VFD's and bearing "Fluting"

The subject comes up now and then that VFD"s can - not always DO - damage the bearings in a motor.

Too often we argue over it when we need not, and vaguely point to photos of the evidence online "somewhere" to support the observation.

Here is something more specific that is easier to find. May even be worth making it a "sticky" so we do not need to discuss it again, and again, every few months, forever:

Shaft Grounding - Helwig Carbon

Note the damaged bearing in the photo. There is a link covering Electrostatic discharge management in general - VFD not necessarily involved, nor even a motor. Also a "white paper" for download.

CAVEAT: Helwig Carbon is primarily a manufacturer of BRUSHES for motors. That implies DC motors, at least "mostly".

They, of course, have competition, both as to brushes in general and use of brushes in the protective devices shown on the cited page.

OTOH, they have spawned a line of business with multiple volume-produced product-offerings to address a problem.

Such a problem must then actually exist, out there in the wild, even if - as said - "not always".

FWIW - Helwig or any other maker - brushes are not the only solution, either.

I've seen this twice on small motors.
It shows up as what looks like corrosion between the shaft and inner race. The fit is looser because of it.
Both of those cases could have benefited from good general shielding/grounding practice. I don't think the erosion would have happened if that had been done. Neither has shielded wiring.

One of those motors I like so I fitted it with ceramic bearings and it's now part of my flex shaft grinder. Ceramic bearings should prevent this too but any such steps are not needed in a good installation.

IMO this is something only seen in poor installations and there's no need for such draconian steps like brushes, ugh.
A product like that could be considered preying on ignorance which is widespread. How many VFD installations must be out there where a VFD was added to an existing motor and the existing unshielded wire left in place.

you should be able to observe the bearing current with a rogowski coil. for this application you can build your own, use a passive integrator, a resistor and capacitor. though its easy enough to build your own integrator.

a voltage probe on the shaft on the other channel of your oscope should help you connect the dots.

it is a real problem. its a problem even for the 608 bearings in your vfd driven heat pump condenser fan.

It's usually a bigger problem for a 480V motor than a 220V motor of the same size. More bearing current through the same capacitance to the rotor.

The windings have capacitance to the rotor, and the pulse currents (common mode) from the VFD cause current to flow through that capacitance, and consequently through the bearings, which are usually the only "ground path". The current causes micro-welds and arc pitting on the races.

Countermeasures:

1) ground the rotor with a contact on the shaft

2) shield the stator windings

3) insulate the bearings, either from the shaft, or from the case of the motor (either usually along with the shaft contact)

Many VFD-ready motors have bearings insulated from the case to prevent that current.

johansen said:

your oscope should help you

Click to expand...

Well there you have it. The frustration that as interested as I was in electron herding, I could only see a tiny fraction of what the little buggers were up to with a meter. Enter the Oscilloscope.

Age 12 before a hand-me down blessed me with one of my OWN. Soon, there were two or more. Long before I could afford a dual-trace model, two cheaper ones had to be made to serve.

Sixty years now, and the only time I have NOT had at least one 'scope was during military service. OTOH, if the mission needed one, better ones than I could afford were "there".

Not a lot of mystery to electron-herding for those many of us who use a 'scope "often".

Bigger mystery is how and why those who do NOT use them form their opinions.

To be fair, a lot of folks do not even have a decent clamp on meter, or probe for their meter.

And, to read the currents involved with the bearing problems, you need a better and generally more expensive probe than the usual types. Sometimes you can read a voltage on the shaft, but usually the grounding is good enough that any voltage is very small. That does NOT mean the currents are not there.

Rogowski coils? OK, but they are not as sensitive as a good wide bandwidth current clamp. They may not be as limited in bandwidth, but...... Just another thing to calibrate... and if you CAN calibrate them, then you probably also can directly measure the current you want the Rogowski coil for.

JST said:

To be fair, a lot of folks do not even have a decent clamp on meter, or probe for their meter.

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I have three. All different. Mind - one is specialized for clamp-around Dee Cee readings, and not everyone needs those.

Meters? Not so much about the dozen or so I am NOT using, but that most things need at least two, if not, three attached.

Otherwise - and this gets hairy at high voltages - one is hopping about connecting, dis-connecting, re-connecting, switching modes and ranges instead of just eyeballing each of several in turn and noting their tell.

As to the current that can damage bearings? Don't care at all about measuring, calibrated or crude. Only want to know if there is evidence it is "there". ANY evidence.

One then has to FIX that.

'Scope doesn't "fix" a problem. Only exposes it AS a problem. ELSE not.

JST said:

Rogowski coils? OK, but they are not as sensitive as a good wide bandwidth current clamp. They may not be as limited in bandwidth, but...... Just another thing to calibrate... and if you CAN calibrate them, then you probably also can directly measure the current you want the Rogowski coil for.

Click to expand...

and how are you going to fit it inside the motor around the shaft?


they certainly are severely limited in bandwidth. a good one you pay money for might have 3db point at 10mhz far as i recall. PEM is claiming 3 db at 20mhz for some of their coils, which are so small in diameter you can fit them between the legs of a transistor with .1 inch pin spacing

for this application, they are easy enough to make. take some coax, strip the shield back, wrap a helix of wire around the insulation solder it to the shield and to the center conductor, and wrap it around whatever you want to measure the current flowing through.

one method of calibrating a rogowski coil involves opening the coil of wire and inserting any standard signal generator in series with the coil, since its relatively easy to get 10ns rise time or less with a signal generator but rather hard to get 10ns rise time of significant current through a conductor..

Now there is something we never thought about....

Granted we have motor/generator action but having some difficulty getting full handle on current flow although it makes sense.

And considering a turns ratio between motor and armature the voltage developed by the rotor would be small but then current high which indeed could arc the bearings.

Proper film in the bearings would insulate some so it seems to make sense but a vfd should not matter as ac is ac.

For our intermittent use likely never to see this but a 480 vac high hp motor running many hours it could be something.

Sent from my SAMSUNG-SGH-I337Z using Tapatalk

Monarchist has a point. There is going to be current unless you can shield the coils.* What you want is to make sure the current is NOT through the bearings, but goes somewhere else.

You send the current elsewhere by insulating the bearings, and then providing a drain for any charges on the rotor.

With that, you will see current regardless, all you are doing is "steering it".

As for the Rogowski coils, the usual power current pickup goes, if it is a good one, no higher than 10 or 20 kHz. I have a 150A Fluke clamp-on that does that, and a meter to match bandwidth. So even 5 or 10 MHz is very good by comparison. And a good high current 60 MHz clamp probe is both expensive, and limited in "window area". I have a 60 MHz pickup, but it is only good to an amp, with a very small window for wire.

A Rogowski coil would be easier to use, but not lacking in its own problems.

Tony Quiring said:

Now there is something we never thought about....

Granted we have motor/generator action but having some difficulty getting full handle on current flow although it makes sense.

And considering a turns ratio between motor and armature the voltage developed by the rotor would be small but then current high which indeed could arc the bearings.

Proper film in the bearings would insulate some so it seems to make sense but a vfd should not matter as ac is ac.

For our intermittent use likely never to see this but a 480 vac high hp motor running many hours it could be something.

Sent from my SAMSUNG-SGH-I337Z using Tapatalk

Click to expand...

No turns ratio is involved. There is no end-to-end current involved, at least that is not the main issue. I'd have to think about it to see what it would take to provide a driver for a circulating current, it would need to be a transverse field, going in a circumferential path. It's probably possible, although the existing steel will tend to "short it out". Might be more of an issue with a larger salient pole machine. Coil imbalances maybe could drive such a field. Driving voltage should be quite low, where the capacitive currents have a very high voltage driving them.

main issue is just capacitance from coils to rotor. Circuit is completed through the bearings.to ground, and back to the VFD.

* Emerson, or maybe Nidec now, has a patent on one way of shielding, invented by a guy who was my boss at one point. It uses metal wedges in the slots, over insulation, of course. There are other ways, and the slot covers do not take care of the end turns, which also have capacitance.

Water lubricated well pumps have a .001" thick tube of stainless steel laser welded to the inside of the motor, then hydraulically expanded while the epoxy injected into the windings cures. The steel tube itself is effectively a "shorted turn" though how much power it sucks up I have no idea.

johansen said:

The steel tube itself is effectively a "shorted turn" though how much power it sucks up I have no idea.

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That thin, its cross-section limits what it could carry, so it must be de minimus, or they'd fail from thermal stresses.

And a lot of stainless is a lousy conductor.... maybe there is one that is no worse than regular steel.

In a well pump it certainly should be effectively cooled.

That shielding should pretty well take care of any bearing currents, though. Assuming the SS is connected to the shell somewhere/somehow..

JST said:

And a lot of stainless is a lousy conductor.... maybe there is one that is no worse than regular steel.

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Not prudent to build a primary-cell battery into any part of a well pump, however inefficient. They have challenges enough as it is.

Fair bet it is a match to the REST of the Stainless used just to keep that part sweet.

msimard said:

I've seen this twice on small motors.
It shows up as what looks like corrosion between the shaft and inner race. The fit is looser because of it.

Click to expand...

I'm not buying to that. Sounds like bog standard fretting and you don't need VFD or electricity for that.
"fluting" marks on the rolling surfaces are the most obvious tell-tale of leakage current.

JST said:

Countermeasures:

1) ground the rotor with a contact on the shaft

2) shield the stator windings

3) insulate the bearings, either from the shaft, or from the case of the motor (either usually along with the shaft contact)

Many VFD-ready motors have bearings insulated from the case to prevent that current.

Click to expand...

4) conductive grease for the bearings.

MattiJ said:

I'm not buying to that. Sounds like bog standard fretting and you don't need VFD or electricity for that.
"fluting" marks on the rolling surfaces are the most obvious tell-tale of leakage current.

Click to expand...

+1

Prezactly.

Fluting shows up where the rolling-elements contact their race.

Anything else is SOMEthing else.

Conductive grease? maybe.... it has to be better than gbearing steel, which is doubtful.....

Monarchist said:

Not prudent to build a primary-cell battery into any part of a well pump, however inefficient. They have challenges enough as it is.

Fair bet it is a match to the REST of the Stainless used just to keep that part sweet.

Click to expand...

Just thinking about the shielding aspect, that would need the metal grounded, preferably, although it would do something even if "floating". The dissimilar metals I assume they have handled, no doubt they have built more than just one......

I managed to edit my own reply about conductive grease and I am too lazy to type it again so I'll just re-copy links here:
In focus - News - Kluber Lubrication
Conductive lubricants will protect the electric motors of the future - Bosch Media Service

And option nr 5. would be dv/dt filter after VFD. Maybe more commonly seen on bigger installations.
Basically it filters the high frequency content of the VFD output and reduces capacitively coupled leakage currents. (And maybe even more importantly makes life easier for the motor insulation, by reducing high frequency content/high voltage spikes)

That's a different issue. Static charges can be easily dealt with by fairly high resistance discharge paths. A megohm or more may be sufficient conductivity, because the charge is built up over time, and not an instantaneous event (the DISCHARGE is an instantaneous event, but that is being prevented).

The VFD issue is completely different, and requires a low resistance path. Current will divide according to the inverse of resistances, so the lower resistance path gets more. That is likely to be the metal to metal contact in the bearing, at the point contact (nearly so) between ball and race.

Greases that will work great for getting rid of static charges are almost surely not good enough to help with the capacitively induced currents, probably by a factor of 10,000 or more..

Yes, a DVDT filter will help. But there is a limit to how much filtering you can do without high costs. Grid-tie inverters use sine reconstructon filters, but they are required to do that, they have harmonic limits, so there is no disadvantage to the fact that they filter the fast pulses as well. VFDs are a tough competitive market, and "better" does not always sell well compared to "cheaper".


You can do pretty well with just an external inductor, but it has to be wound in such a way that it does not just let pulses through capacitively. Sectional winding is best.

With regard to the "fluting" issue, because the VFD pulses are random vs rotational position, I would NOT expect them to be a cause of a regular pattern in most cases. It could happen, but I would suppose a regular pattern would have a mechanical cause in nearly every case.