causes of intermittent motor trips associated with the MOTOR 5

[electricpete]

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).

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[electricpete]

Take the word "credible" out of the question. I just want to brainstorm possible problems with motor only or perhaps the power supply (excluding breaker) that can cause this behavior.

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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[itsmoked]

You have made sure that if the motor is high efficiency the set point is correct and much different from a typical motor? As that would cause the same tripping issue with a correctly functioning 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

 

[electricpete]

Good question.

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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[electricpete]

Another part of the question posed to me is: what tests should we do on this motor to understand/prove whether it is the cause of the problem?

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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[electricpete]

Since it appears the machine will be down for awhile and the interest is high, I suggested we bring in someone to test with pdma style tester or bjm tester. More info can't hurt.

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[itsmoked]

I would agree with your last point. Also the 20 years thing jettisons the 'high efficiency' argument.

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

 

[waross]

How is the supply voltage level. Are there times of the day when the supply voltage is abnormally high?

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

 

[electricpete]

Voltage is 490 vac at the MCC. 460 vac motor with somewhere around 50-100m cable each way between MCC and motor.


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[electricpete]

Also MCC voltage is stable (power plant environment)

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[waross]

Hi Pete; you are usually giving good advice. It's not often you need help.
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

 

[edison123]

Any intermittent cable faults ?

Excessive break-away torque during the first start but no so during subsequent ones ?

What is the connected load ?

 

[electricpete]

Thanks edison.

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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[electricpete]

That makes me think of one thing. What if the pump wer rotating backwards due to backflow on a check value. I would think that could increase the starting current slightly since the current vs slip curve on a typical motor still has a small slope as it hits the s=1 axis (would increase a little bit beyond on the other side).

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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[davidbeach]

Back spinning motor could be an issue. In the Steinmetz model the rotor resistance becomes negative for slips greater than 1. Enough slip and the whole motor impedance could become negative. Plot starting current vs. slip and see where starting current becomes greater than your inst setting. Is that a credible rotor speed?

 

[edison123]

Energizing the motor during back spinning amounts to plugging, which would involve large stator and rotor currents.

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.

 

[itsmoked]

That is the most credible thing I've heard yet.

Pretty easy to check too.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[cdevine]

Can't say I am convinced with this suggestion even before I start as an instanataneous trip normally points to an electrical issue. However...

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

http://www.icande.com.au,

http://www.redbusbar.com

 

[cdevine]

Just had a thought.

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

http://www.icande.com.au,

http://www.redbusbar.com

 

[electricpete]

Here is the motor data
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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