Pump Motor Locked Rotor Condition

[VTer]

We have a general purpose 3HP, 3ph, 60Hz 460V induction motor driving a centrifugal pump for a chiller system pumping water/glycol mix. The motor is started across the line from a MCC bucket roughly 20 feet away. The motor has been recently tripping out on overload. The motor has a FLA of approximately 4.5A (I do not have the nameplate information with me at this moment). We metered the motor current and voltage at the MCC and have observed that the current magnitude in phases L2 and L3 reaches to about 8.5A before the overloads trip. L1 is always steady at ~4.3A. This measurement was repeated multiple times with the similar results. Our electricians decided to swap to a new motor with same nameplate information and same manufacturer. With new motor in place we observed same results - always high current on L2 and L3 before overload trip. We decided to take the measurements closer to the motor at the motor terminals. Upon observing the same results at motor terminals we also noticed that the small part of shaft that was visible between the motor and pump coupling would actually stop rotating. We were able to repeat the results and confirm that the rotor actually stalls when L2 and L3 read about 8A on the meter and right before the overload trips. No questionable noise is heard during this condition. It was also noticed that sometimes the motor would start rotating and slow down to a stall, while at other times it would not even start rotating before tripping the overloads. The chiller and pump supplier confirmed the pump curves and assured us that the 3HP motor is adequately sized. This brings up questions that I hope people on this site with similar experience or ideas can help explain.
1. Why is the current in locked rotor condition only about 2x the FLA
2. Why is the current only high on 2 of the 3 phases and always the same two phases for an overload condition? Shouldn’t we be seeing large current on all three phases?
3. Can inappropriate use of motor be the cause of stall? I am suspicious of general purpose motor being used in this application since the motor is mounted in vertical position with DE bolted on top of the pump. I do not know if the correct type of bearings are used for this application?
I am very interested to see if others have had similar issues. Thanks in advance!


"Throughout space there is energy. Is this energy static or kinetic! If static our hopes are in vain; if kinetic ù and this we know it is, for certain ù then it is a mere question of time when men will succeed in attaching their machinery to the very wheelwork of nature". û Nikola Tesla

 

[ScottyUK]

You are correct that the current magnitudes should be equal if the overload is due to the driven load. Grossly unequal line currents may not be a result of the load but of a problem in the motor or with the supply to the motor. You appear to have eliminated the motor as a possible cause, so I would be taking an interest in the supply voltages - especially at the motor terminals.

 

[waross]

We metered the motor current and voltage at the MCC and have observed that the current magnitude in phases L2 and L3 reaches to about 8.5A before the overloads trip.

And the voltages are??
For safety and efficiency considerations, I would first check the voltages at the MCC bucket, with the motor running.
If you see large voltage differences look for supply problems. Possibly a bad connection somewhere.
If the voltages are reasonably close then check at the motor terminals and look for a bad connection between the MCC and the motor.


Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[VTer]

Thank you for the quick responses. We measured the voltages at the MCC and the voltages are very close and steady throughout this process(within 2V). We do not have voltage measurements at the motor terminals at this time. This will be the next thing we investigate. To add more information, there is a parallel pump and chiller with same nameplate information fed from the same MCC sitting right next to the problematic setup. This parallel setup runs fine without issues. There are also many other loads on the MCC that have no issues. We will look at the motor terminal voltages and report back. In the mean time if any other ideas come up please advise. Thanks!

"Throughout space there is energy. Is this energy static or kinetic! If static our hopes are in vain; if kinetic ù and this we know it is, for certain ù then it is a mere question of time when men will succeed in attaching their machinery to the very wheelwork of nature". û Nikola Tesla

 

[ScottyUK]

As Bill says, check at the output terminals of the starter bucket initially. If the voltages are imbalanced at that point then you have a problem in the bucket, if the voltages are OK there and wrong at the motor then it's almost certainly a termination or joint problem: it's damned unusual for a conductor within a cable to go high resistance mid-way between the ends.

Quick & dirty check - can you swap the buckets over and see if the problem moves or not?

 

[jraef]

Off the top of my head, I think you have a high resistance short in the conductors between the starter and the motor, likely a phase-to-phase short between L2 and L3. So because it is high resistance, the magnitude of the fault current is not tripping the circuit breaker, it is tripping the overload relay. It could also be a high resistance ground fault on L1, with the resulting single phasing condition causing the shaft to not move on an initial start.

So there's an indirect way to tell; if the shaft not moving takes place on a Start command from a stop, it's a GF on L1. If the motor is spinning and STOPS, it is more likely a Ph-Ph short on L2-L3.


"You measure the size of the accomplishment by the obstacles you had to overcome to reach your goals" -- Booker T. Washington

 

[VTer]

Quick update: We measured the voltage at the motor terminals and found that we had an intermittent loss of voltage on L1. We followed the issues all the way back to the load side of the breaker in the MCC bucket. The issue is either in the breaker or source side connections. We cannot get to the line side because our rules do not permit live work which is required to take the bucket out. Shutting down the MCC is not an option at this time because of production. We are getting a new bucket just in case it's needed and plan to correct the issue during a planned shutdown. I will report back once we perform these corrections. Thank you all for your support!

"Throughout space there is energy. Is this energy static or kinetic! If static our hopes are in vain; if kinetic ù and this we know it is, for certain ù then it is a mere question of time when men will succeed in attaching their machinery to the very wheelwork of nature". û Nikola Tesla

 

[ScottyUK]

If we're staking a beer or two on this, I'm backing a problem within the bucket: bad connection, burnt-up contactor or a cooked overload relay being my specific bets.

Anyone else in for this high-stakes Christmas bet?



** EDIT: I'm not going nuts, I wrote this reply before VTer posted his update above!

 

[waross]

My first choice was "burnt-up contactor or a cooked overload relay". Then I reread VTer's 15 Dec 16 19:50 post.
Now it looks like a faulty breaker or bad connection.
If the stab connection is bad, you may also have a damaged vertical bus bar.
It may be well to locate a replacement bar in stock ready for rapid delivery if needed and allow extra time on the shutdown schedule.

Bill
--------------------
"Why not the best?"
Jimmy Carter

 

[ScottyUK]

Ha-ha, that's not fair - inside information!

 

[waross]

It looks like you and VTer were posting almost at the same time, Scotty.
I was in complete agreement with you until I rechecked VTer's post, everything but the time.
Sorry.
I owe you one in the Pub.

Bill
--------------------
"Why not the best?"
Jimmy Carter