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How Long does it Take for Bearings to Flute

09/24/2021 11:08 PM

We recently installed two 150 HP pump motors that are driven with VFDs.

When doing rotation checks and run in we noticed pump #2 was making a growling noise, much louder than #1. My first thought was dry bearings or grease that had not been packed in the bearing properly or a mis-alignment of the rotor shaft. We ran it less than 5 minutes, had both of them up to full speed and shut them both off. #2 actually spun down (coasted) longer than #1. The vendor replaced the motor and told us the manufacturer told them the VFD had caused bearing fluting so they installed a grounding brush on the shaft. Is there a realistic way that a VFD can cause that in a bearing that quickly. We have installed over 100 of these motors and this is the first time I heard of this

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#1

Re: How long does it take for bearings to flute

09/25/2021 4:14 AM
  1. Fluting is damage caused to a bearing due to an electrical current induced on the shaft of a VFD (Variable Frequency Drive) machine.
  2. If a machine is rated at less than 100 horsepower, a carbon fiber brush may be attached to a shaft grounding ring (also called a split ring, solid ring, or a press fit) and installed on the drive side of the shaft. This will effectively redirect the current to ground and will avoid arching between the bearing components which could result in bearing failure.
  3. If a machine is rated at 100 horsepower or more, a non-conductive ceramic bearing may be attached to the opposite side of the shaft in addition to the carbon fiber brush. The ceramic bearing prevents the NDE of the motor from becoming another path for current discharge to ground.
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#3
In reply to #1

Re: How long does it take for bearings to flute

09/25/2021 4:58 AM

I agree.

GA.

VFD's can also cause problems in the motor itself,due the chopped square waves.

A motor not designed for VFD's will also suffer a shorter lifetime due to the chopped

square waves generated by the drive.

VFD designed motors have a variety of design features to prevent EDM of bearings.

Some use grounding brushes,some use grounding bearing seals,others use ceramic

coatings on bearings or ceramic bearings.

A VFD drive also requires a VFD rated cable for best results.

VFD's can also create problems in other equipment not attached to the VFD.

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#2

Re: How long does it take for bearings to flute

09/25/2021 4:53 AM

It generally takes several months for bearings to fail in this situation....the bigger and more important question is why was there no protection installed with the motor, it should have been ordered as a vfd driven motor, which should have included protection...

..."These discharges are so frequent (potentially millions per hour) that before long the entire bearing race surface becomes damaged with countless pits known as frosting. A phenomenon known as fluting may occur as well, producing washboard-like ridges across the frosted bearing race. Fluting causes audible noise and vibration and is an indication of a catastrophic failure mode. Regardless of the type of rolling element or raceway damage that occurs, the resulting motor failure often costs thousands of dollars in downtime and equipment failure related repair or replacement costs.

Failure rates vary widely depending on many factors, but evidence suggests that a significant portion of failures occur in only 3 to 12 months after system startup. All AC and DC motors operated by electronic drives or inverters have the potential of developing this failure in their bearings regardless of motor frame size or horsepower."...

https://www.mep.ca/Public/pdf/AEGISHandbook_English.pdf

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#4

Re: How long does it take for bearings to flute

09/26/2021 12:18 AM

Could the fluting have been caused, not by the VFD but by the motors being stored in an area of vibration such that the balls or rollers fret with the races and produce a washboard effect.

If the motors are not rotated often this can happen and before we put large motors into service the bearings were replaced with new bearings. Some of the larger motors had the bearings removed before storage and then reinstalled before commissioning. The rotors were even removed and sat on rubber while the stator was kept warm with the auxiliary heaters connected to supply.

A recent problem that I know of was with wind generators that sat on a wharf for months before being installed and every bearing need premature failure requiring replacement.

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#5
In reply to #4

Re: How long does it take for bearings to flute

09/26/2021 8:59 AM

This does seem a more likely scenario, that some damage was existing before installation, that could have been caused by any number of things....

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#9
In reply to #4

Re: How long does it take for bearings to flute

09/30/2021 6:10 PM

The issue of bearing damage during storage is called “false brinelling” and is different from fluting due to EDM damage.

The rough sound may be similar though, so identifying it will depend on a visual inspection of the bearings and races.

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#6

Re: How Long does it Take for Bearings to Flute

09/27/2021 5:04 PM

You speak of failure on #2 but not on #1. Assuming the motors were otherwise identical and the driven equipment identical I would look for either of two possible causes:

  1. A pre-installation fault that targeted #2 only. These have been mentioned by others.
  2. A sufficiently different installation in terms of distance from the drive, or differences in programming parameters.

VFD's attempt to produce an output waveform that has a voltage and frequency varying simultaneously, typically in a constant or near constant linear fashion. To do this the DC is chopped and put onto the output as pulses of uniform voltage but varying width and spacing--normally with an oscillator at a high-frequency (called the carrier frequency) controlling this. The output is filtered to smooth it into a waveform that is approximately sinusoidal. However the output has a substantial burden of harmonics, that cause problems when you get to the motor. An output reactor will minimize much of this but if the cable length from the VFD to the motor is long these harmonics can cause a lot of problems. An output filter will eliminate almost all of these harmonics, but they can be large and expensive.

Your vendor did the sensible first thing--putting a grounding brush on the motor shaft. Have a good drive engineer look your system over and see if there are other steps for you, such as changing the carrier frequency, or adding filtering.

--JMM

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#7
In reply to #6

Re: How Long does it Take for Bearings to Flute

09/29/2021 12:20 AM

Thanks everyone, I really appreciate the input.

I am familiar with the larger motors needing rotated during storage and keeping heaters on them, but these were only 150 HP and we have over 200 in service with this being the first occurrence at startup.

THe two at this installation were identical and used the same installation and programming as all the others. The motors were only energized with the drive for less than 5 minutes and surprisingly when we stopped the drives in coast mode the noisy (#2) motor actually spun longer than the other. So it was making more noise and seemed to be dragging on something but it really spun more freely.

I do not know the life story of these motors but I do know once they got to the fabrication shop they only sat around for about 3 months. Looking at the SN they appeared to have been manufactured within the past year

It is possible that there was damage during shipment or dropped harshly on a solid surface but that would have been before the fabrication steps. Once at the fab shop they were together almost the entire time and shipped together

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#8
In reply to #7

Re: How Long does it Take for Bearings to Flute

09/29/2021 10:28 AM

You might want to put a good scope on the outputs from each drive and see if there is an undiagnosed difference. This could point to a problem within drive #2.

--JMM

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