When performing vibration analysis, it is observed that machines with VFD drives have a more frequent occurrence for changing motor bearings. If you’ve been diligent in the installation and maintenance of these machines, ensuring the alignment and balance are within specification and the bearings are properly lubricated, but still have frequent bearing failures, it may be your VFD drive.
I’ll share this one experience and a little information concerning current, how it’s induced along a motor’s shaft, and what to do to fix the problem.
Vibration Testing
It started when bearing frequencies on the motor were analyzed. This motor, like several others on this process floor, experienced a bearing defect. Vibration levels on these machines were low and everything, according to ISO vibration standards, seemed fine. Shown below are the spectra and waveform from the defective bearing.
Determining the root cause
Sometimes you just need another tool. I purchased an Electrical Discharge Detector that measures the electrical flux generated around a motor. This meter is simple to use and doesn’t require intrusion into the switchgear.
This meter takes an average of the flux, over a 30 second interval, encountered around the motor casing and, with value acquired, a determination is made as to whether the probable cause of the bearing defect is electric current passing through the bearing.
In a VFD there are two types of current induced along a motor shaft.
Capacitive Current
If you’re not familiar with how a capacitor works, it stores energy by having two conductive plates with opposite polarization sandwiched around an insulator. In a motor, the rotor and stator generate Capacitive EDM because of their close proximity, polarization of the rotor and stator, and with the air gap acting as the insulator.
High Frequency Circulating Current
Induced by the magnetic flux imbalance around the motor shaft from the windings. This current is thought to be produced only in motors of 100 HP or greater.
What kind of damage would you expect from electrically induced current?
Electrically induced current can cause Electrical Discharge Machining (EDM) which can produce three types of bearing race damage, with only one of these visible to the naked eye.
- Microscopic Electrical Pits: These pits can be created by the current flow through the bearing. This pitting is caused when current passes from the shaft, through the inner race, through the rolling bearing elements, through the inner race, then passing from the motor housing to ground. These pits are generally between 5 and 10 microns, thus requiring a microscope for identification.
- Frosting: Frosting is a grey discoloration that can be generated either by mechanical or electrical means and, again, requires a microscope to identify the source.
- Fluting: Fluting is cause by arcing of the current between the races and rolling elements. Fluting is visible to the naked eye and is identified by the washboard pattern created on the bearing race. A picture of fluting is shown below.
How do we fix the problem?
Because there are two currents often generated by VFD’s, there are also two means of correction.
For motors under 100 HP it is thought that only Capacitive Current will be a failure mode and the fix would require installation of a Bearing Protection Ring on the motor’s DE. This would “short circuit” the current from the motor casing to ground rather than providing a current path through the bearings.
For motors over 100 HP both Capacitive Current and High Frequency Circulating Current are thought to be destructive to bearings. In this case, the Bearing Protection Ring is still required on the motor’s DE to provide a path for the Capacitive Current, but an additional layer of protection is required for the NDE bearing due to the High Frequency Circulating Current. It is necessary, in the case of motors over 100 HP, to install an isolating or insulating bearing on the motor’s NDE.
