VFD Causing Bearing Failure
VFD Causing Bearing Failure
(OP)
I have searched on this and found some discussion, but I am curious as to what anyone has found to be a good solution to the problem. In my case, I had a new 200HP motor/pump setup(pumping chilled water) that the bearings failed after about 6 months. The motor is fed by a new IGBT VFD. A single ground wire initiating at the source is run straight through to the motor and grounded at the termination box. There are currently no chassis grounds.
I have found much discussion online, particularly a white paper by Reliance Motor, but I was looking for some real solutions that have worked.
Right now we are looking at a ring assembly sold by a company called Shaft Grounding Systems. I am also considering adding some chassis grounds, and/or refeeding the motor from the VFD with continuously corrugated cable.
Any input is welcome.
I have found much discussion online, particularly a white paper by Reliance Motor, but I was looking for some real solutions that have worked.
Right now we are looking at a ring assembly sold by a company called Shaft Grounding Systems. I am also considering adding some chassis grounds, and/or refeeding the motor from the VFD with continuously corrugated cable.
Any input is welcome.





RE: VFD Causing Bearing Failure
RE: VFD Causing Bearing Failure
RE: VFD Causing Bearing Failure
RE: VFD Causing Bearing Failure
Does your motor have insulated NDE bearings?
RE: VFD Causing Bearing Failure
RE: VFD Causing Bearing Failure
RE: VFD Causing Bearing Failure
Grounding rings can be installed in place without motor disassembly and have the added benefit of blocking some contaminants from getting into the bearing.
RE: VFD Causing Bearing Failure
http://w
RE: VFD Causing Bearing Failure
I agree, insulated bearings are expensive and I wouldn't generally advise unless they were taking damaged ones out and refitting new ones. It might make sense to do this at this time.
Richanton
Also have a look at this:
http:
designed and developed by one of our own top contributors in Eng-Tips. No affiliation to me but if he is looking for an agent in Australia...
RE: VFD Causing Bearing Failure
Is this a Weg motor by chance?
RE: VFD Causing Bearing Failure
That link to the Beppe meter is very interesting. The Swedish engineer even gave a name to it, EDM.
RE: VFD Causing Bearing Failure
I thought insulated bearings were required on a NEMA "inverter-duty" motor?
"An 'expert' is someone who has made every possible mistake in a very narrow field of study." -- Edward Teller
RE: VFD Causing Bearing Failure
Every manufacturer does their own thing on these terms. For some, like the former Reliance, they are very conservative. For others, any Insulation Class F motor qualifies for at least Inverter Ready.
For my money, stick with MG1 Part 31. A motor with that endorsement on the nameplate has been tested with high frequency pulses up to 1600V and has high temp grease in the bearings. Even MG1 P31 does not require insulated bearings.
RE: VFD Causing Bearing Failure
The following is the section out of MG 1 that pertains to "Definite-Purpose Inverter-Fed Polyphase Motor" bearings.
"31.4.4.3 Shaft Voltages and Bearing Insulation
Shaft voltages can result in the flow of destructive currents through motor bearings, manifesting themselves
through pitting of the bearings, scoring of the shaft, and eventual bearing failure. In larger frame size motors, usually 500 frame and larger, these voltages may be present under sinusoidal operation and are caused by magnetic dissymmetries in the construction of these motors. This results in the generation of a shaft end-to-end voltage. The current path in this case is from the motor frame through a bearing to the motor shaft, down the shaft, and through the other bearing back to the motor frame. This type of current can be interrupted by insulating one of the bearings. If the shaft voltage is larger than 300 millivolts peak when tested per IEEE 112, bearing insulation should be utilized.
More recently, for some inverter types and application methods, potentially destructive bearing currents
have occasionally occurred in much smaller motors. However, the root cause of the current is different. These drives can be generators of a common mode voltage which shifts the three phase winding neutral potentials significantly from ground. This common mode voltage oscillates at high frequency and is capacitively coupled to the rotor. This results in peak pulses as high as 10-40 volts from shaft to ground. The current path could be through either or both bearings to ground. Interruption of this current therefore
requires insulating both bearings. Alternately, shaft grounding brushes may be used to divert the current
around the bearing. It should be noted that insulating the motor bearings will not prevent the damage of other shaft connected equipment.
At this time, there has been no conclusive study that has served to quantify the relationship of peak voltage from inverter operation to bearing life or failure. There is also no standard method for measuring this voltage. Because of this, the potential for problems cannot consistently be determined in advance of motor installation."
RE: VFD Causing Bearing Failure
RE: VFD Causing Bearing Failure
RE: VFD Causing Bearing Failure
Gunnar Englund
www.gke.org
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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
RE: VFD Causing Bearing Failure
One poster was correct the current has to go some where, that is while the chassis of the motor, vfd and the gounding conductor coming in on the supply all have to be bonded, especially between the motor and VFD. If you check the case of the motor to the vfd ground you will find AC current on the ground when they are gounded together. I have seen motors that were being driven by a VFD draw over 100 amps on the grounding conductor. The voltage would be low but the current would be high. I have also been shocked hard from motors that were not grounded properly, this also resulted in grounding currents on the shaft of the motor. Anytime you have gounding current on the motor shaft you will have arcing across the bearings. Shaft grounding kits only work when the currents are coming into the motor from the shaft end. The kit grounds out the currents on the shaft before it can reach the bearings. Same reason you never ground a welder to a structure and then weld somewhere else so current winds up flowing across a bearing surface. The kit can bypass the bearing but only if it is grounded to the motor itself.
A pocket voltage tester like the Fluke tester can help in determining if the motor leakage is high enough to be discharging to the case and grounding is not good enough.
Something else to check is to megger the motor at 1000V (because you are running a VFD) and see what the leakage is. You could be running in the low megohms which will allow the case of the motor to be energized without tripping the motor. I found one like that and it ran for months before it failed. We haven't asked how far the motor run is from the drive or how it is run (PVC, EMT, RIGID, ALUMINUM). The longer the run the higher the ringing voltages are across the motor and this results in higher voltage spikes on the motor insulation. That is one of the things that an inverter duty motor takes care of, it uses a much higher class of insulation (I believe Class H) which reduces the leakage to the motor frame.
I wish you luck and would recommend installing a 3-5% line reactor on the output of the drive if the run to the motor very long.
Mike
RE: VFD Causing Bearing Failure
Keith Cress
kcress - http://www.flaminsystems.com
RE: VFD Causing Bearing Failure
RE: VFD Causing Bearing Failure
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RE: VFD Causing Bearing Failure
If you insulate the NDE and put a ground brush on the DE then the ground brush holds the shaft and the motor case at the potential on the DE and that bearing doesn't necessarily need to be insulated. If you just ground brush both bearings you can possibly end up with circulating shaft currents. If you just insulate both bearings then high frequency components can still transfer across the bearing insulation due the capacitance of the insulation.
RE: VFD Causing Bearing Failure
This link is to a pretty good practical article on the problem and remedies.
http://w
Thanks
RE: VFD Causing Bearing Failure
Gunnar Englund
www.gke.org
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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...
RE: VFD Causing Bearing Failure
Thanks for sharing that.
Dave
"An 'expert' is someone who has made every possible mistake in a very narrow field of study." -- Edward Teller
RE: VFD Causing Bearing Failure
To reduce the audible motor noise and telephone interference factor associated with PWM drive switching rates between 1 kHz and 8 kHz, manufacturers developed VFDs with switching frequencies as high as 20 kHz. While the higher frequencies did reduce motor noise, they also aggravated PWM pulse contribution to excessive motor-shaft voltage and current that can discharge across motor bearings. VFDs that use insulated-gate bi-polar transistors (IGBTs) as high-frequency switches are most likely to cause bearing discharge current
You can install a high-frequency compensated reactor on the load side of the drive, either at the drive end or motor end
lukin1977
RE: VFD Causing Bearing Failure
The grounding rings or brushes being sold are not to simply ground (earth) the shaft; but rather, to shunt (bypass) the harmful currents around the bearing, rather than allowing the currents to flow thru the bearing.
Inverter duty (rated) motors have this provision built in, or use insulated bearings to avoid the problem. Inverter duty motors also have higher voltage insulation to withstand the modulation frequency spikes from the VFD. They also have higher heat tolerance since a VFD causes up to 5% extra heating in the windings, again due to the modulation currents.
Running a 115% S.F. (Servide Factor) motor at 100% of FLA is enough derating for the thermal considerations; but, using a conventional motor on a VFD requires attention to both the bearing current issue as well as the voltage withstand issue.
Hope this helps.
Regards . . .
Jim S. Nasby
RE: VFD Causing Bearing Failure
Bill
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"Why not the best?"
Jimmy Carter
RE: VFD Causing Bearing Failure
http://www.nema.org/stds/acadjustable.cfm
RE: VFD Causing Bearing Failure
A LPS for that useful link.
RE: VFD Causing Bearing Failure
Picture 5-12, for instance, does not at all show a typical shaft voltage for a VFD fed induction motor.
Picture 5-13 is a little better, but still wrong. Typical EDM voltages are between 5 and 15 V. Not around 500 mV as shown in the picture.
Gunnar Englund
www.gke.org
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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...