Preventing Compressor Bearing Failures with Motor Shaft Grounding

Case Study Summary

When routine maintenance revealed electrostatic discharge damage inside a centrifugal compressor, the facility partnered with Rasmussen Air & Gas Energy to identify the root cause, evaluate operational risk, and implement corrective actions to improve compressor reliability and reduce the likelihood of future bearing failures.

Using advanced shaft-voltage testing and analysis, Rasmussen Air & Gas Energy diagnosed stray-shaft-voltage conditions on two centrifugal compressors and developed a practical mitigation strategy centered on shaft grounding and alignment correction.

Facility Overview

A large power generation facility relied on two critical centrifugal compressors to support daily plant operations. Maintaining compressor uptime and long-term reliability was essential to avoiding operational disruption and unplanned maintenance events.

The Challenge

During a scheduled three-year maintenance interval on a centrifugal compressor, Rasmussen Air & Gas Energy technicians identified bearing damage indicating electrostatic discharge (ESD) on the compressor shaft and bearings.

This issue presented several concerns:

• Bearing damage was not detected through traditional oil analysis or vibration monitoring

• The compressor had recently undergone motor replacement and welding activity

• The motor was operating approximately 0.250" off magnetic center

• Continued operation without mitigation could increase the risk of premature bearing failure and unplanned downtime

Because the second compressor operated under similar plant conditions, the facility elected to proactively evaluate both machines for shaft voltage issues before further damage occurred.

The Solution

Rasmussen Air & Gas Energy performed comprehensive shaft voltage testing on both compressors using an AEGIS OSC-9200MB oscilloscope.

Key Evaluation Areas Included

• Shaft voltage waveform analysis

• Grounded vs. ungrounded operating conditions

• Magnetic center alignment verification

• FFT frequency analysis

• Operational comparison between compressors

Compressor Findings

The first compressor exhibited relatively typical shaft-voltage behavior during initial ungrounded testing. However, intermittent discharge events were identified by waveform spikes returning to the zero-voltage reference.

After installing a temporary shaft-grounding brush, the voltage amplitudes dropped significantly, and waveform stability improved.

The second compressor initially demonstrated highly erratic shaft voltage behavior while operating approximately 0.250" off magnetic center.

Rasmussen Air & Gas Energy then adjusted the motor alignment closer to the magnetic center at approximately 0.030". This reduced waveform instability but did not eliminate voltage spikes.

Once shaft grounding was applied, the compressor maintained a stable low-voltage waveform with dramatically improved electrical behavior. Fast Fourier Transform (FFT) analysis on both compressors did not indicate a dominant frequency source contributing to the discharge events.

The first compressor was relatively standard in it’s results. The baseline data was taken with the compressor running and the motor shaft ungrounded. There were a few instances of shaft voltage discharge as indicated by a sharp vertical line back to 0V reference, circled above.

The first compressor had a shaft grounding brush attached and the voltage magnitude decreased as seen above.

The second compressor was tested first without grounding and the knowledge that it was ~.250” from mag center but did have PAG oil in it. The shaft voltage was erratic as shown above.

The second compressor was then moved closer to mag center (~.030”) and tested again. The shaft voltage was less erratic but had many voltage spikes as seen above.

Finally, the second compressor was grounded and maintained a steady low shaft voltage as seen here.

Technical Insight

Stray shaft voltage is a growing reliability concern in industrial rotating equipment, particularly in variable frequency drive (VFD) applications and large motor-driven systems.

Without a proper discharge path, electrical current can arc through bearings, causing:

In this case, Rasmussen Air & Gas Energy identified several contributing factors, including shaft grounding conditions, magnetic center alignment issues, and previous welding activity.

The testing process enabled the team to identify the issue and confirm corrective actions before future failures occurred.

The Results

Following testing and analysis, Rasmussen Air & Gas Energy developed a long-term mitigation strategy to improve reliability and standardize maintenance.

Implemented Recommendations

Based on the testing results, Rasmussen Air & Gas Energy implemented the following corrective actions: 

• Install permanent shaft grounding brushes/rings on both industrial compressors

• Correct motor magnetic center alignment on the affected air compressor unit

• Transition the compressor back to standard plant lubricant after grounding installation

• Evaluate participation in a spare air end program to improve uptime readiness

Key Operational Outcomes

By addressing the root cause of the shaft voltage, the power generation facility realized immediate improvements in equipment stability. These targeted grounding solutions delivered several key operational benefits:

• Reduced risk of bearing electrical discharge damage

• Improved compressor reliability

• Lower likelihood of unplanned outages

• Better long-term maintenance consistency across plant assets

• Increased confidence in compressor health monitoring

Why It Matters

Electrical bearing damage often develops silently and may not be visible in conventional predictive maintenance data until damage has already occurred.

This project highlights the importance of specialized diagnostic testing and engineered corrective actions in protecting critical rotating equipment.

By integrating field diagnostics, alignment correction, grounding solutions, and reliability-focused recommendations, Rasmussen Air & Gas Energy helped the power generation facility proactively address a hidden failure mechanism before it became a larger operational issue.

About Rasmussen Air & Gas Energy

Rasmussen Air & Gas Energy provides engineered compressed air, gas, and energy solutions for industrial facilities across power generation, manufacturing, food processing, agriculture, and other critical industries.

Our team supports customers with:

• Industrial air compressor systems

• Centrifugal and rotary screw compressor service

• Reliability diagnostics and troubleshooting

• Automation and controls integration

• Preventive and predictive maintenance programs

• Turnkey compressed air system solutions

We focus on practical engineering, operational reliability, and long-term customer support.