bearing problems 1

[thewellguy]

I will start by stating I'm not a motor guy, I'm a water well contractor. The problem that I have run into is that I installed a all new vertical water lube turbine with a vertical hollow shaft motor, in a new well and new controls. This is a 150 hp 460 volt U.S. motor and Abb acs550 series drive. The pump runs as smooth as silk (no vibration)The pump load in under the bearing rating, and I can fine nothing wrong with the system.

The problem is that twice in the last six months the top bearing has start make horible noised and has had to be replaced under U.S. warrenty. The motor shop is telling me that the cause of the bearing failure is "shaft current problems" caused be the relationship between the drive and motor. First, does this make sense? Also can some direct me to some reference material that is specific to hollow shaft motors? Finally, any ideas on preventing this in the future? Thanks for your help in advance.

 

[jraef]

Yes it makes sense, it is a well known phenomenon now. In a nutshell, the high speed switching if the transistors in the VFD causes capacitance to form between the rotor and stator. So a current wants to flow, but the only path it has is across the bearings and races. Even though the amount is small, eventually it acts like an EDM system and causes pitting in the races, which deteriorate the bearings rather quickly.

You can either have grounding bushings installed on the motor shaft, or you can attempt to ameliorate it with filtering of the VFD output with very specifically designed load filters for that purpose. The absolute best practice would be both.

Do a web search on "Aegis shaft grounding bushings" and then look at Trans Coil Inc (TCI) or MTE for VFD load filters. All of these sites have additional papers describing the problem in more detail.

 

[sreid]

There are also bearings available with ceramic balls that insulate the inner and outer bearing races.

 

[electricpete]

Somewhere on the GE website is info on hollow shaft motors. I know that because we have some made by GE and I have downloaded the literature for them.

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[roydm]

Some of the large motors I have seen have a grounding brush on the shaft at drive end whereas the NDE bearing housing is insulated from ground.
Roy

 

[petronila]

Hello Thewellguy,

Normally the VFD Driven motors most be specified by the costumer, so if you buy the motor notifiying the use with VD the motor manufacturer will provide you a correct motor.

Is very strange U.S Provide a wrong motor, because normlly they don´t use hybrid or insulated bearings but they insulte the NDE shaft journal bearing, so this is enought to avoid the shaft curent, will be good if you can confirm the shop diagnostic, so you can tell to the shop to do the folowing: 1. Look the bearing grease if is black color and if smell bad. 2. Take a grease sample and watch in a microscope in ordr to find metalic particules 3. Clean the bearings and cut th external raceway with an electric saw then look inn the internal raceway if you find burning signals and small horizontal lines around all the race way. You sayds the motor is verticlly mounted, so this motors normally use special bearings to support the rotor weight, so the problem could be mechanical design too.

Regards

Petronila

 

[ozmosis]

thewellguy
I think you should verify if it is shaft current that is causing the problem. The motor company who analysed it said so, and it is fairly easy to spot once the bearing is out but the type of pump you have, as I understand from the description, is a water lubricated type. This means the method of lubricating is active once the motor/pump starts. My limited experience on VTP is that they have to start quite quickly to get the necessary lubrication to the bearings. If you have a VFD driving it then one of the benefits can be a controlled acceleration time (minutes/hours even) but this could induce a problem if the accel time is too long and the lubrication is not getting to the bearing. 150hp would typically point towards a longer acceleration time as standard and so may be worth looking at.

 

[peebee]

I suspect a search on the terms "fluting", both on this site and on the Internet in general, would be productive for you. That is the term used to describe the scarring of the bearing races due to shaft currents.

An IEEE Spectrum article discussed this within the last few years. As I recall, they stated that a shaft grounding brush was a better solution than insulating (ceramic) bearings, as the insulating bearings will simply move the problem to downstream equipment (to your pump).

 

[thewellguy]

I just got the report from the motor shop. According to them "U.S. is refusing to warranty the motor because the bearing problem was caused by shaft current." My frustration is that I purchased the VFD rated motor from them. Theer's nothing in the manual that stated that I had to install brushes and filters. So thanks all your info was very helpful.

 

[Skogsgurra]

There is a lot to be said about bearing currents. I have been working with these problems for almost 15 years. I put this paper together for a short reference. Working on a hand-book.

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[thewellguy]

I just re-read my post and the last line reads very sarcastic, not what I ment. I do thank you for you help.

 

[itsmoked]

Not sarcastic to me, nor to most others I suspect.

Keith Cress
kcress -

http://www.flaminsystems.com

 

[eyec]

thanks to all who provided replies to this post. though my problem did not involve a VFD motor it led me to some other sources on stray current and we finally found the cause of failure of a motor.

like to give a STAR to all.

thanks guys

 

[electricpete]

ok. Some questions out of curiosity.
eyec = thewellguy ? (same guy?)
The cause of failure ended up being?
The other sources of stray current were?
thanks in advance for any info you can share.

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[eyec]

no, i am not thewellguy

the failure appears caused by stray current through the lower motor bearing which facilitated pitting and final failure of top babbit bearing

possible cause leading to stray current was contaminated lubricant oil and latent moisture

the other sources of info were research and industry papers on stray current, Kingsbury in particular had some good info @

http://www.kingsbury.com/pdfs/universe_brochure.pdf