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Re: AVERAGE Power/Phase angle confusion



Original poster: "by way of Terry Fritz <twftesla-at-uswest-dot-net>" <free0076-at-flinders.edu.au>



On Mon, 8 Jan 2001, Tesla list wrote:

> Original poster: "harvey norris by way of Terry Fritz
<twftesla-at-uswest-dot-net>" <harvich-at-yahoo-dot-com>
> 
> 
> --- Tesla list <tesla-at-pupman-dot-com> wrote:
> > Original poster: "Gary Johnson by way of Terry Fritz
> > <twftesla-at-uswest-dot-net>" <gjohnson-at-ksu.edu>
> > 
> > Sorry to pick nits, but a recent reference to RMS
> > power set me off, rather
> > like fingernails on a slate blackboard.  Our meters
> > measure measure average
> > voltage and current on the dc ranges, rms voltage
> > and current on the ac
> > ranges, but only AVERAGE power (never RMS power).
> Are not these the same thing? If we integrate the
> Power equation curve for a half cycle is not the area
> under this curve the sime area as if the rms voltage
> and rms amperages were multiplied?

Not if they are out of phase. And the area needs to be taken over a full
period and divided by the period to take into account harmonic distortion 
and find the average. The average power is called, funnily enough, average
power. Definitely NOT RMS power, that would be something entirely
different.

> > Average power is the
> > product of rms voltage, rms current, and power
> > factor. 

> I definitely disagree here. If you are making AC
> MEASUREMENTS on any circuit, those measurements
> themselves are already the result of the phase angle
> conditions. If I am measuring a fairly resonant

Definitely not, two-wire devices like multimeters can't measure the phase
difference between two signals now can they?

> circuit the amount of rms amperage times the impressed
> rms voltages IS the power input. In that case the

Only the apparent power input, not real power.

> power factor is near 1, but that does not enter into
> the equation. The power input is merely the measured
> rms amperage and voltage values in multiplication,
> assuming a sinusoidal AC input. Near resonance the
> amount of power input comes close to the value of
> conduction enabled by ohms Law. BY taking the capacity
> out of the circuit, the recorded amperage will go

Depends on the circuit, if it was a parallel LC circuit at resonance the
current will go UP when the cap is removed.

> down, so now the meter reads the amount of reactive
> current present in the inductance. It is THAT rms
> amperage reading of that reactive current that has
> already been reduced by its now large phase angle,
> making a low power factor. The Power factor itself is
> not further multiplied by the previous multiplications
> of (measured,not predicted by OHMS LAW) voltages and
> amperages to derive the power input. By having the
> extra multiplication you are essentially "factoring in
> the power factor twice instead of once." I cannot

Nope, sorry, it was never present in the first place.

> believe how many times this power factor nonscense is
> misinterpreted. Once again,(it is my belief unless

You yourself fell into the trap =)

> someone can explain otherwise); THE POWER FACTOR IS
> THE CAUSE OF OUR METER MEASUREMENTS, NOT A FACTOR THAT
> NEEDS TO BE MULTIPLYIED AGAIN TO ASCERTAIN THE TRUE
> POWER INPUT.
> 
> Sincerely HDN
> 
> 

Darren Freeman