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Across the world, bearings are destroyed in rotating equipment every day. As maintenance professionals, we have all been made aware of poor lubrication, misalignment, unbalance, and other mechanical anomalies that lead to bearing failures. However, over the last few decades there has been an upswing in bearing failures caused by electrical issues. This upswing happened with the widespread introduction of Variable Frequency Drives (VFDs). The problem is electrical fluting.

What is Fluting?

Fluting is a description of the physical effect on bearings after the electrical defects have already caused damage. So, we need to back up for a minute to the root cause and then come back to fluting.

When a motor operates on a Variable Frequency Drive (VFD) there is an excessive amount of capacitance between the rotor and the stator in the motor. This capacitance is caused by the switching frequency in the VFD. A VFD takes Alternating Current (AC) voltage and converts it to Direct Current (DC), then back to AC through a process called Pulse Width Modulation (PWM). This pulsing or PWM is controlled by Insulated Gate Bipolar Transistors or (IGBTs). The purpose of this pulsing is to introduce the ability to vary the frequency and control the speed of the motor. Hence the name VFD. Although VFDs are a great invention that has saved tons of money in energy costs and improved the efficiency of factories across the world, they come with a downside.

When AC power is “recreated” through PWM it can cause a few unwanted forms of noise, including electromagnetic interference (EMI), radio frequency interference (RFI) and common mode current. These unwanted variables can lead to the buildup of capacitance in the motor. This causes voltage to build up in the motor shaft. When the voltage has reached a high enough level, it will arc across the bearing rollers into the motor frame as it tries to find the easiest path to ground. Unfortunately, this arcing occurs directly onto the bearing race!

So, what’s the harm?

Electrical arcing in rolling element bearings can cause major problems over time. At low levels, it can take years to develop physical damage in the bearing. However, high levels of shaft voltage, the arcing can destroy a bearing quite quickly. When arcing occurs in the bearing, it produces small pits in the race. Over time, as these pits accumulate, they become like waves in the bearing race and end up looking like a washboard. As you can imagine, this will generate quite a lot of noise as rollers pass over a surface that was once smooth but now resembles a washboard. The sound of fluted bearings can be extremely annoying! Excessive bearing noise can become especially problematic in schools, office buildings, and hospitals. Not only are fluted bearings an inconvenience for nearby workers dealing with the sound, but they can also fail prematurely. Anytime fluted bearings are identified, they should be changed as soon as possible. Or at least monitored closely with vibration or ultrasonic analysis.

How to Identify Fluting

Unfortunately, fluted bearings are most often identified by excessive noise levels. This is a major problem because the damage is already done by the time we can hear it. Damage in the bearing can be picked up early with vibration analysis as well as ultrasonic analysis. The vibration spectrum will identify a race defect with many harmonics that are very widespread across the frequency range, and very similar in amplitude. While using these techniques is better than waiting for audible noise, it’s still too late. The damage has already been done!

Electrical testing can be performed to see if the potential for fluting is present. A shaft riding device can be used to measure voltage on the motor shaft. We recommend leaving this to a professional. An oscilloscope can be used on the leads going to the motor to measure common mode current. This is a good indication that bearing fluting is imminent.

How to prevent Fluting

Although early detection is best for predicting bearing failures from fluting, it is best to put measures in place to prevent fluting altogether. Most Google searches and articles about fluting will discuss many Band-Aid fixes that mask or delay the effects of fluting. However, fluting can be mostly avoided if installations are set up properly from the beginning.

Here are some measures that can be put in place if fluting is already a problem:

• Shaft grounding devices: These can come in the form of brushes or rings. They are quite effective at grounding the shaft and avoiding arcing in the bearings. They give the voltage an easy path to ground. However, they are a bit challenging to install and require constant maintenance.

• Inductive Absorbers/Filters/Chokes: There are several products on the market for reducing unwanted common mode current right at the VFD. These are often called filters or chokes. These chokes simply slide right over the leads going to the motor and absorb the unwanted electromagnetic interference (EMI) as well as radio frequency interference (RFI) before it even gets to the motor. These are very easy to install and require no further maintenance. One of the more popular brands of these filters is CoolBLUE®.  

• Use Proper Lubrication: Some lubrication is more conductive than others. Always use non-conductive grease in motor bearings. This will greatly reduce the amount of arcing that occurs in the bearings.

• Insulated bearings: These bearings can eliminate the arcing from the roller to the race. They can be coated or made of insulating material such as ceramic. Although insulated bearings can solve the fluting problem, they are quite costly and can be difficult to obtain.

Although the above practices can greatly reduce the effect of unwanted shaft voltage, it is best to avoid the problem during installation by ensuring that the machine is set up properly. Here are a few tips to consider when installing equipment that runs on a VFD.

• Make sure the VFD is sized properly for the machine. Consider horsepower, amperage requirement, load, etc.
• Communicate with the VFD manufacturer or distributor and use the VFD that is recommended for your specific application. Be sure to include all details so the proper set-up can be provided.
• Use VFD-rated cabling to the motor. Using this type of cabling greatly reduces the effect of common mode (stray) current, radio frequency interference (RFI), electromagnetic interference (EMI), and any other unwanted noise from the VFD.
• Ensure that your grounding system is sufficient. The better your grounding system is, the less effect fluting will have on your facility. Keep your ground system in good working order and keep your power clean. It won’t do much good to ground motor shafts with rings or brushes if there isn’t a good path to ground from there.
• Avoid running ground cables in the same conduit as the power leads. If the cabling is run together, the power leads can induce current into the ground cable and exacerbate the problem of stray currents. This can ultimately cause a direct circuit for stray currents to flow from the VFD, through the motor, into the motor frame, and back to the VFD. Try to run ground cables separately to the machine.

How to correct fluted bearings

So, what do you do if your bearings are already fluted? It goes without saying that fluted bearings should be replaced as soon as possible. If harmonics are observed in the vibration data, or noise is heard from the bearings, there is already physical damage to the bearing race. It is best to change the bearings as soon as possible to avoid unwanted downtime from a catastrophic failure.

Often, when maintenance professionals find fluted bearings, they want to jump straight to grounding or filtering. This is smart, but if bearings are already fluted, the corrective methods for VFD noise will not correct the physical damage already present in the bearing. It must be replaced, period.

It would be wise to not place new bearings into a machine that has issues with excessive shaft voltage. The shaft voltage issue must be corrected first. Damage to bearings can occur quickly and continue to progress long after the electrical issue has been resolved. With that said, don’t change the bearings and run them for two months while you wait on filters or grounding rings!

Key Takeaways

• Always follow manufacturer’s recommendations when purchasing and installing VFD operated machines.
• Ensure that each motor is VFD rated or “Inverter Duty”. This will be indicated on the motor’s nameplate.
• Always maintain a good grounding system in your facility. Be sure to periodically inspect the grounding system as modifications are made and new machines are added.
• Once machines are installed, perform early electrical testing to identify any issue that may lead to fluting.
• Install grounding rings, brushes, filters, or absorbers as needed if problems are detected.
• Maintain a robust reliability program including vibration analysis and/or ultrasonic testing. This can identify any damage that may have slipped by.
• Always use the correct lubrication for motor bearings to minimize the effects of electrical damage.

Conclusion

Electrical fluting is thought to be a new problem that has developed over the last 20 years with the rapid increase in VFDs. However, fluting is not a new thing at all. Many DC motors run on drives that convert AC power to DC to vary the speed. Fluting has been an issue in DC motors for quite some time. Furthermore, fluting was also around before any drives were created. Certain electrical circuits and motor constructions can create the perfect environment for fluting to occur.

Now that VFDs are extremely common across all industries, we must get used to preventing, minimizing, and correcting issues that lead to fluted bearings.

Download our 4 Stages Bearing Failures infographic for a basic reference guide to understanding the stages of bearing failures. Perfect to hang at your facility too! 

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