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What is Electrical Fluting?

Fluting is damage that is caused to a bearing due to an electrical current induced on the shaft of a variable frequency drive (VFD). Fluting is common in VFDs because if the motor in question is not VFD rated, then the current induced upon the shaft travels through the bearing, which can cause bearing failure at an exponential rate. To make sure your VFD machines are running efficiently, it is critical that you know how to properly diagnose and repair fluting. Running your machines at optimal efficiency will significantly extend the life of your bearings.

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A surefire way to physically see if a bearing is experiencing fluting is to simply check the inner ring for damaging ridges caused by an electrical current. However, there is an even easier way to detect fluting before it needs to be replaced– using modern bearing inspection tools and software.

Case Study – Detecting Electrical Fluting using Ultrasound Analysis

Thankfully, there are many tools and software that maintenance teams can use to diagnose electrical failure, such as fluting, in its early stages. By using ultrasonic instruments such as UE System’s Ultraprobe 15.000 or the remote monitoring system 4Cast, it is possible to record and analyze sound files by checking sound trends and looking at visual data displayed in graphs.
In this case study, the maintenance department suspected that fluting was occurring on a couple of their bearings. Here’s what they were working with:

• 15 kW motor, driving a fan mounted directly on the shaft

• 3540 RPM on a VFD running at 29.8 Hz; actual speed 1750 RPM

• Non-Fan End Bearing is type 6309

• Fan End Bearing is a 6208

Bearing Outer Race Fault Frequencies data. Harmonics of 5635 cpm can be seen; a generic 8- ball bearing frequency is normally 5600 cpm showing that the Non-Fan End Bearing is most likely the cause.

The maintenance team decided to inspect the bearing using an Ultraprobe 15.000. They recorded the sound from the bearing for further analysis using the Spectralyzer software. The goal was to try and confirm their suspicions of fluting by diagnosing the bearing condition via sound spectral analysis.

Findings from Ultrasound Analysis

The sound file and graph data collected shows multiple harmonics at 5639 cycles per minute (cpm). The bearing opposite the fan end (6309) is an 8-ball bearing. Based on that information and the normal operating speed of 1750 rpm, this would indicate that there is an early outer-race bearing defect (5600 cpm for 8 ball bearings) on the non-fan end bearing. While checking the sound file, there were indications that electrical fluting may have been occurring, which would have then caused the defect.
When looking at data collected one year after, the overall noise floor has continued to rise, surpassing the previous reading in overall energy level. This is characteristic of a late stage bearing failure, which would warrant immediate prioritization for repair.

Data showing an early bearing defect with potential signs of electrical fluting found in the sound file (red). Data from one year after, showing an increase in the raised noise floor indicating a late stage bearing (white).

Conclusion

By using bearing inspection tools and software, the maintenance team was easily able to diagnose fluting present in their bearings and proceeded to replace them before they caused even more critical damage to their assets.
After reviewing the findings, the team took action and immediately corrected the issue. Moving forward, they are confident that they will be able to diagnose and resolve any fluting issues they may face. Installing a consistent bearing monitoring and inspection process will help their machines continuously run at optimal efficiency, which will then allow them to extend the life of their bearings, ultimately saving them from preventable significant costs such as unexpected downtime and critical repair down the road.

Text: UE Systems  Photos: UE Systems