Phase failure detection

[ardhuru]

Hi,

I am building a circuit that needs to monitor the 3 phases in the
mains, and detect if (a) any one phase fails, and (b) if the phases are connected improperly.

After some search I found http://www.picbasic.org/forum/archive/index.php/t-2134.html to be pretty close to what I need.

What would be the best way to go about it?

Would this logic work?

Detect positive edge on phase 1
Start Timer 0
Detect positive edge on phase 2
Read Timer 0 as variable 1
Detect positive edge on phase 3
Read Timer 0 as variable 2
Stop + Reset timer
If Variable 1 is equal to (almost) = (Variable 2)/2 = 6.67 ms (for 50 hz) then all is ok.

Start again.

Am I on the right track, or is there a better/easier way to do this?

Regards,

Anand

[BrianT]

Be warned that phase failure detection can be very difficult in my experience.

On a remote antarctic base we had a 3 phase generator that was working properly and driving some lightly loaded 3 phase motors before a phase dropped out. The lightly loaded motors kept running with no perceptable problems except a bit of a growl. If a new 3 phase motor tried to start we instantly knew there was a problem as it tripped its overload breakers. The motors that were still running appeared to act as generators and supplied the missing phase with a lower voltage and slightly time skewed voltage.

Have a close look at the commercial phase failure detectors before you jump in I suggest.

HTH
Brian

[ardhuru]

Hi Brian,

Thanks for sharing your experience.

You mentioned the missing phase did show a slight skew in the waveform; this is exactly what I'm hoping to detect with the technique I described. Of course, without actually seing the waveform or actually trying out my circuit I wouldnt know if it would work,

The reason I'd rather not use a stock unit is that I also need to incorporate some other (non phase related) features into it.

Regards,

Anand

[Archangel]

The lightly loaded motors kept running with no perceptable problems except a bit of a growl. The motors that were still running appeared to act as generators and supplied the missing phase with a lower voltage and slightly time skewed voltage.


Brian

That is the operational theroy behind a Phase converter, Basicly a 3 phase motor and some large capacitors to generate the third leg.
JS