Motor Failure Analysis 4

[BaliHai]

Is anyone an expert on failure analysis for motors..need to put in place a system which will work well for analysing failures, ie why they happened, what is the root cause and what can we do to fix it. Examples, flowcharts with a series of questions,etc.

 

[motorman]

Assuming that you are most interested in those mysterious insulation failures, the following website is a good place to start. However, in the real world, my experience is that half the time it is impossible to separate the cause of the insulation failure from the effect because all the evidence has been destroyed by the failure. For example, you open-up the motor and there is a big whole in the core iron of the stator. Was this the result of a failure in the ground wall insulation? Or did it start as a failure in the turn insulation that then went to ground. But by far the best diagnostic tool is to have data loggers that are recording key motor operational parameters. Reviewing the log by itself might not tell you that it was failure in the ground wall or turn insulation but would tell you, for example that yes there was a ground failure but leading up to that failure the operator started and stopped the motor 20 times in an hour last night when the motor is only designed for a max. of 8 starts per day.
Anyway, here is the website:

http://www.firstelectricmotor.com/motor failure.htm

 

[don01]

here here Mr Motor man the person who really knows why that motor went down is indeed an expert !!

But BALIHAI, do you want to record maintenance issue then cross reference history ,and or provide trouble shooting charts or create a predictive maintenance system (if thats not an oximoron I'll be supprised)

Give us a little bit more of an idea what you envisage There may be some commercial stuff or low cost home grown systems to suit your desires.

Regards Don

 

[BaliHai]

Don
Thanks for your comments.
I am actually after troubleshooting charts or rather, failure analysis charts. At the moment, the technicians/fitters are recording the following as 'cause of failure'

"Motor burnt out" - which is not really helpful when it comes to analysing the possible causes and putting in place steps to reduce this or eliminate this.

Can you suggest a reference, book, or website?
Thanks

 

[jbartos]

Suggestion: Visit

http://www.ch.cutler-hammer.com/ess/Partial_Discharge.htm

http://www.joliet-equipment.com/electric-motor-repair.htm

for more info.

 

[THooper]

Hi BaliHai,

From the editors of Electrical Apparatus Magazine they offer a book answering some of the most frequently asked questions about electromechanical maintenance, repair, and application called Let's Solve Your Problems. This paperback answers real questions posed by real people in manufacturing plants and repair shops around the U.S. and abroad. As a repair shop plant manager I use this frequently for reference. A good winding failure chart for failures in three phase stator windings that has pictures of the most common winding failures like, single phased, phase to phase, turn to turn,unbalanced voltage, overload, locked rotor, winding grounded in the slot, etc.. with detailed pictures can be purchased from EASA, Electrical Apparatus Service Association. I use a bearing failure chart from SKF bearing mfg that has descriptive bearing failures.

I hope this helps! To purchase Let's Solve Your Problem call 312-321-9440 the cost is around $29.95 To purchase EASA winding failure chart call 314-993-2220 and to get skf bearing failure charts contact your nearest skf bearing distributor.
Motorhead1

 

[electricpete]

Good answers from all.

Some more references:
EPRI 100898 - Random Wound Motor Failure Analysis - only availalbe to EPRI members.

I understand that IEEE had a project underway called P1415 - "Induction Motor Failure Evaluation Guide". I also understand EASA has a motor failure manual... haven't seen either of these myself.

A few photo's of various failure at:

http://www.joliet-equipment.com/failure_in_3ph_stator_windings.htm

The ideal situation would be to find someone experienced in failure evaluation and hire him to do a few for you (perhaps your motor repair shop has an "expert&quot. You would of course have to provide him the history and all info available from time of failure (what relays targeted, what was weather etc)... but watch the way he does, and ask questions to understand his thought process and try to develop into an expert yourself. Flowcharts etc are nice, but there's no substitute for direct experience alongside someone who knows what they're doing.

 

[electricpete]

Here's another link to an article specifically on failure analysis (I hope it works). There's quite a bit of good motor info in the Support Section of the CSI website... I think it's under "App Notes"

http://www.compsys.com/drknow/aplpa...14510be34cdc29a0852567ec005ad246!OpenDocument

 

[DHSJ]

Knowing the Root Cause of a failure is important, but I feel that a predictive approach to motor failures is a much better means of preventing failures. Being able to track and trend the items that cause failure will give you more insight to preventing motor failures. The ability to monitor, track, and trend; resistance to ground, resistance phase to phase, inductance phase to phase, motor current signature analysis, eccentricity, starting current, and power quality prior to failure provides much more insight to actual causes of motor failures.

 

[don777]

how do i test a de-energised 3-phase electric motor with a multimeter (will not run, blowing fuses)

 

[electricpete]

If all you have is a multimeter, then use the resistance scale. All of phase-to-ground resistances should be off-scale high. Phase-to-phase resistances should be fairly low and should be similar among the three phases.

Prefered method to check the phase to ground insulation is to use a megger instrument designed to supply higher dc voltage.

 

[electricpete]

DHSJ - I think I can see where you're coming from but my own opinion is different.

We invest fairly heavily in predictive technologies and preventive maintenance.

The purpose of these investments are to prevent failures. In order to best use our limited resources to prevent failures, we need knowledge of:
What kinds of failures are we susceptible to?
How do operational factors contribute to failure?
How effective are the various predicitve technologies for the various failure scenarios?
How effective is the preventive maintenance in preventing failures?
How effective are purchase and repair specifications/ practices in getting a product not prone to failure?

The common denominator in all the above questions is that they are related to failures. The better we can understand failures, the better we are equipped to make good decisions.

Since you single out predictive activities I'll focus on that. One of the glaring questions following a motor failure should be "How did we miss this?".

Some questions I have for you:
How do you evaluate the effectiveness of predictive technologies?
Do you think the test equipment vendors are reliable sources of information regarding the effectiveness of their particular box?
If you rely on advice from other users, how have they formed their opinions? If the answer is that they have studied motor failures then I'd say you're getting good advice. If the answer is that they rely on the vendors than I'd say you may want to reconsider your approach.

Ok, I'll admit this is a little bit of pie-in-the-sky. The correct approach is going to vary depending on lots of factors which may be unique to a company. If you happen to work at a plant which has an load of approx 120MW worth of induction motors, where the consequences of failure for some of these motors is approx $2 million in plant downtime, where the cost of preventive and predictive motor activities is approx $1 million per year, then the investment of time to understand motor failures is worthwhile in order to intelligently manage both failure and preventive costs. I can imagine that many other companies do not have the luxury of devoting time to failure analysis for very practical business reasons.

 

[electricpete]

DHSJ - I'll hope you're not offended by my previous post... it was only intended to present a different opinion, not to criticize yours.

Based on the motor parameters you mentioned I'm guessing you use a tester made by P**A (name disguised to protect the innocent). We've been thinking about buying one. What has your experience been? Have you found any problems with that equipment?

 

[don777]

got readings between phases but nothing form earth to phase(3kw electric motor), what is a possible problem ?

 

[electricpete]

don777 - what is expected from phase-to-ground for a normal 480vac motor is very high resistance of at least 1.5 megaohms temperature corrected (old standard)... preferably 5 megaohms (new standard).

When you say you got nothing, it sounds like maybe your meter is telling you it can't read that high resistance. If so, then your instrument is not showing any fault with phase to ground insulation (although a higher-voltage megger instrument might possibly show a fault).

The three phase-to-phase resistance readings should agree to within approx 10%. If not, there is indication of a likely shorted turn.

If you have passed all the above tests, I can't think of any other checks that can be done with a multimeter.

Out of curiosity - did the motor work initially and just started blowing fuses, or is this a new installation?

 

[nbucska]

If the motor burns, only an autopsy may dive some answer--
whatever it is worth. It is hard to tell which short was
the original one.

If the protection saves the motor BEFORE it overheats too
much -- assumung you have only a multimeter:
a.) With smaller motors you may measure the inductances
by using a lightbulb or e.g. hot plate in series with
the line and measure woltage drop and current.

b.) With a large motor you may need more accurate
methodes and instrumentation.

<nbucska@pcperipherals.com>

 

[electricpete]

Good suggestion by nbucska.

You might want to also want to try uncoupling the motor from it's mechancical load to see if it continues to blow fuses during start. If that's not possible, rotate by hand to check for mechanical binding. For radial pump check fluid system lineup to ensure very limited flow during starting.

for new installations:
- Check fuse size
- check motor matched to mechanical load to provide satisfactory starting performance
- check motor wiring vs nameplate.