Al,

On Wed, 10 Mar 2004 07:11:19 -0800, al smith wrote:

> I did some measurements, where I found peak current for 122VAC single phase
> to be 1.554A being pulled on the AC, and then did the same test using 212VAC
> (two legs of a 208 3 phase) and found the AC current draw to be 0.935A

This has always perplexed me: surely if you're connecting across 2 of 3 phases (something we don't do in the
UK) then the voltages are at 120 degrees phase angle to each other, so you will be getting a distorted
waveform, not a sine wave?

> Thus wattage is 189.6 for single phase, and 198W for the '3phase'.  Pretty
> close in value, although I might expect them to be pretty much the same,
> since the same load was applied.  So thats the first curious item.

Unless you're using "True RMS" meters, I think the distortion I mention would mean that the readings would be
skewed.

> Then, using the formula  A=W/(V*pf*1.73)  I get a pf of 0.58, much lower
> than I might expect. So is there a relationship of the pf related to
> voltage?  I would tend to think not, as its the comparison of apparent vs
> real power, or the efficency of the supply.  I wonder if running it using
> two of the 3 legs of a 3 phase power has anything to do with it?  I do have
> a variac, so maybe I will run the same test, using a single phase 212VAC and
> see if there are any differences.

I'd be interested to hear the results - and to see a 'scope trace of the waveform, so I can see if I was
right! :-)

Incidentally, is power factor correction (PFC) a Good Thing for the user (ie. does the meter go round less for
a given power used) or for the supplier (the meter goes round more)?

Cheers,

Howard Winter
St.Albans, England

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