VFD Drive - 3 Phase Motor - Bearing Life Issues?

Hi,

I am fairly new to the forum. I have a 10K lathe currently without power. I have researched the variable speed mods and reviewed the VFD / 3 phase motor and the DC motor / controllers.

The local electrical motor shop warned me about VFD drives as he has seen bearing failures due to electrical arcing thru the bearings. I presume this is due to the excessive eddy currents and under speed conditions perhaps. He said this is an issue even if the motor is inverter rated.

My question is: Has this been an issue for smaller 3/4 or 1 HP motors in "hobby use" as with the South Bend lathes?

Thanks

John

While your mileage may vary, I have run many motors of many different sizes for 25 years + and never had a bearing failure attributed to a frequency drive. I would guess that if there was any foundation to that theory, the culprit would be the carrier frequency of the drive passing through a bearing.

Stay tuned for the Guru's of VFD's!

Stuart

I have seen several bearing failures in 15 HP VFD-controlled motors. The cause was arcing due to the very fast rise time of the VFD pulses. We installed grounding brushes to give the transients a return path other than through the bearings.

But these motors run 24/7 for many days at a time.

I expect you will have no problems with a fractional HP motor run infrequently. And if you do, bearings are cheap and easy to replace.

Jon (definitely not a VFD Guru)

AKJOHN62 said:

Hi,

I am fairly new to the forum. I have a 10K lathe currently without power. I have researched the variable speed mods and reviewed the VFD / 3 phase motor and the DC motor / controllers.

The local electrical motor shop warned me about VFD drives as he has seen bearing failures due to electrical arcing thru the bearings. I presume this is due to the excessive eddy currents and under speed conditions perhaps. He said this is an issue even if the motor is inverter rated.

My question is: Has this been an issue for smaller 3/4 or 1 HP motors in "hobby use" as with the South Bend lathes?

Thanks

John

Click to expand...

It can happen. There are even photos of it while it is going-on online in maker's 'white papers'.

You don't have to re-engineer the motor, though. Just look around for a basic dv/dt filter choke to place between VFD output and motor. The VFD maker will publish the correct size, small ones are not terribly costly, and they are 'out there' in the used market - eBay & such..because...

Most modern VFD have cleaned-up their act and.. don't often need them any longer.

"short answer?" Just make sure you buy a decent VFD - new - wire it by their book, and fageddaboudit. "Ancient" VFD are a bad dice-roll, anyway, even if old motors are still good ones.

Old VFD as has had the "F" abused, neglected, or aged clear out of it, all you have left is a case load of "VD", and that ain't ever good, is it?

Bill

I've done more than 10 VFD installations in the past 7 or 8 years using motors ranging from 1/3 to 3 HP. All are still running and none has suffered a bearing failure. I've read the literature about the bearing issues with VFD's and it all seems plausible but I've not seen any problems. I think that motors much larger than this may be more prone to trouble because of the much larger currents involved. For home "hobby" operation with smaller motors I doubt you'll ever see a problem. Keep in mind that replacing a bearing on one of the smaller motors is really easy to do if a bearing problem should develop.

Monarchist said:

It can happen. There are even photos of it while it is going-on online in maker's 'white papers'.

You don't have to re-engineer the motor, though. Just look around for a basic dv/dt filter choke to place between VFD output and motor. The VFD maker will publish the correct size, small ones are not terribly costly, and they are 'out there' in the used market - eBay & such..because...

Most modern VFD have cleaned-up their act and.. don't often need them any longer.

"short answer?" Just make sure you buy a decent VFD - new - wire it by their book, and fageddaboudit. "Ancient" VFD are a bad dice-roll, anyway, even if old motors are still good ones.

Old VFD as has had the "F" abused, neglected, or aged clear out of it, all you have left is a case load of "VD", and that ain't ever good, is it?

Bill

Click to expand...

The bearing failure rate can be reduced by cleaning up the carrier signal, but ultimately, due to the way VFD's control the motor speed, there's no way to completely eliminate the static build up. Installing a filter choke may help, but you'd be better served by getting a VFD from a company that deals with airhandler manufacturers like ABB, Danfoss, Yaskawa and Novatorque; their built-in chokes and filters are fantastic. I'd look at them in that order, as well. We use ABB as a standard VFD in our fanwall systems,

The elimination options are: shaft grounded bearings (which has been mentioned), ceramic bearings (which is expensive and requires opening up the motor case) and the use of a Faraday shield on the bearings. The first two options are the most common.

Just to be safe, though, I would simply slap a shaft grounding kit on to the back of the motor and call it a day. There are two that I would suggest:
1) Helwig makes an outstanding shaft grounding kit that is low clearance so you shouldn't need to permanently remove the fan (if you put it on the back) and may comfortably sit on the face of the motor next to the impeller shaft depending upon where the motor's installed in the housing of the lathe.
2) & Aegis which is a low/no maintenance alternative and may actually be perfect for you since you're not going to be running your lathe 24/7/365

Cheers,
~J.

Bah, I couldn't find the edit button.

I deleted a part of the post above which makes the paragraphs seem to jumble upon each other. Between the second and third should be:

[...]are the most common.

Click to expand...

The VFD/bearing failure issues that I have typically seen or dealt with, involved the larger (5hp+) motor which makes sense since the larger motors tend to the 440v/3phase voltage which requires a higher carrier frequency to replicate through the inverters. With the lower power motors (remember, Power(hp) = Voltage(V)*Current(I)), like the ones used in lathes, it's less likely to become an issue.

Just to be safe, though, I would[...}

Click to expand...

An ounce of prevention is worth a pound of cure.

Air handlers that I have seen with fretted bearings on the motors are 24/7 operating and have computers controlling their speed. These machines often stay at an exact speed for long periods of time. Just the opposite of what you do wit lntermittent use and manual control.