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Two motors burnout in two weeks


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#1 phild

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Posted 27 July 2005 - 09:29 AM

As stated two 132kw motors burn't out in two weeks on conveyor. Last motor suffered bearing failure and severe meltdown.
Protection is CET3 and Siemens thermistor relay at MCC.
Checked settings on CET3 at rated current of motor, tripping time 10 secs. thermistor cct 260 ohms at 22 C.
Changed CET3 on suspicion as no real way checking EOL for overload operation, thermistor relay did operate with o/c of thermistor cct.
Cannot find any info for S+S CET3 on net, we are slowly replacing the CET3 with CET4's throughout the plant.
Until that happens we are stuck with the CET3's, so if anyone can shine some light on CET3 it would be appreciated.

Regards

PhilD

#2 marke

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Posted 31 July 2005 - 02:28 AM

Hello phild

That is certainly bad luck.
I do not know that I would be pointing the finger at the CET3 however, especially if you have thernistor protection as well. Perhaps ther is something wrong with your trip circuit as it would be unusual for both the CET3 plus the thermister protection relay to fail at the same time.
I have seen many installations where the protection circuitry has been defeated due to the occasional trip, rather than find the root cause of the problem.

If you have a motor failure due to a bearing failure, that is something that you would not necessarily expect a protection relay to pick up unless the motor is at full load. The bearing failure coul result in a lot of extra heat being dissuipated in the motor without overloading the motor.

Have you determined the mode or cause of failure in the first case? This may provide a clue as to why the protection did not operate.

Best regards,

#3 GGOSS

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Posted 31 July 2005 - 11:19 PM

Hello phild,

Although I agree fully with Marke, I know the CET3 product very well and can provide some assistance there is you qualify exactly what you need.

Also, you mentioned you were changing these over to CET4. Are you aware the CET5 has been available for over a year now?

Regards,
GGOSS

#4 phild

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Posted 01 August 2005 - 12:11 AM

:)Hello
From what you said, I wish it was as simple as that.
I was not made aware of the first motor burning out until the second one burn't out. The first one blew the main contactor tips to pieces, possiably the motor was two phasing, but more likely a voltage imbalance allowed the motor to keep running long enough to destroy the motor.
This is with the CET3 having the features plus the siemens thermistor relay wired separate to the CET3 EOL cct.
This plant when built was wired for sound.
The second pope motor suffered a bearing failure, the motor was basically seized.
From the experiences over the years, us electricians have learnt to cheat with these CET3 as they are prone to brown out problems with nuisance tripping.This is first time I have seen where no protection operated except the panel earth leakage relay, which alerted us to the problem.
The third motor we installed is a reconditioned toshiba from another mine, now I have been told we have to replace there motor with a new motor.
A toshiba is not pope, so it took us 2 days to fit this motor onto the drive train. I managed to track down a CET3 manual, didn't throw much light to what we already know, the only thing I wasn't aware of is when you use the test button on the CET3 it is Ie * 6 for 10 secs(inertia time setting). So after 10 secs the EOL should trip.
The CET3 we replaced didn't trip until about 40 secs, but this still has me thinking, two separate overloads, one using a thermal model and the other using PTC thermistors. Neither worked, the motor is working on a cyclic load of @ 50 to 60%. We have looked and tested the alarming of the motor at the panel and PLC and cannot fault. Sent an email to have the repair shop to try and ascertain from the damage what might have caused the problem.
Regards
PhilD

#5 GGOSS

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Posted 02 August 2005 - 12:36 AM

Hello phild,

The statement "The first one blew the main contactor tips to pieces" strongly suggests an earth fault or short between phases at the motor. Under those conditions the CET3 and/or Siemens thermistor relay will not trip. In fact there isn't a product on the market that can protect against those things, some will however detect the condition after it has occured and trip to prevent further damage. The CET3 can be set up to do that via an earth fault protection option that is available.

As far as the second motor failure goes, (and as indicated by marke) bearing damage will cause an over- temperature condition in the motor, but unfortunately that cannot be sensed or modelled by the CET3 or any other electronic motor overload relay. To protect against this sort of thing RTD's would need to be fitted to the motor, and connected to an RTD monitoring device/relay. An RTD scanner is available for the CET5, and possibly for the CET4 although I cannot confirm that.

It would appear that the motor failures occuring at around the same time is moreso a coincidence than anything else. There doesn't seem to be any information linking the two together.

Hope the above helps in some small way.

Regards,
GGOSS

#6 marke

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Posted 02 August 2005 - 03:21 AM

Hi Phild

I have not heard of problems with reliability with the CET3. From my knowledge, they have been very reliable. I think that I would tend to look at the possibilty that things have been "temporarily" disabled at some stage, possibly due to brown out problems or the like.
To check the operation of the protection system, just disconnect the thermistors from the thermistor relay. With the inut open, the relay must trip. The big question is, does it drop out the main contactor. If not, why not. It may have been disabled.
A thought, does the protection directly control a contactor coil, or is it via a PLC? I am a fan of direct control!!

Your description in the forum of the contacts being blown off sounds like a catestrophic failure rather than a minor overload situation. If there is a sudden insulation breakdown that causes a major fault curent to flow, the protection systems can not stop this from happening. The protection in the CET3 and the thermistor protection are designed to protect the motor from a continuous overload that will cause the motor to run too hot and result in the insulation deteriating. It is possible that the motor was operating under overload conditions for an extended period of time and this resulted tin the "blow up", but it is also possible that it was just one of those things that happens due to moisture or similar. If this motor was not operating for quite some time and then failed when or soon after starting, I would look at the condensation potential as a cause. If the motor was operating and had been for a considerable period of time, then look at the protection systems incase there is an issue.
The motor rebuilder should be able to give you some good clues provided that he knows his stuff. If the windings showed signs of "cooked" insulation, then they has probably been an overload before the blow. If the windings are basically clean except around the blow, then it is an insulation failure.
If only some coils are cooked, then it is possibly a negative phase sequence problem due to imbalance. I would not expec two phase operation to result in damage to the contactor. To do this there is a lot of fault current an this sounds like a phase to phase breakdown.

It is very easy to draw the wrong conclusion without more information!!

Best regards,
Mark Empson.

#7 phild

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Posted 02 August 2005 - 10:20 AM

;qHello

thank you for comments.This raw coal reclaim conveyor runs 7 days 24 hours a day at 60% of rated load. Availabilty or access is zero until down days.
All motor protection is hardwired to the main contactor to trip on emergency stop, motor EOL, thermistor O/L and earth leakage.
Alarming is also generated through the PLC to the central control room.
Iam back on day work tomorrow, there may be some info back from the repair shop, regarding the possiable cause of burn out to the second.
motor.
Besides changing out the EOL the other day, all protection functions where operated and noted. Each tripped the motor contactor.

Regards
Phild

#8 marke

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Posted 03 August 2005 - 12:44 AM

Hello phild

One question to ask the repair shop is whether the failures are the result of a continuous overload, or are the result of a catastrophic breakdown. If the first then is is reasonable to expect that the CET3 and thermistor should have protected the motors. If the second, then they would not provide any protection at all.

Best regards,

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Posted 03 August 2005 - 12:21 PM

hello mark

Until they return the info, I will ask the question.
Iam still not convinced the protection operated on both motor burnouts.
We accessed the alarm processors today looking at the alarm events on this drive at the time of each burn out. No motor EOL or motor over temperature alarms, only the earth leakage and the MI alarms coming up.
We work rotating shiftwork so contact is done through email with the various people that are now employed to do the maintenance.
One of you made a comment about a lightly loaded motor, which I would qualify this motor to be, besides the poor PF this motor would be operating under. Is there a scenario with a large lightly loaded ac polyphase induction motor running continually without many starts that contributes to a situation where there is a possiablity of motor burn out due to voltage fluctuations or imbalance.

Regards

Phild

#10 jraef

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Posted 03 August 2005 - 04:51 PM

Negative sequence currents caused by a current imbalance have an increased heating effect that does not show up in thermal models unless they are specifically designed to look for them. I know nothing about your EOL, but I do know that many EOLs (and ALL electro-mechanical OLRs) do NOT have biasing of the Thermal Model to compensate for disproportionate heating from negative sequence current. This is typically covered in the IEEE device number 46 classification. However, your PTC relay should have picked it up if it were designed correctly.

BTW, I agree with Marke as to the PTC or EOL not being able to pick up on a failed bearing. I have seen that many times. The PTC is invariably located too far away from the bearing that will fail (Murphy's Law), and the failure can happen without causing a significant enough increase in motor current to be detected by an OL relay until it is too late (if at all).

This site http://www.joliet-eq...or_windings.htm provides some photo guides to assist in determining the cause of failures in the windings. I have found it to be very usefull. Use the left hand column to click through individual photos of the various failure modes.
"He's not dead, he's just pinin' for the fjords!"

#11 phild

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Posted 20 August 2005 - 10:15 AM

:) Hello again

The answers to the question, why two 132kw motors burn out in quick succesion.
The first motor, no answers for the motor failure as no follow up information from the repair shop or follow up by the mainetance for the failure, except that the motor age and damage, is not to be repaired and to be scrapped.
The second motor, fan end bearing collapsed, rotor poled onto stator, damaging the slots, causing the motor windings to short to the frame operating the earth leakage protection only. When we opened the dog box, the motor was on the nose.
All together two motors scrapped, another motor from another mine has to be replaced with a new motor plus buy one replacement for the two scrapped motors. Altogether four motors total cost $au 45,000.
Lost production costs $au 200,000.
The replacement motors,due to the pope brand no longer in existence, they are talking of replacing them with of an australian made brand, but for several reasons namely heresay about the reliability of this brand they have settled on buying Toshiba's which are used on other mining sites. Fair enough compatiability with other sites, but this will cause problems fitting the larger size toshiba' into the popes with connection of cables into the dogboxes plus the cable marshaling plate would have to drilled and fitted.
This raises another question besides the actual failures experienced, on these mining sites there are motor stores. On our site, the motor is given a drive number which is allocated a stock code, this stock code could be used on numerous drives of the same rated motor hp, frame and speed.
The question has been asked of the history of these motors namely how many times these motor has been repaired rewound etc. The answers is no records has been kept on any repairs.
The other point taking the failure of the 132kw motor which had only been in service for less than 2 weeks with a collapsed bearing. The question is a motor sitting in storage for any lengh of time, gathering dust and vibration, plus the grease in the bearings is doing what.
What procedures would you take with an unknown motor of any size sitting for an unknown time coming out of storage and more than likely going straight into to replace another motor.
Iam not sure if the term is bearing brindling is correct,caused by vibration by passing traffic, overburden blast coming from a open cut in the vicinity.
The unknown electrical well being of the motor, would it benefical to have test equipment besides a megger, namely the all test pro™31.
To trouble shoot for phase balance, fault to ground, emi and rotor test.
If you can comment on these questions, it would be appreciated.

Regards

Phild

#12 Carl

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Posted 20 August 2005 - 10:53 AM

One of the causes of bearing failure is the misalignment of the motor shaft to the load ...you have a bigger alignment tolerance if there is rubber coupling.

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Posted 20 August 2005 - 11:11 AM

Hello

Whether the bearing failure was caused by a misalignment Iam not sure as the drive was laser aligned and the coupling is a voith coupling which has a spider and dog fitting, which would allow any small misalignment, although last time I was near the motor you could feel vibration coming up through the grate floor, this is not unusual as a disc brake rotor is mounted on the drive train and these have habit of making the surroundings vibrate.

#14 marke

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Posted 20 August 2005 - 08:45 PM

Hello phild

One of the problems with motors sitting for a long time, especially when subjected to vibration etc, is that the bearings can be damaged. If you are concerned about long term reliability, I would look hard at the bearings before refitting the motor.

Best regards,




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