×
INTELLIGENT WORK FORUMS
FOR ENGINEERING PROFESSIONALS

Log In

Come Join Us!

Are you an
Engineering professional?
Join Eng-Tips Forums!
  • Talk With Other Members
  • Be Notified Of Responses
    To Your Posts
  • Keyword Search
  • One-Click Access To Your
    Favorite Forums
  • Automated Signatures
    On Your Posts
  • Best Of All, It's Free!
  • Students Click Here

*Eng-Tips's functionality depends on members receiving e-mail. By joining you are opting in to receive e-mail.

Posting Guidelines

Promoting, selling, recruiting, coursework and thesis posting is forbidden.

Students Click Here

Jobs

Corrosion (Rust) in Rotor (electric motor)

Corrosion (Rust) in Rotor (electric motor)

Corrosion (Rust) in Rotor (electric motor)

(OP)
Dear Friends,

3 years ago we purchased 200 numbers of 3 phase induction motors. Due to some reason, we did not use the motors and was in our ware house. Now we want to use the motors. Before assembling it on the machine, we opened one motor and found corrosion in rotor.

I request motor experts view on this.

1) Can we use the rest of the motors?
2) If there is corrosion on rotor, what are the effects of it during operation?

Thanks,

An

RE: Corrosion (Rust) in Rotor (electric motor)

Open a few from different areas in the warehouse and show us some pictures. It could be harmless or fatal corrosion we don't know without seeing it.

Keith Cress
kcress - http://www.flaminsystems.com

RE: Corrosion (Rust) in Rotor (electric motor)

I would expect no issues from that level of corrosion. Hang out and see what the consensus turns out to be.

Keith Cress
kcress - http://www.flaminsystems.com

RE: Corrosion (Rust) in Rotor (electric motor)

I agree with Keith.

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

RE: Corrosion (Rust) in Rotor (electric motor)

Check the windings for earth/ground fault.
There is only superficial rust on your pic.
Should be ok.

RE: Corrosion (Rust) in Rotor (electric motor)

I have worse machines in service. No problem.

RE: Corrosion (Rust) in Rotor (electric motor)

(OP)
Friends,

Will the long storage impact the grease in the ball bearings.
Two Ball bearings used are sealed type. Will the long storage degrade the grease and impact the function of motor?

What type of test is done to check the quality of grease in the bearing?

thanks,

Ana

RE: Corrosion (Rust) in Rotor (electric motor)

Depends on the storage environment. If there's continuous vibration, even at low level, then the bearings themselves may be in trouble. I'd be tempted to give one a try - modern grease has a long life.

If you're not happy with that then a bearing change is certainly an option although the labour costs may approach the value of a new motor if these are small types.

RE: Corrosion (Rust) in Rotor (electric motor)

I agree with Scotty. Try a couple out first.
They should run quiet and smooth.You will soon hear if the bearings need changing.
There's not a lot of grease in sealed bearings usually.Just enough to lubricate.

RE: Corrosion (Rust) in Rotor (electric motor)

(OP)
Opened the bearings and checked the moisture content in the grease. Weight % of water in the sample was approximately 1.
Is this OK?
what is the allowable range?

RE: Corrosion (Rust) in Rotor (electric motor)

There is Always some humidity in grease. One percent is perfectly OK. I expected more. How did you test - by weight Before and after heating?

If your Windings read better than one hundred megohms after drying, then I would regard the motors as good as new.

Keith's consensus seems to be "connect and run - no probs".

Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

RE: Corrosion (Rust) in Rotor (electric motor)

I don't believe I've ever seen a rotor that shiny in my whole life. They all tend to be mostly tan with fine rust, with only a few tiny glimmers of partially unrusted laminations. I've seen motors that ran underwater for two weeks after a flood and kept running after the water receded for another 10 years.

Keith Cress
kcress - http://www.flaminsystems.com

RE: Corrosion (Rust) in Rotor (electric motor)

Good for you. That paper is surely impressive...

But, how did you measure the water contents? And what are your error margins?

And, why do you think that there is a problem with water or corrosion? is the warehouse damp?

Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

RE: Corrosion (Rust) in Rotor (electric motor)

It is not unusual to see rust on shafts. The shaft is not protected by a lubricant film like the bearings are.

It looks to me that the paper discusses bearings immersed in oil with dissolved water. Grease is a different situation. Grease is supposed to be a little better at excluding moisture. I don't think grease absorbs water as readily as oil. I have seen from motors stored in bad conditions, rust occurred in the bearing only at locations where grease was not present.

The run and check strategy is pretty good in my view. Modern vibration equipment and techniques are s very sensitive to the tiniest of defects (you need to look at more than just the overall level). If you get a good vib check I feel good about the bearing. The small reservation I might have is condition of the lubricant. It is a tricky question, the bearing manufacturers, lubricant manufacturers and even motor manufacturers tend to have very strict recommendations regarding storage. Something like 2 - 8 years storage life even in good conditions and with periodic rotation. Experience suggests those limits are overly conservative, but it's up to your comfort level how to approach it.

=====================================
(2B)+(2B)' ?

RE: Corrosion (Rust) in Rotor (electric motor)

Pete, that is the rotor surface - not the shaft. But even so, there's no need for alarm.

I think that it is important to have an answer to my question about how the OP measured the water contents of the grease and how wide the measuring tolerances are. You may be worrying about something that has no significance at all.

Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

RE: Corrosion (Rust) in Rotor (electric motor)

(OP)
water content was measured using karl fischer titration method

RE: Corrosion (Rust) in Rotor (electric motor)

OK, ambitious and good. Then we can rule out uncertainties there. It is entirely up to you then to do what you feel is best.

Personally, I would use the motors for all but the most demanding purposes (like critical applications in nuclear plants).

Gunnar Englund
www.gke.org
--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

RE: Corrosion (Rust) in Rotor (electric motor)

The rotors generally do not handle the same voltage as the stator. It is much lesser. As such the rotor bars are shorting the at the ends. Thus some minor deterioration of insulation of rotor laminations should not be a matter of concern. Hence using these motors shall be electrically safe, provided the stator winding insulation is healthy.

Also the motor bearings are sealed for life. I guess, the "sealed for life bearings" do not need any re-greasing. They are moisture protected by their construction.

Why not take the no load trail for few hours satisfy before putting them into service.

RE: Corrosion (Rust) in Rotor (electric motor)

I agree, the interlaminar insulation on the rotor is not as critical as on the stator due to the lower frequency of the fundamental flux wave to which rotor is exposed. If similar rust were observed on stator, I might ask for a core test (not too much trouble since the motor must have been disassembled at the time rust would have been seen, anyway).

To follow up on the comment about condition of lubricant itself following storage in possibly non-ideal ideal conditions:
http://www.reliancebearing.ie/wp-content/uploads/2...

Quote (Reliance)


Storage periods for rolling bearings
Rolling bearings should not be stored for longer than 3 years. This applies both to open and to greased bearings with sealing shields or washers. In particular, specifically greased rolling bearings should not be stored for too long, since the chemical-physical behaviour of greases may change during storage. Even if the minimum performance capacity remains, the safety reserves of the grease may have diminished. In general, rolling bearings can be used even after their permissible storage period has been exceeded if the storage conditions during storage and transport were observed. If the conditions are not fulfilled, shorter storage periods must be anticipated. If the periods are exceeded, it is recommended that the bearing should be checked for corrosion, the condition of the anti-corrosion oil and the condition of the grease before it is used.
Whatever concern for the bearing itself can be evaluated from a test run, but the concern articulated in paragraph above is that the lubricant itself may have degradedduring storage. You can find similar sentiment echo'd in literature from motor manufacturers, bearing manufacturers, lubricant manufacturers. For them it's an easy way to CYA and it costs them nothing to make that recommendation. You could easily argue the other way since many iinstalled motor have had grease in the bearings for decades, sometimes combination of running/standby service and sometimes in poor conditions. If we watch the housing temperature upon installation and have a regular vibration monitoring program, one would think we could catch any consequence of the degraded grease before it starts to threaten the equipment reliability. Depends on your circumstances, experience and opinions.

=====================================
(2B)+(2B)' ?

RE: Corrosion (Rust) in Rotor (electric motor)

(OP)
Thank you all for your inputs.
Based on the motor manufacturing year, we decided to scrap some motors and not take risks.


Red Flag This Post

Please let us know here why this post is inappropriate. Reasons such as off-topic, duplicates, flames, illegal, vulgar, or students posting their homework.

Red Flag Submitted

Thank you for helping keep Eng-Tips Forums free from inappropriate posts.
The Eng-Tips staff will check this out and take appropriate action.

Reply To This Thread

Posting in the Eng-Tips forums is a member-only feature.

Click Here to join Eng-Tips and talk with other members!


Resources