In the age of variable speed drives bearing fluting damage is becoming a dominant failure mode in electric motors.

I have experienced motors as low as 37 kW see damage in a matter of weeks. We strive to extend motor service life between overhauls to years and this attack is unacceptable.

This failure mode can be difficult to detect with Vibration Analysis as no defined impact zone exists but a board spread of damage. Trending the 1-20kHz alarm is by far the most effective. VA only detects the problem doesn't eliminate it.

Lots of remedies are discussed in the industry such as Insulated Bearings, Grounding Brushes, Motor Grounding, Drive chokes,  etc.

First a little experience we had with a 400kW motor which we detected bearing fluting. I grabbed an old temperature probe approximately 500mm from the rubbish heap and ground a point on the end, wrapped the probe shaft with electrical tape and headed off with our electrical supervisor with his Crow Oscilloscope.

I jammed the probe into the fan cowl and made contact with threaded shaft end and held a good pressure on it while the motor was running. Our supervisor attached the Crow and we measured 110 Volts DC spiking 3 to 4 times per second to ground. That means the bearings were seeing welding style current, no wonder they don't last very long.

This triangular voltage pattern is from rectifiers is another Common mode source and is caused by the clipping of voltage as the power passes through the diodes. Measured with a scope meter.

This graph above shows the Common Mode Voltage in a balanced power supply, if voltage imbalance, phase vector shift, or a lazy diode then this common mode voltage increases significantly.

We were recommended an insulated bearing by a major bearing manufacturer, well a short time after the problem came back. The PT100 temperature probe had cracked the outer protective layer.

We installed grounding brushes which worked really well until they wore out, placing them end on the fan end shaft is way better option. All these remedies are a band aid to the problem.

So what is the cause of Bearing Fluting?

The source of Bearing Currents are the switching devices most commonly used in Variable Frequency Drives.

When the Drive reforms the Sine Wave AC cycle at your desired frequency it does this by pulses of energy from the DC capacitors inside the drive.

Pulse Wave Modulation, the drive method of reforming power to the motor is not a sine wave but a series of timed pulses. The characteristics of the cable smooth the current to form a sine wave.

The voltage output remains as the series of voltage steps, any sharp changes of these pulses cause High Frequency Bearing Currents to flow.

I use a SKF Tked1 to measure these Voltage pulses in the MHz range, and this instrument is great indicator.

Experience shows that the most dominant factor in reducing these Bearing Currents is Proper Grounding.

This grounding must provide a low impedance path back to the Drive and beyond to the transformer Star point (MEN). Typically more than 75% of the problem is addressed in this step.

The remaining High Frequency current is then eliminated AT THE SOURCE, being the drive. Following this strategy means your Motor Insulation is protected which suffers from the Voltage Pulses.

Shaft Grounding & Insulated Bearings will help, provided the grounding is addressed first. Otherwise these devices struggle to be effective in the long term and don't protect Motor Insulation.

3Phi Reliability have had great success in eliminating these remaining Bearing Currents with Nano Crystalline Cores (Not Ferrites). We have tested Ferrites as being approximately 30% effective.

These EMF cores burn High Frequency current above 20kHz, so do not affect the power supply or communications.

The results of the this combination of proper grounding & testing first, followed by installation of EMF Cores shown a >99% reduction in Bearing Currents, protecting Bearings, Lubrication and Motor Insulation.

3Phi Reliability Stock these EMF Cores and can provide advice & Testing.