We have based the taking of weekly amps readings on the asssumption that if there is possibility of bearing failure then due to increased friction the motor will start drawing more current.Is this a fair assumption or have we missed something???

 

The extra loading a failing bearing on such a motor (37KW) will be so small by comparison with the load the motor is driving, you are very unlikely to see any change in the current.

 

The phase amps alone are not going to be a good predictor of bearing condition. The bearing condition will be far into a failure state before the motor amps indicate a problem. Better predictive technologies exist, such as vibration analysis, monitoring bearing temperature, etc. Check www.sfkcm.com for a start into the world of predictive condition monitoring for rotating equipment.

 

You missed two things.

1) Amps is a very poor indicator of load on a motor. So poor in fact that an operator's accustomed ear is a better indicator. To truly know your load, you need an instrument that monitors amps on all phases, phase-phase volts, and the phase angle between amps and volts, or power factor. There are lots of suppliers of these, but two off the top of my head are Ohio Semitronics, "http://www.ohiosemitronics.com":http://www.ohiosemitronics.com and Load Controls Inc, "http://www.loadcontrols.com":http://www.loadcontrols.com
Both sites have info explaining why current montioring alone doesn't work. These instruments are not very expensive.

2) Bearing failure sufficient to cause a discernable and assignable cause load change is already advanced failure. The bearings are already just moments away from catastrophic failure.

If this is really critical, I suggest you look into acoustic emission monitoring or vibration monitoring of the bearings.

 

Hello. Try vibration and heat ups.
Kinetico.

 

With a motor of such size it is nearly impossible to detect bearing wearing by looking at the amps.
If your motor drive a critical process the best is to make constant weekly measurement and analisis of vibration. This can be done by an especialized company. This procedure really will show you the wearing of the bearings and give you time to take desicions on maintenance. This is named predictive maintenence.

 

It is difficult to predict bearing failure through load,bering failure in motor can be monitered by temperature,sound,vibration level at drive end and non drive end,keep tract of this parameters

 

The proper method is to watch bearing temparature and look for noise in a bearing.Going to be bad bearings will not reflect on motor current easily.The best is to regularly replace bearings on a total motor run time or time based. All modern motors have very small air gap to improve magnetic coupling and there by motor efficiency. A bearing failure can make the rotor rub against stator and cause more damage. For 2 pole motors we replace bearings twicw a year (24 hours shift).
It is also the duty of the operator than any body else to watch for machine odd behaviour since he works with the machine every day. Look for all possible indications to predect failures than focusing into motor current. Many maintenance people have the habit of patroling along the machines and watch for abnormalities. Many companies also have keep standard frame size motors for doing periodical maintence. They swap the motors and overhaul the replaced motor and keep as a spare for other machine.
Hope this helps,
Sekar

 

WE have done the same thing at a re-man mill we do the PM at for the last 5 years, there are some human and mechanical factors to also consider. The human factor can include TO MUCH GREASE, or the pipe fitting opposite the grease nipple was not open before it was greased, also if the belts were recentl changed and the millwight tightened them way to much, putting excesive pull on the shaft bearing, will definatley cause a bearing failure extremely fast. Also, was the machine operator changed, are they running it harder, overloading it!? Could they be trying to start the motor when it is in a locked rotor state!? A preventative Maintenance program will only be a success if everyone knows and understands the load and operating parameters that the said machine is capable of opertating under, sudden changes in operation will throw your history off and send you back to first base. But the PM system works, in the first 6 months we ran ours we cut down time by 78% and redused the number of burn outs by over 100% compared to the two years before. Recently though we have gone through a major people change, a large percentage of people had to be hired and trained and as a result we have burnt out nearly $50,000 in motors in less than 5 months. New people means they don't understand the process, worker education is just as important as PM.

 

The friction in the bearings contributes nearly nothing to the total mechanical load of a motor, and even less if it's a big one. I guess it's tenth of percent. So I suppose you might notice a change in current not before some seconds before the failure.
Some weeks ago we had a visit from the representative of a manufacturer of a system for
preventive mechanical maitenace (motors, gears, etc.). He has a handheld device with a contact microphone. You put it on the housing of a bearing and the device records the (frequency weighted?) noise level together with the number of the measuring point. Next week or month or day, you inspect the points in the same order, and it will tell you where noise increased. So may know where to expect damages.

 

In my opinion is is better to registrad the frequenties of the mechanical vibrations in the motor rather then monitoring the current. The variations of these frequenties is most important.

 

Couple of things. Put an RTD in the bearing casing and monitor the temperature. Much more reliable. You say the motor failed. Is it stopping and starting too many times per hour? Cutler Hammer and GE have motor monitors with a thermal model built in. Programme the thermal model and if the thermal capacity of the motor reduces to a dangerous level, the monitor will lock the motor out.

 

Weekly amperage readings on a motor are of little value by themselves, and certainly under any condition they will not alert you to bearing problems other than catastrophic failure of the bearing.

A combined voltage and amperage reading would be useful for some things, but not bearings. At least with volt/amp readings a knowledgeable person could monitor the system functions for trouble.

If you have been having bearing problems then focus on that problem by looking at the following (this is definitely not a complete list):

1. Ambient temperature problems, high temperatures would require different greases.

2. Are you greasing the bearings properly, are you removing the vent plug when greasing? This is one of the BIG crimes of lubricating bearings. If you grease a bearing without removing the vent plug you damage the bearing covers and seals and cause bearing failure and winding failure due to grease intrusion into the motor.

3. Rare but not when it is your problem are bearing problems arising out of the use of VFDs.

4. Using low quality motors or incorrect types of motors on work loads that present thrusting loads on radial bearings. Close coupled or submersible pumps can especially be vulnerable to this. If this is a problem use a motor with thrust bearings in addition to the radial bearings.

If you are looking for predictive failure maintenance then special testers can be used. These testing machines electrically test the motor windings in different ways, typically they test the insulation.

Lots more needs to be discussed on your issue, work hard at it and you will see improved bearing performance.

Measuring amperage is not worth your effort if that is all you are going to do.

 

Yes this a fair assumption. Failing bearings will "drag down a motor". You do not give any specifics of your application.
Failing bearings on the load of the motor will also give you a higher amperage reading.(ie.pump,conveyor etc.) Most electric motors are Thermal Protected, Friction=heat.
There are other factors that can contribute to high amperage ratings besides the motor bearings.
Scooter

 

Bearing lubrication and bearing failure analysis is a discipline unto itself, requiring much more information than you provided. Such information includes: bearing type; duty; lubrication type; method of lubrication; detailed operating history; etc.

Assuming a web search and the motor vendor were not helpful, I can only suggest the following:

See if you can relate failures to a change in environmental or operating condition. Also check technical school or college libraries.

An excellent reference is:

"Motor Application and Maintenance Handbook"
Author, R.W. Smeaton
Publisher McGraw-Hill Co.

Regards,
Phil Corso, PE
(Boca Raton, FL)

 

Pulse Width Modulation motor voltage control causes bearing eddy current - this was recently shown to be one of the major bearing failure mechanisms.

-Alex
[email protected]

 

A decent resource on this topic is at http://www.abb.com/GLOBAL/seapr/SEAPR035.NSF/viewunid/67BD0B8B45590059412568890037BD7D!OpenDocument

 

Bearings are damaged by FORCe(s)/MOMENTS that produce too high STRESSES for the material used in bearing. Temperature lowers significantly the material strenggth too. These are mechanical parameters. You need to find a SOURCE of these forces, their directions (e.g. axial ones on bearings that are nor designed for them) and:

(1) remove them or
(2) change type of bearings.

All other methods suggested in previous resp's could help to reveal sources of unusual stresses that are nothing to do with electrical parameters from the electric feeder (voltage, current, even harmonics).

Good assignment for electromechanical investigator.

When solving your problem please share your "expertize". Some of us will appreciate it!