480v 3-phase pump control

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jgurley

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I'm assisting a local non-profit with their irrigation system, and have found a mystery (to me). I'm a Electrical Engineer, not an Electrician.

The 3-phase 480v pump control panel isn't working, and the problem is that a low voltage relay is burned out (open coil). Why this relay is there at all is the mystery.

As with most such panels, there's a mechanical breaker with mechanical interlock, then a contactor, followed by an overcurrent device. Two inputs (of the three) terminals on the contactor go into a 480v:18v transformer. This low voltage is used to drive the contactor coil, once the correct switches are thrown and buttons are pushed. The mystery relay coil is also driven by the transformer output, and when closed allows all the low voltage wiring to work. It's a simple SPST relay, and if I just manually close the relay, everything fires up as expected. It's suffered a lot of heat damage, so an exact replacement is tricky, since the numbers are mostly gone. If the mechanical breaker is on, the relay is on - no other conditions can stop the relay from energizing. There's no sign of the wiring having been modified, since everything is nicely harnessed.

I can think of no reason the relay would be there except maybe it would prevent a brown-out on two of the phases (the ones driving the transformer and coil) from energizing the pump. If that's true, then the specs on the replacement coil are probably more critical than "18v SPST relay".

Am I missing something? Any reason to retain the silly thing?
 
Without seeing the controller I can only make some assumptions.

You say that the relay seems to be there to provide power. While I can foresee a few situations where you may have an interposing relay I can't really say for sure here.

Can you say more about the relay? does it have a base and the relay is removable? Does it have pins or blades? is it integral to the controller in that removal would require alteration of the controller? What sort of pumping system is this? Float operated, remotely operated or from push buttons?

Without more information all I can really ask are questions...
 
jgurley said:
I'm assisting a local non-profit with their irrigation system, and have found a mystery (to me). I'm a Electrical Engineer, not an Electrician.

The 3-phase 480v pump control panel isn't working, and the problem is that a low voltage relay is burned out (open coil). Why this relay is there at all is the mystery.

As with most such panels, there's a mechanical breaker with mechanical interlock, then a contactor, followed by an overcurrent device. Two inputs (of the three) terminals on the contactor go into a 480v:18v transformer. This low voltage is used to drive the contactor coil, once the correct switches are thrown and buttons are pushed. The mystery relay coil is also driven by the transformer output, and when closed allows all the low voltage wiring to work. It's a simple SPST relay, and if I just manually close the relay, everything fires up as expected. It's suffered a lot of heat damage, so an exact replacement is tricky, since the numbers are mostly gone. If the mechanical breaker is on, the relay is on - no other conditions can stop the relay from energizing. There's no sign of the wiring having been modified, since everything is nicely harnessed.

I can think of no reason the relay would be there except maybe it would prevent a brown-out on two of the phases (the ones driving the transformer and coil) from energizing the pump. If that's true, then the specs on the replacement coil are probably more critical than "18v SPST relay".

Am I missing something? Any reason to retain the silly thing?
Click to expand...

It sounds like a Phase Loss/ Phase Monitor relay, I am assuming there is a 480 v 3 phase pump? Do the 3 phases of the 480 voltage terminate on the relay base?

Post some pictures please as well as a wiring diagram if you have it.

If its not a PM and the control voltage is broken through a NO on the relay and the relay coil is energized right of the secondary side of the control transformer the only other answer would be that they are using the relay as an interposing relay to switch a different voltage.

If it came from the factory like that, I'd assume it has a purpose. Can you call the manufacturer?
 
That was my 1st thought (phase loss)n but the OP states the "relay" is supplied by the transformer. A diagram would certainly help but another "guess" would be that the relay is actually a timer to provide delay in restarting.
 
augie47 said:
That was my 1st thought (phase loss)n but the OP states the "relay" is supplied by the transformer.
Click to expand...
I suppose that could be the supply to drive the coil rather than supply sensing for phase loss. Or, as you say, it could be a timer.
More information needed.......
 
This panel is probably 20-30 years old and judging by the drilled holes etc. looks custom. It drives a 75hp centrifugal pump with 10" plumbing. It will take a few days to provide any pictures.

I like the idea of a time delay function, but the relay is a very straightforward unit - a 1" diameter coil with lugs on the wires and a contact with screw terminals. Looks like a generic relay. All the time delay relays I've touched have enclosed coils or are in cases of some sort.

I've attached a crude schematic and picture of a close relative to the relay.
 

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If there were two supplied by different phases, I'd say it's a phase loss detector. But it can't be. If it were a more complex relay module, then I'd say it's a restart delay timer. But it seems it isn't. That leaves brownout detection (under-voltage lockout). Normally that would be done with a special relay too, but it's possible the manufacturer just found a common relay that happens to have an appropriate dropout voltage. Or the original timer/brown-out relay failed some time ago and was replaced with this plain relay instead...
 
18V is really odd, that makes me think maybe there is something you are not aware of in this system. An odd low voltage like that, and the fact that it's a pump panel, makes me think this might be some sort of a low votlage "Probe Relay" like a Warrick / Gems or BW / Ametek relay. None of them look like what you posted, but the way your diagram looks, I could see that being used as a Low Level Cutoff relay where a probe was hung in the well at a point a little above the pump intake / foot valve. If the water level dropped below that point, the relay would drop out and not allow the pump to be started, preventing it from sucking air and losing prime. If the panel was removed but the probe left behind, you may not have seen where and how the probe was connected.
 
Jraef said:
18V is really odd, that makes me think maybe there is something you are not aware of in this system. An odd low voltage like that, and the fact that it's a pump panel, makes me think this might be some sort of a low votlage "Probe Relay" like a Warrick / Gems or BW / Ametek relay. None of them look like what you posted, but the way your diagram looks, I could see that being used as a Low Level Cutoff relay where a probe was hung in the well at a point a little above the pump intake / foot valve. If the water level dropped below that point, the relay would drop out and not allow the pump to be started, preventing it from sucking air and losing prime. If the panel was removed but the probe left behind, you may not have seen where and how the probe was connected.
Click to expand...

The water is being drawn from a pond, which acts as a buffer for a canal from a pushup dam on a creek, and there's no "wiring" or pressure sensors at the pump or intake - just a 20ft 10" aluminum pipe to a foot valve. I think the 1200gpm that is moving through the system requires a wrangler "in the area" to act as a low water detector or they just guess how long they can run, based on the creek level. If I was building it, I'd have some sort of pump protector against running dry - maybe it's difficult with a 75hp pump? Or maybe the pump isn't bothered by running dry.

The 18v is simply what I measured with a digital meter, so it might have been nominally 12v. The part number on the contactor is obscured, so the coil voltage is unknown besides that it happily closes with 18v. The extra relay is literally a glove fit onto the transformer (single mounting bolt to transformer, very short coil leads) so I think the circuitry is original, but this is a grazing operation that has gone through a lot of owners. The combo of transformer and relay is mounted into the box with self-tapping screws through the housing, which seems to indicate it might be a field install?
 
I agree with Jaref here, 18v seems odd. A 480v 75 HP motor would need a size 4 starter (if NEMA). Usual control voltage would be 120v. I believe something else is going on here.
 
You're right, there is something fishy here. I got the transformer and relay back from the site manager and I recovered the numbers off the relay, and it has a 120v coil, so my 18vdc measurement must have been measuring stray current or something. My diagnosis of a bad relay is still okay, but I'm sure confused, and look forward to a visit to the panel when I get the new relay. I would think the relay might be buffering the contactor coil from the switches, so maybe my tracing of the wiring was as bad as my voltage measuring. Sorry for the trip down a rabbit hole for all those that provided feedback.
 
Might just be an interposing relay then. The relay handles the power to the contractor coil so that the push buttons need only handle the relay coil. That's done often in bigger starters because the coil inrush current can be a lot for small cheap push buttons to handle.
 
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