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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