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Motor bearing failures with VFD

Common mode currents are motor bearing killers. 1) use as low a carrier frequency as the VFD will allow. 2) make sure that one bearing is grounded and the other is insulated 3) use a dv/dt filter 4) make sure that the bearings installed are proper for the load, roller or ball bearings 5) Use an Aegis or Hedwig shaft grounding device.

We have recently changed the lubricating grease for these machines and had discussions with the machinery manufacturer regarding grease type, quantity and interval to make sure we are maintaining everything correctly. The first failure we experienced saw the bearing balls "eroded" away to look like pebbles on a beach. The second failure on the same motor was a seized bearing on the DE (motor is directly coupled to a compressor air-end with a flexible rubber insert). This unit was an 80kw motor which since we replaced the motor has run for two years without any problems. (The customer insisted on a new motor after the second failure took place only 4 weeks after the first repair). Our re-winder told us there was no need for insulated bearings as the motor end cap and retaining bolt holes were already insulated. I am still skeptical about that. Unfortunately I haven’t been able to find the bearings we removed from the failed unit to have them examined. We have had a similar problem on a 37kw and a 55kw version.
Unfortunately the smaller one cannot be greased as it uses "sealed for life" bearings. We are now scheduling in the re-bearing of this motor at about 6000 hour intervals so as to prevent an out-of-warranty failure. The lead length from the inverter is quite short on all units - max 2 meters. The frequency range varies depending on the kw rating of the machine - motor rpm is approx 1500-7000, once again depending on kw rating.

These failures usually will occur faster in a high torque situation at low to medium speeds like an extruder.
Also, if you are using a gearbox directly coupled to the motor, take the same precautions with it. Common mode can be transferred through it and cause similar issues.
Concerns others have raised about lubricants are valid. Make sure the lube is NON conductive.

On these large horsepower motors typical design for variable frequency drive power would be to insulate ODE, opposite drive end bearing and provide a shaft current device to mitigate any bearing currents (Aegis). Also maintain per the motor manufactures guidelines amount and frequency of greasing, dependent on temperature, running hours, atmospheric conditions/application. The recommended grease for electric motors has been Siemens, Reliance for a number of years as typical industrial applications.
The other thing not mentioned is lead lengths and switching frequency. It is not uncommon with various manufactures and drive designs to see common mode noise which will become an issue quickly with lead lengths in excess of 100 feet. There are numerous ways to help reduce this using reactors on the drive output, reducing switching frequency, dv/dt filters, etc.

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