VSD and motor bearings

[heelex]

Hello, I received the following email from an electrical engineer that has designed some of our facilities:

As commented yesterday at the meeting we have had some failures on bearings on motors with VSDs on some of our projects, as a result we were provided with a report on the cause of these failures. To summarize the failure is seen on motors of 10hp and over that are controlled by means of a VSD, with a failure rate of approximately 20%. The solution to this issue is to grease the bearings with dielectric grease and provide grounding brushes on the bearing. The cost that has been preliminary quoted to use is approximately $200 for a factory install of the grounding system. Please comment on how you would like to proceed.



Thank you,

Has anyone here heard of such failures? It seems suspect to me that the engineer has come up with this.

Any thoughts appreciated.

 

[iwire]

Has anyone here heard of such failures? It seems suspect to me that the engineer has come up with this.

Any thoughts appreciated.

Yes, VFDs can take out motor bearings due to electrical currents flowing through the bearings.

I am not sure his cure will work. The brush will only take part of the current the bearing will still take the remainder.

The dieelectric grease will not stop the current flow between the bearing races and rollers. When I have been involved in this ceramic coated bearings where put into the motors to stop the problem.

Google has plenty of info about this problem.

http://www.google.com/search?hl=en&q=fluted+bearings+VFD&btnG=Google+Search&aq=f&oq=&aqi=

 

[heelex]

Thanks for the quick response. I will forward the information to all involved.

 

[Jraef]

The Aegis web site provides some very informative white papers on this subject. I have used their system on many occasions and it has always cured the problem, at least within my lifetime.

By the way, I have never used grease that is specifically called "dielectric". I mean, the opposite would be to use conductive grease? Most conductive grease is what you use on mating electrical surfaces, not bearings, and it typically has metallic particles in it, NOT what you would want in your bearing race. So in other words, I think ALL bearing grease is, at some level anyway, dielectric. In fact it is the dielectric of the grease that helps contribute to the problem. The voltage essentially builds up in the rotor like in a capacitor, until it OVERCOMES the dielectric of the grease and starts jumping the gap between the bearings and races. It is in that jump, which is similar to an EDM arc, that the bearings are damaged. I think that part of his statement is just a red herring. The only thing that would stop that from happening would be to use CONDUCTIVE grease, and therin lies the problem.

 

[weressl]

Hello, I received the following email from an electrical engineer that has designed some of our facilities:

As commented yesterday at the meeting we have had some failures on bearings on motors with VSDs on some of our projects, as a result we were provided with a report on the cause of these failures. To summarize the failure is seen on motors of 10hp and over that are controlled by means of a VSD, with a failure rate of approximately 20%. The solution to this issue is to grease the bearings with dielectric grease and provide grounding brushes on the bearing. The cost that has been preliminary quoted to use is approximately $200 for a factory install of the grounding system. Please comment on how you would like to proceed.



Thank you,

Has anyone here heard of such failures? It seems suspect to me that the engineer has come up with this.

Any thoughts appreciated.

Usually this problem does not occur on small motors, like 10HP. Larger motors with ASD specification often have one of their bearings isolated, so no current can flow. Are the motor leads below the manufacturer's recommended maximum length and are they the recommended type?

The lower your carrier frequency is the smaller these currents will be.
Eaton makes standing wave traps that is installed at the motor end that prolonges winding life and reduces the stray currents generated in the rotor/shaft.
Try searching Google for shaft grounding, I tagged one on my office PC, but I am not there today. That will drain the current away from the bearings.

 

[iwire]

Try searching Google for shaft grounding, I tagged one on my office PC, but I am not there today. That will drain the current away from the bearings.

How would that stop current on the bearing?

It seems it would only reduce the current going through the bearing not stop it.

 

[weressl]

How would that stop current on the bearing?

It seems it would only reduce the current going through the bearing not stop it.

I did not say it will stop it. (Parallel resistances; the current flow is inverse proportion to the resistance. The brush is magnitudes smaller resistance than the bearing.) It will provide a sufficiently low AND constant drain that the bearing will not suffer damage. As someone pointed out earlier the bearing current is not continous since the lubrication film is an insulator the shaft voltage needs to rise to a level that causes the breakdown of the film insulation level resulting in arcing. The arcing will pit the runner and create an even higher resistance path, so the wear escalates, not unlike the thermal runaway of a bad connection. (To add insult to injury, the arcing carbonizes the lubricant which in turn becomes an abrasive component.)