causes of intermittent motor trips associated with the MOTOR
causes of intermittent motor trips associated with the MOTOR
(OP)
OK, here is the situation.
2-pole motor ~ 50hp fed from ungrounded 480 vac system.
Motor tripped on instantaneous overcurrent a few months ago.
As part of investigation - Meggered/bridged motor (sat). Did a start while monitoring current on O-scope (sat). Checked molded case breaker instantaneous trip (sat - within expected bounds based on nameplate LRC and up to factor of 2 increase in 1st peak due to dc component). Breaker was replaced at that time.
Motor has not shown any unusual vibration or other characteristics.
Now this weekend - Motor tripped again on start (instantaneous). Motor meggers and bridges sat again. Voltage is normal.
They called me up at home and want to know: "Is there anything about the motor that can make it susceptible to causing a trip?"
My response is: No. The highest instantaneous peak varies in a random manner based on phase at closing (as we all know). None of the previous tests have shown anything abnormal. I have never heard of a 460vac motor fault that would lead to an instantaneous trip, and then motor subsequently operates fine. I think you need to check your setpoints and make sure they are high enough.
Their response is: the setpoints have been fully checked and are well above that theoretical worst case. We are on the verge of changing out the motor instead of re-revaluating the setpoint. We want to know what is it about the motor that can cause variation in starting current beyond what is predicted by analysis using 2* factor and nameplate data.
Can you help with this question? Any credible faults on the motor that cause an assumed (by others) abnormally high instantaneous current during start?
(note that is the narrow focus of my question - I am not interested in talking about the setpoint).
2-pole motor ~ 50hp fed from ungrounded 480 vac system.
Motor tripped on instantaneous overcurrent a few months ago.
As part of investigation - Meggered/bridged motor (sat). Did a start while monitoring current on O-scope (sat). Checked molded case breaker instantaneous trip (sat - within expected bounds based on nameplate LRC and up to factor of 2 increase in 1st peak due to dc component). Breaker was replaced at that time.
Motor has not shown any unusual vibration or other characteristics.
Now this weekend - Motor tripped again on start (instantaneous). Motor meggers and bridges sat again. Voltage is normal.
They called me up at home and want to know: "Is there anything about the motor that can make it susceptible to causing a trip?"
My response is: No. The highest instantaneous peak varies in a random manner based on phase at closing (as we all know). None of the previous tests have shown anything abnormal. I have never heard of a 460vac motor fault that would lead to an instantaneous trip, and then motor subsequently operates fine. I think you need to check your setpoints and make sure they are high enough.
Their response is: the setpoints have been fully checked and are well above that theoretical worst case. We are on the verge of changing out the motor instead of re-revaluating the setpoint. We want to know what is it about the motor that can cause variation in starting current beyond what is predicted by analysis using 2* factor and nameplate data.
Can you help with this question? Any credible faults on the motor that cause an assumed (by others) abnormally high instantaneous current during start?
(note that is the narrow focus of my question - I am not interested in talking about the setpoint).
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RE: causes of intermittent motor trips associated with the MOTOR
I don't think voltage unbalance causes increase in instantaneous peak during starting.... does it?
High source X/R ratio... yes,.... but only up to the max factor of 2 which is supposedly accounted for.
Any others?>
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RE: causes of intermittent motor trips associated with the MOTOR
Is this an old installation that has just started acting up, or did the first trip happen shortly after installation?
Keith Cress
kcress - http://www.flaminsystems.com
RE: causes of intermittent motor trips associated with the MOTOR
The same motor has been in place for 20 years and just started acting up a few months ago. Breaker was changed after that first trip a few months ago. I don't know the history of the breaker before that first trip, but I suspect it may have been changed out with a new one from another manufacturer (different setpoint strategy) due to obsolescence (we have been going through all our switchgear doing that). If that's the case, certainly seems to me like it's a no brainer that the breaker setting is the problem.
At any rate, the breaker and its setpoint is out of my field of focus for the time being. I was asked very specifically about whether the motor itself can be the cause the trips assuming the breaker is set "properly" according to motor nameplate info. Setpoint is someone else's responsibility. And I have pointed out that a setting strategy can include bumping the setpoint up when "spurious" trips are seen (as this appears to me), but that is not the input they are seeking from me. They just want to know if there can be something wrong with this motor (or perhaps the supply system).
I also just found out we don't have a spare motor that meets the particular quality requirements for this motor. That tends to increase the importance of understanding whether there may be a problem with the motor since swapping the motor doesn't seem to be an option at this time.
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RE: causes of intermittent motor trips associated with the MOTOR
We have done megger and winding insulation resistance.
I am hesitant to ask for surge test since that is potentially destructive and this motor does not have a spare.
There are some low voltage testers out there that might be slightly more sensitive to turn faults. We don't have any of those but I think we could get a motor shop in with that equipment. I tend to think it is a waste of time since if there is a turn fault, it is clearly not continuous and wouldn't likely show up. What do you think?
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RE: causes of intermittent motor trips associated with the MOTOR
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RE: causes of intermittent motor trips associated with the MOTOR
Meggering should have found anything that can occasionally mechanically short.
How about a loose connection in the supply. Opens occasionally and over amps the remaining phases enough, while single phasing, to trip the instantaneous. Not sure that would raise the current enough though. More of an overload thing.
Really is a dooser.. ??
Keith Cress
kcress - http://www.flaminsystems.com
RE: causes of intermittent motor trips associated with the MOTOR
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: causes of intermittent motor trips associated with the MOTOR
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Eng-tips forums: The best place on the web for engineering discussions.
RE: causes of intermittent motor trips associated with the MOTOR
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RE: causes of intermittent motor trips associated with the MOTOR
Can you monitor the phase to ground voltages? Probably not.
Have you checked and inspected the contactor? There may be some tracking that goes to ground or phase to phase. When running it would be warm and dry but after having sat for awhile and cooled of, you may have a combination of a carbon track and a little moisture.
If there is a problem in the contactor it will take a long time to find it in the motor. (LOL)
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: causes of intermittent motor trips associated with the MOTOR
Excessive break-away torque during the first start but no so during subsequent ones ?
What is the connected load ?
RE: causes of intermittent motor trips associated with the MOTOR
We meggered the motor from the switchgear, so no sign of hard cable fault (but intermittent... I can't say).
The connected load is a centrifugal pump. Driven through an Omega coupling.
I have always considered that load torque should not come in to the picture when we are discussing instantaneous trips. Even if the rotor were locked, I would think we would get a thermal trip, not an instantaneous. If this is wrong, let me know.
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RE: causes of intermittent motor trips associated with the MOTOR
It is easy enough to check, so I'm just going to ask them to check it. But if anyone can speculate about whether this affects LRC, I'd be interested to hear your thoughts.
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RE: causes of intermittent motor trips associated with the MOTOR
RE: causes of intermittent motor trips associated with the MOTOR
I use plugging (with lower voltages) in my repair shop during no-load trial runs (for in situ balancing) and current is very high. The current stays the same for a particular plugging voltage till the rotor comes to a stop indicating the speed has nothing to do with the current.
RE: causes of intermittent motor trips associated with the MOTOR
Pretty easy to check too.
Keith Cress
kcress - http://www.flaminsystems.com
RE: causes of intermittent motor trips associated with the MOTOR
Commissioning centrifugal pumps for the first time is often the most arduous starting duty that the pump will see due to the need to fill the pipe line. That is, the head is low and the flow high (refer to power requirements of a standard pump curve). Of course this condition can resurface if the pump loses prime. If the pump loses prime due to say a faulty foot valve or rubbish in the foot valve you get reverse flow through the pump and the need to refill the line.
It wouldn't be the first time that the electrics have been blamed when a bit of maintenance was all that was needed.
Chris Devine
www.icande.com.au, www.redbusbar.com
RE: causes of intermittent motor trips associated with the MOTOR
Is it possible that you have a intermittent phase to phase short. It may not show up if you have done an insulation test phase to earth or low voltage motor winding tests. Possibilities include MCC and cable. Burn marks would be expected also if due to surface tracking or vermin. Can you inspect the cable throughout its length?
Chris Devine
www.icande.com.au, www.redbusbar.com
RE: causes of intermittent motor trips associated with the MOTOR
460vac
60 hp (NOT 50 hp)
KVA code G
I will try to dig up setpoint info and breaker style.
I compute LRC is expected to be 421 - 473 at 460vac
Interview with personnel present indicate they were watching it at the time it tripped... was not rotating backwards prior to trip. Shaft did not move either direction (trip occurred before motor moved).
We started the motor 5 more times - trip did not recur.
We recorded two starts attached. The one labeled unit 1 is a sister motor that has not had a problem. The one labeled unit 2 is the one that tripped (did not trip during this recorded start).
I see the highest peak current < 1000 Amps. To me this seems normal - well within 2*sqrt(2)*LRC.
Do you agree?
Any other commnets?
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RE: causes of intermittent motor trips associated with the MOTOR
Currently set on G.
Do you think there is justification to bump it to H?
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RE: causes of intermittent motor trips associated with the MOTOR
(that is assuming of course that there is not a problem with coordination with upstream breaker)
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RE: causes of intermittent motor trips associated with the MOTOR
Circuit breaker sensitivities extend to harmonics and higher ambients also. I suspect that the higher ambients only impact on the thermal trip elenment. Electronic trip elements that are true RMS sensing overcome crest factor issues.
If you are looking to increase the magnetic trip setting a check of downstream cabling for short circuit performance is the only other thing I would suggest. You may weld a contactor as well. On balance its worth a shot.
Chris Devine
www.icande.com.au, www.redbusbar.com
RE: causes of intermittent motor trips associated with the MOTOR
Do you think this can possibly cause an increase in instantaneous inrush?
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RE: causes of intermittent motor trips associated with the MOTOR
That can result in an out of synch reconnection of a running motor. Several in rapid succession. Same thing that causes Wye/Delta starter systems to throw occasional head scratching instantaneous trips. Only in your case you'd get several shots at a fully out of synch reconnection on each start.
I'd either;
1) With the motor locked out, mess with the start circuit and try to figure out why that's happening.
or
2) Swap contactors and watch the problem switch motors.
or
3) Replace the contactor.
One issue is the switching circuit that is running the contactor coil power. If it has issues any contactor would have the problem.
Keith Cress
kcress - http://www.flaminsystems.com
RE: causes of intermittent motor trips associated with the MOTOR
I am referring to the springs that maintain contact pressure when the contactor is closed.
I don't fully agree with Keith but I think that he is on the right path.
I see two phases establishing a voltage division and phase angle for two windings (assuming a wye connection) and then the third phase abruptly changing both the phase angles and voltage and current division.
I like Keith's idea of rolling starters or motors and see which the problem follows. I am not convinced the problem is in the motor. I still suspect a possible contactor problem. The late make on one phase may be a problem or not. But I suggest looking for mechanical and/or tracking problems in the starter.
Does one coil operate all three poles of the starter or is each pole powered independently?
I agree with Keith's suggestions.
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: causes of intermittent motor trips associated with the MOTOR
RE: causes of intermittent motor trips associated with the MOTOR
davidbeach; I recognize the low angle reducing things but at a few RPM isn't the angle going to be changing really fast? 30 degrees is represented by only 1.3ms. Bouncing contacts are going to be bouncing at about 50ms? Meaning a more random situation? Granted their won't be much EMF available to send back into the system.
Keith Cress
kcress - http://www.flaminsystems.com
RE: causes of intermittent motor trips associated with the MOTOR
Can this cause above normal currents? I don't know but I wonder. Is there anything here that may push one or more windings into saturation?
Bill
--------------------
"Why not the best?"
Jimmy Carter
RE: causes of intermittent motor trips associated with the MOTOR
At slips close to 1, the condition at the initial start, there is no rotationally induced voltage in the motor field and closing angle doesn't matter, you just get more of the starting level current, but that isn't enough to trip the instantaneous. At slips closer to 0 you begin to have to be concerned with the internal voltage and how it synchronizes with the line voltage but now it takes a while for the phase angles to get too far out of sync because the rotor is moving at near synchronous speed.