# Phase loss monitor doesn't like generator



## Peewee0413 (Oct 18, 2012)

gottspeed said:


> Weird call today.
> 
> 
> I was called to a water pumphouse the other day to connect a large trailer mounted 600v 400a backup generator because the genset inside the plant dropped a valve and has to go for service.
> ...


Is there a rotation monitor somewhere? Beyond my expertise, but something is missing..

Sent from my VS995 using Tapatalk


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## varmit (Apr 19, 2009)

The electronic phase monitors are more "picky" about true ABC sequence. ABC, BCA, or CAB connections are all the same to most electrical devices. (will result in same rotation) The phase monitor requires line 1 = A, line 2 = B and line 3 = C For the output to turn on. Unless you have a scope to use, you will need to do a trial and error connection of the phase monitor. VFDs do not care about the line side phase arraignment.


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## varmit (Apr 19, 2009)

After rereading you OP: It sounds like you will need to swap the generator phasing to match the utility power phasing -EXACTLY. Again ABC = BCA = CAB for most things, but not a phase monitor, they are ABC only.


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## JRaef (Mar 23, 2009)

varmit said:


> After rereading you OP: It sounds like you will need to swap the generator phasing to match the utility power phasing -EXACTLY. Again ABC = BCA = CAB for most things, but not a phase monitor, they are ABC only.


I agree. Having built one in the past, the issue comes down to how you measure it electronically. We can't measure a zero-cross point, because you can't measure zero effectively. So what you do is measure a point where the phases cross each other. This is called a Phase-Locked-Loop (PLL) detector circuit. But to save money, most of the low cost PMs will only use two PLLs, so to determine rotation sequence, you must pick a starting point, look for it, then time the others from that. So with that design, it MUST be A, followed by B, followed by C. Even though B-C-A and C-A-B are the same RELATIVE sequence, the problem is the time between the detection of the two phases and where it expects the time gap for the third one is off. 

There are other phase detection designs that don't have that issue, but that's the cheapest way to do it so it's the most common.


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## Jay Freeman (Aug 2, 2017)

I never had this problem but in a situation like this, how does one determine that BOY= ABC?

Would any PM which uses 3 instead of just 2 PLL's do it?


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## paulengr (Oct 8, 2017)

I'm not buying any of the explanations.

We are basically ARBITRARILY designating one phase as "A" or "L1" or "U" or "T1" or "Brown" or "Red" or whatever you want to call it. Phase rotation only makes sense when we talk about the timing of the peaks or zero crossings relative to each other. And realistically the major reason for using a phasor display is strictly for figuring out whether or not all the CT's and PT's are wired correctly.

https://www.schneider-electric.cn/l...nstructions_on_how_to_use_the_phasor_tool.pdf

"A", "B", and "C" are ARBITRARY. As long as timing wise B lags A, C lags B, and A lags C, everything is identical. ABC = BCA = CAB. If it is the opposite where the phases LEAD each other than we have the opposite rotation and ACB = BAC = CBA. If I turn the display off, move ALL three leads by say one position so that for instance we have ABC to BCA and then use a power meter to check again, it looks IDENTICAL.

There is really only three ways that I can see a phase monitor not working correctly. The first one is that the phase rotation is reversed and nobody noticed until now. The second one is that the phase monitor has some screwy way that it is sensing one particular phase different from the others, such as if there are separate power inputs labeled say "L1" and "N" and that "A" input MUST be the same as "L1", or that the delta has a ground fault somewhere so that the control power input is "zero" so that instead of say 277/277/277 we have 480/480/0.


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## JRaef (Mar 23, 2009)

Jay Freeman said:


> I never had this problem but in a situation like this, how does one determine that BOY= ABC?
> 
> Would any PM which uses 3 instead of just 2 PLL's do it?


Yes, a design using 3 PLLs would not know or care if it was ABC, CAB or BCA. It's just that the majority of cheaper units don't use 3. To make it work, you simply roll the conductors into it until it's satisfied. That was probably in the instructions, but who reads those...


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## Jay Freeman (Aug 2, 2017)

Ok... I figured as much.


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## Remy_p (Sep 17, 2021)

gottspeed said:


> Weird call today.
> 
> 
> I was called to a water pumphouse the other day to connect a large trailer mounted 600v 400a backup generator because the genset inside the plant dropped a valve and has to go for service.
> ...



Hi, first post here for me...

same thing for me this week... today i instaled 2 new carlo gavazzi Phase Lost Monitor in 2 control panels, 









every thing work fine on utility power....

but it's an other story with the generator power.

in panel #1, PLM work fine, 
in panel #2, PLM don't work, the output make on/off evry 2 secondes , i switch with the new one PLM in panel #1 and with this one, my output stay on. so i think 1 of my 2 new PLM are defect....
( Gavazzi says extremly rare defect on their products, and want it back to anlyse.. so they will not have my generator circuit to test it.. so they will say their product is not defect. )

but the old generator was delta , and the new generator is wye.... so it's maybe the source of my problem...

what's the solution???:
change carlo gavazzi for a siemens where i can adjuste up voltage / down voltage, and set a time delay reaction...? ( carlo have no adjustment)
harmonic problem ? expensif to fix..

thank you for your help.


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## paulengr (Oct 8, 2017)

Seen a third way a phase monitor can fail now. It has happened more than once. For some unknown reason multiple Omron phase monitors blowing their guts out. Very strange. I can only think of overloaded contacts or over voltage on either side.


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