Re: Question about calculating power factor...

On Wed, 9 Jul 2008 10:24:13 +1000, "Phil Allison"
<philallison@xxxxxxxxxx> wrote:


"Default User"

Let's say I start with 120VAC and 2A, both RMS. If I generate samples for
one cycle @ 60hz, I will have voltage samples raning from -169.7V to
169.7V, and current samples from -2.83A to 2.83A. Let's say I then make
the current lag behind the voltage by 45 degrees, and I multiply the V
sample by the A sample to get a VA sample, I get these three graphs from
Excel:

http://home.earthlink.net/~alank2/pf.gif

The top one is voltage, the middle one is amps, and the bottom one is V*A
(watts).

If I average all the samples in the V*A chart, I believe I will get True
Power (watts) from this measurement.

My question is, without the V or A samples, just the V*A samples, can you
calculate the Apparent and Reactive Power?

It seems to me that looking at the graph (bottom one) that there should be
a way, but I can't figure out how to do it.

I was able to calculate the RMS value of the V samples, and the RMS value
of the A samples, and then multiple that together and call it Apparent
Power.

Can this be done by looking at the V*A samples alone? If so, how? If
not, why not?


** The phase angle is given by the fraction (F) of each period when the
"V*A" samples have negative sign.

phi = F x 360

Apparent power is then = Watts / cos( phi)

Assuming all sine waves.


....... Phil


If the voltage and current are 180 degrees apart, the product is
always negative, so F = 1, so F*360 = 360. That doesn't look right.

If they're 90 apart, F=0.5, and F*360 = 180.

Is the correct expression phi = F * 180?

John




.

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