Preventive Maintenance Strategy for Motors

[adityalagudu3291]

Would appreciate it if you can provide your thoughts on the Preventive Maintenance Strategy for Motors.

I am particularly looking for 3 phase Motors under 100HP. What should be the replacement period for motors under 100HP, based on the criticality level of A, B, C? Which standards are a good reference to look into it?

 

[petersonra]

For most motors, wait until it dies and have a shelf spare.

Some motors are more critical and may require a little more thought.

 

[adityalagudu3291]

For most motors, wait until it dies and have a shelf spare.

Some motors are more critical and may require a little more thought.

Assuming those are A and B critical motors, how do we come up with a replacement period? I believe according to NETA maintenance procedures, these motors should be replaced for every 6 years. Please help to provide your comments.

 

[petersonra]

Assuming those are A and B critical motors, how do we come up with a replacement period? I believe according to NETA maintenance procedures, these motors should be replaced for every 6 years. Please help to provide your comments.

For most motors it just does not matter much.

 

[adityalagudu3291]

For most motors it just does not matter much.

Thanks for your input. I am looking for any documented reference. Please let me know if you come across anything like that.

 

[Russs57]

I'm not going to waste my time looking for documentation. I'll draw on my forty something years of experience instead.

Most motors that die an early death do so for reasons that aren't the motors fault. You had voltage spikes. Someone over greased and/or didn't know to remove plug opposite grease fitting and grease with motor running. The motor was overloaded and/or didn't have proper running protection. The motor overheated because nobody bothered to wipe the dust/dirt/grease obstructing ventilation openings. Addtional loads have been added leading to uneven phase voltages and therefore uneven current in the motor. Etc, etc.

The best thing you can do is keep records from day one and look at trends. Are the megger readings starting to decline? Are amp readings changing and/or no longer balanced? What about temperature? Are you willing to pay for vibration analysis?

If you have an area with reasonable power quality....have phase loss protection and overload relays that are set correctly and work......and you keep the motor clean, cool, dry, and lubricated......IMHO you should get 20+ years out of a quality motor. I have many that are 60 years old and still running 24/7/365 where I work.

Stick a little 1/4 HP made in China motor out on a hot roof under an exhaust fan cover.....leave it covered in dust and grime......run it into service factor amps and beyond with nothing but auto reset thermal overload protection......well you might not get 6 months out of it.....and you might keep on doing it until someone points out that you are using an "air over" motor outside the air stream

BTW, VFD's big a whole nother set of problems to the table. Ditto for motors that start and stop very often.

I would prioritize based on lead time to acquire a new motor and what the loss of that motor means to your operations. For example a 100 HP chill water pump motor might not be in stock locally and could render you unfit for business on a hot summer day. However a 5 HP AHU motor might be delivered and changed out in under 4 hours and only effect a small area.

 

[tom baker]

I'm not going to waste my time looking for documentation. I'll draw on my forty something years of experience instead.

The best thing you can do is keep records from day one and look at trends. Are the megger readings starting to decline? Are amp readings changing and/or no longer balanced? What about temperature? Are you willing to pay for vibration analysis?

I would prioritize based on lead time to acquire a new motor and what the loss of that motor means to your operations. For example a 100 HP chill water pump motor might not be in stock locally and could render you unfit for business on a hot summer day. However a 5 HP AHU motor might be delivered and changed out in under 4 hours and only effect a small area.

Excellent reply and I will add based on similar years of experience.
I had 20+ years as an instructor and taught many motor classes, and found the following to be usefull...

NEMA has a lot of good information on motors, https://www.nema.org/standards/view...edium-squirrel-cage-induction-motor-standards
Motor manufacturers are a great source of info, at Baldor is the 100 page Cowern Papers by a former Baldor district manager and technical writers, its excellent https://www.baldor.com/mvc/DownloadCenter/Files/9AKK107303
NFPA has 70B, recommended practice for electrical maintenance. https://www.nfpa.org/codes-and-stan...s/list-of-codes-and-standards/detail?code=70B
IEEE would be worth checking, you would need to do a document search and them pay for any articles of interest

Biddle has A Stitch In Time, Guide to practical insulation testing, excellent

https://www.instrumart.com/assets/Megger-Guide-to-Insulation-Testing.pdf

Take good readings from day 1, open circuit volts, running volts, running amps, and megger readings, A one time reading does not tell you the trend. Meg the motor, correct for temperature and plot on the special graph paper. Its much better to do a Dielectric Absorption Ratio test as it tells you a lot about the motor condition, much more than a 1 time megger test.

The other comments are spot on, correct overload settings, keep the motor clean and dry, don't overgrease, use the correct grease, do vibration analysis and balancing. I also like to record the RPM when running loaded (its related to Back EMF)

A motor is dumb, it will keep pulling current (review what back EMF is) if ioverloaded, and it will generator excessive heat (not enought back EMF).

Make sure the motor operates in its +/- 5% of voltage range. Keep it clean and dry.

So there is some homework for you.

On motor life, I haven't seen such a recommendation. Suggest you contact Baldor or Reliance or ?? and ask them

I had motors that were in installed in WWII and lasted 70 years, these were the big old T frame that had lots of copper and ran cool.

Let us know how it goes and what you would recommend.

 

[retirede]

I had a GE engineer tell me that their design resulted in a 10% failure rate in 10 years when operated at service factor load, max ambient, at balanced nameplate voltage. Nothing in writing.

 

[Russs57]

I'll admit to one sad fact I have observed a lot.

You may have someone near the top who writes really good specs on what testing should be done. If you are lucky he has enough power to make sure the right tools/meters are bought and the people doing the tests are properly trained.

The disconnect is that often the data is gathered, and entered, but nobody that really knows how to interpret it is overseeing. I suppose everyone assumes" the guys doing the testing know what readings indicate a problem. But chances are very high they don't have the lifetime history of the readings and the time/software to be looking at trends and/or predicting MTBF.

It used to be that maintenance was about doing work to make sure things didn't fail. It is quickly becoming dealing with failures after the fact that could have, and would have, been prevented in the past before downsizing and value engineering.

Okay, rant off.

 

[paulengr]

To begin with, NETA is created by testing companies for testing companies. Most NETA accredited companies do zero repair work, certainly not motors. NETA has limited credibility. They can’t even follow EASA or IEEE standards for motor testing. Use NFPA 70B or an IEEE standard.

In fact this is a good test. If your testing company does not have a repair shop, get rid of them. They just test. They have no practical experience what the results mean. That’s the issue with NETA and why the criticality stuff is meaningless.

Second you need to read Nolan and Heap. The ancient replacement strategy is not recommended. The bathtub curve does not apply. For AC induction motors the ultimate limit on life is insulation breakdown which takes on average 20 years though it hits anywhere between 15 to as high as 25. Few motors get there simply because external environment destroys them well before then. You can check this easily. Once a year run an insulation resistance and graphical PI test using IEEE 43. Do not use the garbage NETA version that is still quoting the 1970s procedure back when we used asphalt and mica insulation and varnish instead of epoxy. Measure temperature and correct. You can easily determine ahead of time when breakdown is towards the end. 50+ HP rewind. Under that run to failure. All sizes are going to be stock anyways.

Getting there is the issue. First, I can’t stress enough how critical alignment meeting ISO standards on a structurally sound support free of soft foot is. Second lubrication is just as critical. Use ultrasonic greasing or change oil and get samples tested to monitor condition. Quarterly vibration analysis to detect problems before they occur and take corrective actions period. 25-50% of motor failures are bearing failures.

On the electrical side make sure it is fused or breakers to meet “no damage” criteria and overloads set properly. Check contactor wear once a year, more with heavy use like cranes. IR scans for weak joints. Check power quality once a year (voltage and current unbalance, motor load). Again correct problems.

Make sure it gets plenty of air. Clean if it’s dirty. Maintain fan shrouds. Fix fans if needed. Make sure the seals are maintained. Internal contamination shows up on your insulation resistance test.

Motors do not follow a bathtub curve. There is no wear out period nor much of infant mortality (under 0.1% based on our rebuild shop ISO 9000 data). Following Nolan and Heap it runs until insulation breakdown at 20 years average or more. But about 90% of motors never make it that long because of everything else around the motor. So concentrate all your efforts on predictive/preventive maintenance.

As an example say you see the PI drop over time along with capacitance to ground (if you measure it). The PI graph flat lines and PI is around 1.1 although it is still over 5 megaohms insulation to ground. This is a common pattern. It may be insulation breaking down but most likely contamination. Swap out the motor and send for evaluation. It it’s just dirty they can clean and bake and replace the bearings. Switch to Inpro seals of fix the contamination problem to extend the motor to full life inexpensively. This isn’t motor failure yet. It’s all predictive maintenance.

Another example is a 1 HP motor fails on a VFD. It has been in service 10 years. Replacement lasts 6 weeks. Shop inspection shows first couple turns shorted on all 6 coils. Motor is 200 feet from VFD and has output filter. Solution? Output filter worn out. Replace filter and motor (not worth rewind).

This is speaking from experience. This is what we offer and do. Other than insulation breakdown or rare manufacturer defects, motors last indefinitely. You should take the attitude of trying to make them last beyond your retirement.