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'[EE]: Phase failiure Detection in 3 Phase'
2002\01\31@093009 by Jeethu Rao

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I have designed a 16f628 based timer for 3 phase loads (using a contactor).
Now, here in India, its quite a common occurance that 1 or 2 phases in the
3 phase supply fail. And it could quite easily kill the 3 Phase Motors.
I want to design a system for detecting phase failiure and to signal the PIC
about this. I've got only one Input pin left on the PIC (MCLR!). Any
suggestions
or Ideas ?

I thought of using a 3 input AND gate fed by each of the phase via a 1 meg
resistor and
a 1N4007 diode. and connecting the output of the AND gate to the PIC. Is
this ok, or do
I need to use 3 opto couplers for isolation?

TIA,

Jeethu Rao

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2002\01\31@094638 by Lawrence Lile

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It sounds like you are on the right track.  I would suggest TWO resistors in
series, so in the unlikely event of a resistor failure your circuit is still
isolated.  Resistor trick is used all the time.  Will killing MCLR also kill
your circuit's output to the motor?  If not, you may want to rethink it,
because this would be a good fail-safe condition in any case.

Filter the output and add a bleeding resistor, so that a realtively brief
interruption of one phase won't kill your device.  There are small
interruptions of power in the grid more often than you think, because of
circuit-breaker-like devices called reclosers.  I would suspect that an
outage of 500 milliseconds would not bother your motors.

--Lawrence

{Original Message removed}

2002\01\31@095144 by Spehro Pefhany

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At 07:57 PM 1/31/02 +0530, you wrote:
>I have designed a 16f628 based timer for 3 phase loads (using a contactor).
>Now, here in India, its quite a common occurance that 1 or 2 phases in the
>3 phase supply fail. And it could quite easily kill the 3 Phase Motors.
>I want to design a system for detecting phase failiure and to signal the PIC
>about this. I've got only one Input pin left on the PIC (MCLR!). Any
>suggestions
>or Ideas ?
>
>I thought of using a 3 input AND gate fed by each of the phase via a 1 meg
>resistor and
>a 1N4007 diode. and connecting the output of the AND gate to the PIC. Is
>this ok

This is actually non-trivial because rotating machinery will generate a
missing phase while it operates. There will be an imbalance though, and
the motor may overheat. It won't start properly, of course, and will
burn up quickly if stalled.

Maybe you could explain the relationship between your circuit ground and
the three phases, and where the power for your circuit is coming from.

Best regards,

Spehro Pefhany --"it's the network..."            "The Journey is the reward"
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2002\01\31@132433 by Harold M Hallikainen

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       If this is a 3 phase Y (or star), you can just have three resistors with
one end tied together. The other ends are tied to each line. The voltage
between the junction of the resistors and neutral will be zero if the
line is perfectly balanced. You can set a limit as to how far off balance
you'll accept.
       If it's a 3 phase delta with either the center tap of one side or one
corner tied to neutral, the same circuit will give some voltage when all
is well. It will change on loss of a phase.

Harold


On Thu, 31 Jan 2002 19:57:37 +0530 Jeethu Rao <.....jeethurKILLspamspam.....SANCHARNET.IN>
writes:
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2002\01\31@232616 by Patrick J

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Use optocouplers.
And you could just have the 3 optocouplers conneted in series on the PIC-
side and thereby get the AND gate function as a little bonus !

Btw, just out of curiosity... do you have 3 phases and a 0-line and also
protective ground as we do here in Sweden ? (total 5 wires)

/PJ

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'[EE]: Phase failiure Detection in 3 Phase'
2002\02\01@023803 by Vasile Surducan
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If the distortion regime on indian supply lines are the same like in
Romania, any semiconductor devices ( ie optocouplers ) will crash after a
while. I suggest you a much simple detection mechanism: three relays
connected from phase to neutral. Al closed contacts in series to your MCLR
pin used as IO input. Resistor to vcc, series end to gnd.
If one of the phase fails the io pin see high level.
Take care about the incomplete failure of a phase ( ie: 30% voltage or
less can be still active, because of inductive 3-phase loads, also the
phase can be differet from 120 degree in malfunction regim )

Vasile



On Thu, 31 Jan 2002, Jeethu Rao wrote:

{Quote hidden}

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2002\02\01@085922 by Chris Loiacono

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Not to boast, as you will see, but here in the good ol'USA, SE corner, I
thought we had about the WORST electrical supply in the world. This region's
portion of man's largest machine (as the US electrical grid has been labeled
by the MIT Tech. journal) is a hodge-podge of generators and wiring methods
that changes every 50 to 100 miles or so. Some of the overhead wiring goes
through quite inhospitable terrain and is outdated, most of it done before
there was much standardization of methods or materials. Much is buried in
wet swamp land and some is new, state of the art. I get calls from customers
often telling of weird voltages, Neutral connections coming and going, and
old oil-filled transformers being swapped out on poles nearby as they fail-
for new ones with different configurations (as in: the N connection is moved
from one point to another, or a 'wye' becomes a 'delta') from one day to the
next without notice.
One day some weeks ago, I lost a PC (was on a transformer isolated UPS) a
nuke box and a water softener in one episode that occurred at the stroke of
midnight when a series of surges went through the lines (BTW, the PC's on
APC back up pro's did fare well). I have installed hundreds upon hundreds of
phase angle power controllers with 7500V optical isolation in US, Japan and
Europe (guess I'm spoiled), and have never had one fail due to opto
isolators crashing. I suppose part selection has much to do with this....But
I have to admit, I don't think any of mine are in India or Romania. That
situation sounds pretty scary.....

How could the series resistor scheme protect a circuit from fast spikes
better than optos? Are we saying that they do, or that since they pass more
transients, they don't fail as often? I suppose that would be OK if a motor
winding was on the other side of the spike, but a micro I/O pin?
Hmmm....unless those tiny protection diodes really do work well....
It seems I learn something every day....
Chris

{Original Message removed}

2002\02\01@094015 by p.c

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a lo-cost cheap way to detect phase failure is to connect a resistor
from each phase to a common point (star) then measure the voltage
at that common point, use a diode to rectify and add simple RC
filters /attenuator to set a simile trigger point
Peter Cousens
62,springfield ave
London N10 3SY
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2002\02\04@030606 by Jeethu Rao

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Vasile,
       I was outta town for a few days.
       Your thinking is just like my dad's. He too had suggested the same
       Idea. But there is a problem. I don't think I can get Relays with
       High Voltage coils locally. Step down to 12 volts using 3 small
       transformers or Use Optos? I'm a bit inclied towards Optos.
Thanks,

Jeethu Rao

{Original Message removed}

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