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

 

[electricpete]

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/2010/08/Handling-and-Storage-of-Bearings.pdf

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)' ?

 

[paramathma]

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