VFD bearing failure 1

[Lewish]

Is anyone experiencing bearing failure in VFD motors? If so, what have you done to solve the problem? Is it worse with some VFDs than with others? What are your motor sizes? How long are your motor leads? Any other useful thoughts would be appreciated.

Lewis

 

[teco]

lewish:- yes its becoming a big problem with newer vsd drives that do not have filters in the output.we have modified over 200 machines,it mainly occurs with drives and motors from 110 kw upward.normally you get a electrical discharge through the drive end bearing for close coupled machines or the non drive end bearing for belt drive.
to over come the problem:- the bearing that is faulty has to be replaced with either an insulated bearing or insulated housing. a carbon brush is then fitted to the housing of the good bearing and runs on the shaft, this completes the earth path.
should you require further assistance or information please advise "e"mail or fax no.
kind regds.

 

[Marke]

This has become a greater problem with the VFDs using IGBT technology. Modern VFDs are employing switching devices that are able to switch very fast. i.e. they have a high dv/dt. This high dv/dt couples from the windings to the rotor and causes a small capacitive current to flow. The greater the dv/dt, the greater the capacitive current that flows. Also, the higher the switching frequency, the higher the current that flows. This is less of a problem with motor that have a large airgap. The capacitive current flows from the rotor to the frame and earth and the current path is typically through the bearings unless some other path is provided. One solution is to use insulated bearings and provide another discharge path between the rotor and the frame.
Inserting filters into the output of the VFD reduces the dv/dt and therby reduced the bearing current problem. It also reduces EMI problems but it costs money.
You may be able to add dv/dt filters externally to reduce this problem. Mark Empson

http://www.lmphotonics.com

 

[tmahan]

Lewish -
Where this problem is most commonly observed is in clean room applications. In clean rooms the drive carrier frequency is usally turned to a maximum (16-20kHz) to try and move motor noise to an above audible range. This causes the damage occurs faster. The damage is also more easily noticed in the clean room enviroment since the bearing damage first manifest itself as noise in a quiet enviroment.

If you have already have the drive and motor see how low the carrrier freq can be set and turn it down to that level. (typically around 1 to .5 kHz)

MarkE-
When I was working for a drive manufacture several years ago they were experimenting with using a conductive grease for the bearings as a solution to this problem. I don't follow drives very closely anymore,, do you know if conductive grease is a practicle solution.

 

[BJC]

Here is a paper that may help.

http://www.ab.com/drives/techpapers/ieee/IAS96 bearing.pdf

The company is Shaft Grounding Systems the number is 541 997 4068

 

[Lewish]

BJC, thanks for the link to the paper. This is more like what I was looking for. I am aware of the problem and its causes, but I still would like to hear from people who are actually dealing with failures in the field. What size motors are most often seen?
Thanks.

Lewis

 

[BJC]

Lewish

The Big I (Intel ) uses them on all there VFD motor applications. The company is Shaft Grounding Systems, When I called this am they said their web site was broke and it sounds like they don't know how to fix it. If you call them they will fax you lots of stuff but other wise there back in the stone age. The papers they have are very good.
The guys who invented the system came to an IEEE IAS meeting here and gave a very good presentation. They know the problem and what to fix it with.

 

[gsanders]

We are having bearing failures on 300 and 100 hp motors using siemens drives. We have 4 toshiba 300 hp motors 2 us electric 300 hp motors and 2 siemens 100 hp motors all on siemens vfd's. Alot of people have been telling me it could be a ground issue but they can not tell me why or how to solve it. Any suggestions or hints.

 

[electrageek]

Call and talk to Hugh Boyanton, the President and owner, of Shaft Grounding Systems. He might be able to help. They have their headquarters in Albany, Oregon. I don't have a phone number but directory assistance should have it.

 

[jbartos]

Suggestion: The common mode currents have to be dealt with, e.g. by the proper grounding of the rotor and stator, right principle of variable speed frequency drive, etc.
Some previous postings are availablve in this forum including the complete list of AB papers written on this topic.

http://www.ab.com/drives/techpapers/ieee/ieee.html

 

[rodar]

The best way to solve the ground issue
is to carry a unique ground wire to
each motor that is bundled with the
phase wires for that particular motor.
The idea is to keep the loop area between
the phase wires and the ground wire small.
This minimises the inductance of the return
path for common mode currents.
Carry this ground wire into the panel
and land it on the ground terminal of
the drive that is associated with it.