Current through compressor bearing

[Gerben]

Hello all,

three weeks after overhauling a Grasso S3 2500 compressor, the bearings of the driven rotor looked like there were small welding spots on the outer and inner ring, the balls were damaged.
It seems that the (also) overhauled electrical drive motor is generating current which is transported through the rotor of the compressor, and then throuhg the ball bearings to earth. That is the only explanation what I can think of.

Anyone faced this problem before, or has another explanation?

Thanks, Gerben

[Grizzly]

Yes.
I had a conversation with a Siemens engineer either last year or the year before.
About the exact same problem with early Sabroe screws with variable speed drives.
I can't remember exactly what they did to cure the problem.
But I know a man that will likely remember, so i will speak to him next week and see if i can come back to you on this one.
I think it is down to the style of the bearing and there being no route to earth for the induced voltages.
As you rightly suggest Gerben.
Grizzly

[Gerben]

Thanks for your quick reply Grizzly,

like you mentioned, on a variable speed drive, I have handled with the problem that the motor bearing was damaged. We solved it by using insulated bearings.
Now it is a 3000 rpm machine, of 1992, with no problems up till now.
We only did a standard overhaul, like always, so nothing has changed.

Gerben

[US Iceman]

I'm not surprised by the bearing issue. Do the spots look like bearing spalling in the race? Like small arc points from a welder?

[NH3LVR]

Not an expert on this, but a couple of thoughts.
I worked on a cleanroom job where all the Air Handling units had variable speed drives. This was my first experience with shaft grounding being installed on the motor bearings. This might solve your problem.
Another possibility might be to install a coupling with a non-conductive element between the motor and compressor. Woods brand is commonly used here.

[Mongrel]

Most variable speed driven Screw Compressor packages built "Down Under" in Oz, are fitted with insulated bearings in the motor and earthing brush on the motor shaft to prevent this occurrence.

[Gerben]

Hi Iceman,

it is like you mention, small arc welding spots. I do not know what you mean with "bearing spalling", but it must be a kind of damage.

Gerben

[Mike W]

I have seen 2 things that cause arcing, VSD's and people welding on or near the compressor set with an arc or tig welder.
The VSD problem can by eliminated by fitting a EDM kit which is usually available from the motor supply. This consists of a insulated bearing being fitted to 1 end of the motor and an earthing bush fittted to the near the opposite bearing to ground the rotor.
Extract From CMG site

THE power supply from the grid, according to CMG , is generally smooth without harmonics but because the supply from a VVVF drive is not, high frequency harmonics are generated.

These high frequency harmonics cause capacitative voltages to be stored in the stator and rotor laminations which discharge periodically.

The stator discharges are simple as they go to earth. However, the discharges from the rotor while trying to get to earth, discharge through the bearings and cause EDM (electrical discharge machining).

Once a bearing has been subject to EDM, a non reversible failure will occur in the future - possibly three months but it can be as little as three hours.

The three choices available are:

1) Do not use VVVF drives. This is very unlikely as the advantages outweigh the disadvantages.

2) Improve the design of VVVF drive so that these harmonics are not produced. As drives improve with new designs, the effect of EDM is lessening but the problem will still arise as long as drives do not produce a smooth supply waveform.

3) Cure the symptom. CMG fits a VVVF drive kit to motors operating on VVVF drives. This kit insulates one bearing of the motor to stop discharge through that path and a shaft earthing brush is fitted at the opposite end of the motor. This allows the capacitative voltage to discharge safely to earth and bypass the bearing.

[RANGER1]

Have some verbal info from motor manufacturers that 150 kw and smaller motors don't necassarily need insulated bearings etc .
Gerben has mentioned for other reasons , if any why failure may have occured .
I'm not familiar with Grasso but can only think of basics like -
Have you taken bearing/s to bearing manufacturer for inspection ?
Could drive shaft have been heavily impacted in transit or fitting causing bearing damage ?
Was it getting enough/any oil ?
Could any debris have entered bearing or wrong preload of bearing cause clearance problem ?
What are you going to do next ?
Hope i'm not suggesting to much crap , just trying to give ideas .

[Tycho]

Most variable speed driven Screw Compressor packages built "Down Under" in Oz, are fitted with insulated bearings in the motor and earthing brush on the motor shaft to prevent this occurrence.

we had this problem on some of our frequency driven compressors as well, but it was only the motor bearings getting damaged.

we solved it same as Mongrel, put a copper brush on the motor shaft and grounded it on the motor base, no problem since