Corrosion (Rust) in Rotor (electric motor)

[paramathma]

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

 

[itsmoked]

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

 

[paramathma]

See attached picture of rotor

 

[itsmoked]

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

 

[waross]

I agree with Keith.

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

 

[theode]

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

 

[ScottyUK]

I have worse machines in service. No problem.

 

[paramathma]

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

 

[ScottyUK]

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.

 

[theode]

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.

 

[paramathma]

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?

 

[Skogsgurra]

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

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[itsmoked]

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

 

[paramathma]

I found a paper in which they have mentioned that 1% of water will lead to corrosion.

http://www.stle.org/assets/document..._Ball_Bearing_Steels-Experimental_Results.pdf

 

[Skogsgurra]

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

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[electricpete]

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)' ?

 

[Skogsgurra]

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

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[paramathma]

water content was measured using karl fischer titration method

 

[Skogsgurra]

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

http://www.gke.org

--------------------------------------
Half full - Half empty? I don't mind. It's what in it that counts.

 

[krisys]

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.