phase loss detection

rpoet

Member
Join Date
Jun 2008
Location
New York, NY
Posts
536
Hi All,
I have a portable power distro that serves power to up to eight Parker Gemini servo systems. These controllers are most happy running on three-phase 208v power, but are also able to be used on 240v single phase, with reduced capabilities. We will mostly be using the distro on three-phase power, but there may be times when we need to use single phase.

We can account for the reduced capabilities of the servos on single phase if we plan for it, but it would be a problem if we drop a leg (and it has happened here) and we didn't know it; the servos will run but will not be able to generate the torque necessary for some applications. The easy solution is a phase-monitor relay, but I've only found these available for three-phase applications.

Since there may be call for running from single- phase, I'm leaning towards three relays, each with coils wired between two phases to sense a dropped leg. The N.O. contacts would be wired to the PLC and tell it to drop the power contactors to the drives until the situation is fixed. I'll add a selector switch to the PLC to let it ignore the third relay if we're running on single-phase power.

Alternatively, I'd entertain a dedicated phase-loss / power quality relay that can be set for either single phase or three phase input with a selector switch. I just haven't found it if that critter exists. Any thoughts would be appreciated.


-rpoet
 
I don't think that detecting voltage on the lost leg is reliable. You'll most likely get induced voltage on the open leg as well.
What about current detection on all 3 legs? Or perhaps a couple of opposite connected phase direction relays could work? Hmm...o_Oprobably not because of the induction.

Kalle
 
Kalle,
I suppose I could use 120vac relays and essentially "ground the star point" of my three relays, though that would put current on the ground conductor as there's no neutral in the cabinet, and no hope of getting one. I knew it wasn't this easy...


-rpoet
 
We can account for the reduced capabilities of the servos on single phase if we plan for it, but it would be a problem if we drop a leg (and it has happened here) and we didn't know it; the servos will run but will not be able to generate the torque necessary for some applications. The easy solution is a phase-monitor relay, but I've only found these available for three-phase applications.

rpoet,

CR magnetics has some models that might work for you.

The CR4560 (4 to 20)and CR4550 (0 to 5V) has 3 analog outputs (one for each phase) so the voltage level as well as which phase is active can be identified.
http://www.crmagnetics.com/Products/Assets/ProductPDFs/CR4500_Series.pdf

The CRD4570 (D) is a rs485 output model that monitors 3 phase voltage, current, power.
http://www.crmagnetics.com/Products/Assets/ProductPDFs/CRD4500 Series.pdf
 
Milldrone,
The CR Magnetics transducers look great, but seem like overkill for this project. I just need to prove power exists, and not power quality. I think three simple relays will work for this application, though I'll keep the CR stuff in mind for the future.


-rpoet
 
I always use a Schneider power failure relay. I also make sure I use one with assymetry detection as well as motor loads can fool the standard ones. Typically RM4TA02.
 
Why not use a simple phase monitoring relay wired to a PLC input and your selector switch also wired to another PLC input?

If the phase monitor is in the faulted state, but the selector switch is set for single phase, then the PLC can make the required adjustments and keep the machine running. If the phase monitor relay opens, and the SS is set to 3 phase, then the PLC can shut the machine down.
 

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