Shaft Grounding Safety Issue 1

[Mysterrose]

Mechanical Engineer here trying to wrap my head around electrical question. I have a 1000HP, 600VAC VFD driven motor. I understand the VFD induced bearing currents fairly well, the part I'm having trouble wrapping my head around is related to a possible safety issue.

Lets for now assume no attache equipment for simplicity and just look at motor. If I insulate both bearings, that stops any bearing currents from damaging the bearings, but does not stop the voltage building up on the shaft. The shaft voltage will still be present on the shaft and trying to find the lowest impedance path to ground.

My question is, does this pose a safety issue? For example if somebody were to come close or perhaps even come in contact with the shaft, would they get hurt by the voltage present on the shaft? If not for this motor, at what hp/voltage motor might this become a safety issue?

 

[Skogsgurra]

You are actually touching (pun? perhaps) upon something that I have asked the Authorities to look into for several years now.

The voltage may reach quite high values even if there's no defect in the machine. I have measured around 70 V peak on a large motor with insulated bearings. That is bad per se, but the real problem is that most Codes require that "Every externally accesible part that may come in contact with live circuitry carrying lethal voltage shall be grounded for safety reasons"

The exact wording differs between countries, but the gist of it is that anything that can carry more than ELV/SLV shall be connected to ground.

I did put the question to the Swedish National Safety Board several years ago. No answer so far.

The problem is that the question comes as a total surprise to most people. Non-insulated bearings do the grounding in case of a motor failure where the windings comes in contact with the rotor. The quality of the "grounding" through the bearings has been taken for granted and has never, to my knowledge, been questioned. When the bearings are insulated, there is every reason in the World to question the quality of the non-existing "grounding"

In DC Machines, the situation is even more alarming because the Windings are placed on the rotor and the risk of a failure that puts the rotor under voltage is really high.

The last frame in this presentation:

http://www.gke.org/presentationer/files/GE Bearings and their electric environment w O and E.pdf

says something about the problem.

It is a very good question you put there. I really hope that someone will take it seriously at last.

Gunnar Englund

http://www.gke.org

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

 

[LiteYear]

Here in the mining industry in Australia it is indeed considered a safety hazard by many. Generally it's unlikely that contact with the shaft would be possible in the first place - the more obvious danger of a rotating member means permanent guards or interlocks are generally required. And generally speaking, any bit of metal that could possibly be touched is grounded. However, the voltage hazard remains if the shaft enters a hazardous zone - in that case, discharge of the shaft voltage may take place in the form of a spark which of course is a major issue in a hazardous area.

There was a serious explosion at an underground mine in New Zealand a few years ago. No proof of the cause has been established, but one possibility posited was that a voltage discharging from the shaft of a VFD driven motor caused a spark in the hazardous zone. In this case the VFD and motor were in the safe zone, but the shaft passed through into the hazardous zone.

As Gunnar's data suggests, the HP of the motor is not well correlated with the severity of the hazard. The voltage is a better predictor, though still not the whole story. The division ratio formed by the capacitance of the bearings is a key parameter. I've performed measurements on a 5.5kW drive that induced higher voltages than a 300kW drive due to those factors.

 

[PersonalProfile]

This may be of interest:

http://www.eng-tips.com/viewthread.cfm?qid=316664

Regards,
Lyle

 

[squeeky]

We have had much greater voltages. You should only insulate one side to stop the circulating currents. If you insulate both, as you say, the voltage will build up from static electricity. Now there is only a thin film of lubricating oil electrically insulating your shaft from the ground of the bearing. Should there be a sudden load change and this film gets thinner due to shaft loading, there may be a flash over and bearing damage. This flash over can occur even if there is no oil film distortion if the voltage builds up high enough. If you look at the bearing you may see a series of small lines or a line of holes. This is the discharge. Mechanical people may say there was some dirt caught in it.

The voltage may not come from a VSD. Your supplier may fit chokes and the like and tell you it is not from his equipment. People use clock gauges with magnetic bases to align the shaft. The vibration guy comes with his probe and connects it with a magnet. If this magnetizes the steel locally near the shaft, you now have a magnetic field near a rotating conductor.

Improseal, Wyko and others have come up with a circular grounding brush. The normal carbon brushes are no good no matter how high the copper content is. They are designed to glaze over. The harder copper brush may damage your shaft. A gold bristle brush I find the best and connecting a high wattage precision 1 ohm resistor in series allows you to connect an oscilloscope to view discharge currents.

I tried to up load this once and failed. I am trying again. The old thread has a download which confirms what I say. I have had this happen. One photo shows the end of the turbine mechanical hydraulic speed adjusting link. After failing again I will upload one picture at a time. This one shows large blow holes from a lightning strike which caused a part of the shaft to become magnetic. We had to de-gause the shaft.

 

[squeeky]

Thrust bearings. This is not a mechanical issue.

 

[squeeky]

Going for two this time. Top bearing frosting and shaft damage.

 

[squeeky]

The end of the turbine hydraulic system.

 

[squeeky]

Close up of the shaft after they did some polishing. This is what started my investigation on this one.

 

[Skogsgurra]

Sorry to say this, squeeky.

But you are the victim of snake oil salesmen and wishful tinking.

Those magnetic devices, that you mention, have no detrimental effect on either shaft voltage or bearing currents.

There are quite a few other opinions of yours that neither I nor the majority of knowledgable people in the business will subscribe to.

You can better than that. I hope.

Gunnar Englund

http://www.gke.org

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

 

[DRWeig]

Gunnar,

With my enlarged prostate, I am also a victim of wishful tinking at times.

Sorry, couldn't help myself.

Best to you,

Goober Dave

Haven't see the forum policies? Do so now: Forum Policies

 

[controlsdude]

why cant you have a set of races with bearings attached to one side where the ground attachment is on the race, then the insulated bearing is in between these two sets?

race with ground

insulated bearing

race with ground

 

[LionelHutz]

The shaft could build-up stored energy so it could be a personnel hazard if they were the path for the discharge, But, I'd also say that in practice it's not a concern because the fixed metal parts of the machine are grounded and the rotating parts are guarded by grounded metal guards so no-one should ever come in contact with a rotating shaft.

FYI, shaft voltages are more common in VFD applications, but they can also occur without a VFD. We have a 5kV soft-starter at a site that experienced multiple rapid motor bearing failures with the classic washboard failure pattern indicating electrical discharge was the cause. So, they installed insulated bearings in the motor and then experienced similar rapid failures in the fan bearings. They couldn't believe the motor design was responsible for the shaft voltages and were convinced the soft-starter was causing the issue even though the shaft voltage was present with the motor running directly from line power using a vacuum contactor. I believe they finally got the motor manufacturer to install a decent shaft grounding system to stop the failures.