless, but it is,
unfortunately, a fairly common way to refer to average or true power IME.


>>>>>>>>>>>>
and now John says:

I'll add my SARands 0.02 cents in here.

When I tried to take issue over this with the `powers that be` at my
former employer over 20 years ago I was given the justification that
`RMS Power' was valid for certain applications.

The job was to size motors and plant for *big* mine winding drives.
These things have ratings up around the 4 - 5 MW area, thus very
long thermal time constants. Their electrical (and mechanical) loading
is quite short-cycle by comparison, hoisting rock up from depth in a
cycle that might take 10-15 minutes.

`RMS Power' requirement was estimated at the mine design stage for a
period which approximated to the thermal time constant of the machine.
The mechanical power requirement was predicted & efficiency taken
into account, giving rise to a strip-chart of electrical power vs. time
over a period of hours.
This was RMS'd to arrive at a motor rating figure.

I never bought the argument that it was more useful than an average
power requirement, but got smitten down. I've a residual feeling that it
gave rise to an over-estimation in the sizing of plant & that this
suited the consulting engineers of the day.
More conservative design, less risky etc.

Never saw a winding motor get a darn bit warm at all...


        best regards,   John

e-mail from the desk of John Sanderson, JS Controls.
Snailmail:          PO Box 1887, Boksburg 1460, Rep. of South Africa.
Tel/fax:            Johannesburg  893 4154
Cellphone no:   082 741 6275
email:                jsand@pixie.co.za
Manufacturer & purveyor of laboratory force testing apparatus, and related
products and services.

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