Chris, you have right, i've made a mistake, mostly because i was thinking
at abnormal mains regime. Maibe next ideea will be better understood:
if a simple frequency measurement is required, what you say here is
perfect; but if these measurements must be performed to catch a harmonic
oscillation on a not simetrical mains than amplitude and frequency
measurements must be done on every phase and on every line. If a decimal
point resolution is need for frequency, usual is used a pll tehnique, else
it takes too long time for one measurement and deformant oscillations
could be lost.
Of course the better way for this sort of measurements are fourie
analyses but also are much expensive.

best of,
Vasile


On Tue, 12 Feb 2002, Chris Loiacono wrote:

> Vasile:
>
> I have a PIC based product that samples the 3-phase line frequency on POR,
> then assumes this will not change. Checking each time the unit is powered on
> or off is sufficient for this device. The primary purpose is to determine
> what frequency the line supply is at, ie; 50 or 60 Hz. so as to be able to
> calculate phase angles correctly. For line dropout, I monitor the current on
> two of the three lines. It's not a motor controller, so I'm not concerned
> with rotation. I also needed to make this work without a Neutral connection.
>
> I am not sure I understand the idea of figuring the freq on each phase.
> Isn't it safe to assume that the three come from the same generator,
> rotating at the same speed?
>
> Chris
> The problems appears when the reference phase have a malfunction, or the
> system becomes non-symmetricaly because of the load or neutral slide, so
> the whole theory about frequency measurement must be changed taking care
> of phase modification ( because your measuring neutral is fixed ), thus is
> better to check the frequency on every phase and compute an average.
> Is not an easy job.
>
> best regards, Vasile
>
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