Submersible well motor nuisance trip 1

[gtwy]

Hello-

I enjoy reading this forum but dont work w/ motors regularly so I haven't posted here yet-

Last week we had a customer who's 75 hp 460 volt submersible well motor went bad (old age). It has been started for years from a soft start w/ internal bypass contactor. Soft start has an initial torque setting of 45% and a ramp time of < 3 sec. That's what Franklin seemed to recommend, too.
The well guys were out this week and replaced the motor. Additionally the motor manufacturer (Franklin) will add warranty if their overload relay (Submonitor, functionally for the price it looks pretty nice)is added to the controls.
Franklin does not necessarily recommend this combo due to nuisance tripping but customer wanted to see if it could be made to work.
The motor does start and run but trips now and again on motor overheat, which we were told it might. Just wondering how the Submonitor decides the motor is overheating (the motor does not have temp sensor wired in at the controls).
I am not positive but it may be tripping during a start period vs. up and running.

 

[jraef]

Franklin makes their own motors, they are not built to typical NEMA specs so they are much more susceptible to damage than we might otherwise expect, but they are also designed to be under water continuously as well. That's why they offer an extended warranty if you use THEIR motor protection system.

Without the Subtrol (the thermal sensor in the motor), the Submonitor looks at the motor kW to decide everything, which makes it better than most current-only based OL relays. The down side is, during the first 3 seconds it is looking at current, voltage (level) and imbalance only. So if your soft starter is not at 95% of full MOTOR RATED* voltage within that 3 second window, it ASSUMES you are in a situation where the motor windings will be damaged. That's kind of why they don't recommend using soft starters with the Submonitor. Not all soft starters are created equal so they hedge their bets by just saying no.

Try backing off the ramp time to 2.5 seconds (if you have that granularity) and / or increase the initial torque by a few percent until the nuisance tripping goes away.

* Another fly in the ointment is that if the motor is rated for 460V and the line is normally at 480V but sometimes drops to 460V, the soft starter voltage is a percent of LINE voltage, whereas the Submonitor is a percent of MOTOR design voltage. So a low line voltage condition can make it APPEAR that the soft starter output voltage, however accurate and repeatable it is from ITS perspective, looks like it is too low. That's how you end up with it working sometimes and not other times.

"Dear future generations: Please accept our apologies. We were rolling drunk on petroleum."
— Kilgore Trout (via Kurt Vonnegut)
For the best use of Eng-Tips, please click here -> faq731-376

 

[gtwy]

Thank you for the reply. This all transpired Thurs. into yesterday. I figured that we might be tweaking, tweaking, tweaking to get it to "stay lit". As fate would have it, their is a neighborhood neaby so the power co. is sensitive to starting current at this site.

 

[Marke]

Great response Jeff

Mark Empson
Advanced Motor Control Ltd

 

[gtwy]

Yes, good info about Franklin. The very little I've dealt w/ them they seem highly concerned over the type of motor protection being used esp. if it's not theirs. Balanced w/ the cost to replace a deep well motor, I can see why.

 

[gtwy]

I did forget to mention that the soft start does shutdown if the motor is not up to speed at time out. I guess that the Submonitor has a less forgiving shutdown calculation than the soft start.

 

[thewellguy]

Another note on things overlooked on submersible motors. All of them that I am aware of require class 10 overload protect. Many of the motor starters that are not maded/supplied be the oem miss this detail.

 

[LionelHutz]

I'm fairly certain there are submersible motors which require much less than a class 10 overload for proper protection.

 

[gtwy]

From what I can tell about the Submonitor, once it has determined that there a shutdown condition, it disconnects the motor in three seconds.

 

[jraef]

More accurately, if everything isn't perfect for the motor circuit WITHIN the first 3 seconds, it shuts down.

Lionel,
Some submersibles are Class 10, usually lift station pumps and the like. But you are right, Franklin submersibles require Class 5 if you use an external OL relay, but for all intents the Submonitor is the equivalent of Class 3 (see above) and if you want the full extended warranty you have to use it.

"Dear future generations: Please accept our apologies. We were rolling drunk on petroleum."
— Kilgore Trout (via Kurt Vonnegut)
For the best use of Eng-Tips, please click here -> faq731-376

 

[gtwy]

Actually as the smoke clears what may have taken the motor out was a bad pressure switch at the tower controls that had the pump cycling on and off. The Submonitor has a false start feature that may have prevented this.

 

[racookpe1978]

So, the old motor might have been good?

Or was it burned out by the cycling, but the new one should last correctly?

 

[gtwy]

The motor was shot. It was at least 8 years old but the cycling probably did it in. I haven't heard yet to what extent it was cycling but evidently the pump kick-in pr sw setpoint had shifted up near (or past) the pump cut off so my guess is that it was probably cycling to the whatever deadband the cut off pr sw has plus the time delay relays installed in the controls- at least one at the tower and one at the pump panel.

 

[jonr12]

The Submonitor does not work fast enough to catch a bouncing pressure switch. The pump will usually bounce on and off a half dozen times before the Submonitor will kick out on rapid cycle. Then if you don’t have it purposely set to stay off after this trip, it will reset itself and let the pressure switch start bouncing again.

Using time delay on and off, you will see the “allotted” times, and the Submonitor will not trip. So set the Submonitor to look for times longer than the time delays.

I also do not think 3 seconds from 0 to full speed is quick enough for start up. That would mean that the motor is running below 1800 RPM for 1.5 seconds. That could be enough time without water between the plates to grind off the thrust bearing a little on every start.

 

[gtwy]

I haven't been involved w/ determining the root cause but I dont think it was contact bounce- more like a severe duty cycle. The controls may have been messed up for quite some time. Indeed the Submonitor may have been programmed not to auto reset- I wasnt involved with the set up. I was under the impression that the Submonitor would go into lockout (needing a manual reset) when it saw the programmed number of "false starts", whatever it considers that to be.
As far as the starting goes, unfortunately the power co. will not allow across the line starting so a soft start (or reduced voltage in some manner) must be employed. Franklin seemed ok w/ the 3 sec/45% init torque setting. Depending on the cable capacity, they want to see 65% of full v if the cable is under 50% and 80% of full v if loaded over 50%.
There is actually a lot of "fur" growing on this episode- actually two well pumps went south within two weeks of each other- a real nightmare as they are owned by a small municipality.

 

[jonr12]

I would have the motors checked for cause of failure, turn the soft start down to 2 seconds, and make sure the Submonitor is programmed for manual reset only.