Special Instruction and Special grease for VFD Electric Motors

[petronila]

Dear All.

I am looking for technical information(if it exist), in order to know about special instructions (relube times, frequency, etc) and special grease condition (wich type and how is composed)used in VFD Electric motors.

Thanks in advance for the inputs.

Regards

Carlos

 

[BrianPetersen]

It's just an electric motor; why would the fact that it's connected to a VFD have any relationship to the bearings on its shaft?

Most electric motors have sealed-for-life bearings anyhow. When the bearings are shot, you rebuild the motor and put in new sealed bearings. Or, if it's a small motor (not worth fixing), you throw it out and buy a new one.

Explain why your electric motor is any different???

 

[Skogsgurra]

There's more to it than one thinks.

The PWM causes EDM and the coupling mechanisms can be capacitive or inductive or you can have frame voltage.

If the coupling is capacitive (mostly in smaller Machines) then it might help to use conducting grease. It doesn't help at all if the coupling is inductive (low source impedance) or if you have frame voltage (also low source impedance).

But otherwise, there isn't much in terms of grease that you can do to help. Motor reactors, common-mode filters, earthing brushes are the proven methods to reduce EDM in bearings. Sine filters could also be used but often cause other problems.And, of course - there's always the sine inverters.

PersonallyI do not believe in conductive grease. The conductive stuff aggreates and causes indentations and vibration, which isn't good either.

Gunnar Englund

http://www.gke.org

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

 

[1gibson]

Additional consideration for shaft currents in a VFD driven motor. Ground one bearing, not sure if it needs special grease.

 

[BrianPetersen]

I understand the electromagnetic concerns, but on most motors (with sealed ball bearings) there's not much you can do about them anyhow.

If the shaft/rotor needs to be grounded, then ground it. Doesn't change the bearings.

If the motor needs reactors or filters in the power supply, then do that. Doesn't change the bearings.

 

[flexoprinting]

Hello Petronila

I was told to give a continuous running printing press motor about a pump of grease once a month but had no expert advise to back up that rule so I found your question interesting. Looking through the "Cowern papers" I expected to get my answer there, Dang! no mention of it. Found this Goggling you may find helpful.

http://www.mobilindustrial.com/ind/english/files/tt-electric-motor-bearing-lubrication-guide.pdf

Chuck

 

[MikeHalloran]

I hadn't thought about it before, but there might be a justification for _not_ using some types of grease when a VFD is present.

For example, the first grease I reach for, usually, is a light silicone grease, which is usually perfect, or at least satisfactory, for most any service, and is usually compatible with whatever's already there. It's not normally conductive, so that's not an issue.

However, if there's an electrical discharge, or other super high temperature excursion, however localized, the silicone in the grease will break down. Unfortunately, its normal decomposition product is not carbon, as in the case of dinosaur grease, but ... silicon, fine white sand. Definitely not good for bearings.

Please note: I haven't seen a documented correlation between use of silicone grease and bearing failures in VFD-equipped motors. I'm conjecturing that it _could_ happen, based on my limited experience with silicones.








Mike Halloran
Pembroke Pines, FL, USA

 

[waross]

Years ago I worked in a large lumber mill. The motors typically ran for two,8 hour shifts a day, 5 days a week.
Once a year, all small motors up to about 10 HP got one shot a year in each bearing,
Motors up to about 60 HP got two shots and
Motors over about 60 HP got three shots a year.
If in doubt, skip a year.
If the records were incomplete or missing, correct the records and skip a year.
We had very few bearing failures.
Once a month seems a little excessive.

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

 

[Skogsgurra]

That transition from silicone to sand has been seen in quite a few situations. EP (Extreme Pressure) grease, which I believe contains silicone,shows that clearly. Such grease should not be used above 85 - 90 C and if there is EDM in the bearing, it is quite possible that local temperatures (enough to vaporize steel) also produces "sand". I have seen microscope photographs where the sharp little particles are embedded in the runways.

Gunnar Englund

http://www.gke.org

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

 

[flexoprinting]

Hi Bill

I agree once a month is very excessive, so much for listening to Folklore. I assume from your post other than the point Gunner made that the addition or use of a VFD does not change the frequency of greasing as was the concern of the OP and myself.

Thanks Bill

Best regards, Chuck