November 2025
Maintenance, Reliability and Inspection
Mitigate electrical discharge in VFD motors: Best practices for bearing protection
A variable frequency drive (VFD) motor is an advanced electric motor controlled by a VFD that adjusts frequency and voltage for precise speed and torque control. This enhances energy efficiency, reduces wear through soft starting and improves process control in industrial applications. However, high initial costs and issues like electrical discharge (FIG. 1), which can damage bearings, are drawbacks. Despite these challenges, VFD motors remain popular for their efficiency, longevity and cost savings, making them a preferred choice for industries seeking optimized performance.
FIG. 1. Capacitive discharge current. Source: Electro Static Technology.
Observation. In one of the author’s company’s plant units, an air handling unit (AHU) primarily operated by VFD motors was commissioned in 2022 and became fully operational in May 2024.
The first bearing failure occurred in January 2024, when a condition monitoring (CM) survey detected abnormal noise and high vibration envelope readings [9 gravitational envelope (gE)] at the drive end (DE). Greasing and a test run showed no improvement, with the vibrations envelope rising to 25.5 gE. The electrical team was advised to replace both bearings and inspect the end cover tolerance. The DE bearing and grease appeared black, with minor raceway markings, but all tolerances were within limits. Insulated bearings were recommended and to monitor electrical discharge using a handheld electrical discharge detector pena. This pen is used to detect and count electrical discharges (bearing currents) in electric motors, helping identify early bearing damage caused by shaft voltages—especially in VFD-driven motors. After maintenance, all parameters were within acceptable limits.
In April 2024, the motor experienced a second failure due to the absence of insulated bearings and the plant's continuous operation at full load, which caused the VFD motor to run at full speed. As a result, bearing fault frequencies emerged within three months. A CM survey detected rising vibration levels, reaching 8.6 gE, despite regular greasing. Upon disassembly, the motor drive end (MDE) end cover and shaft seating were found to be blackened. Further inspection revealed deep electrical fluting marks on the MDE bearing (FIG. 2), along with light markings on the inner and outer races (FIG. 3).
FIGS. 2 and 3. A sample flow of ISO 15243:2017 (FIG. 2, left)1, and fluting on the outer ring raceway of a deep groove ball bearing (FIG. 3, right).
Action taken. New insulated bearings were installed at the DE, while a standard bearing was installed at the non-drive end (NDE) (FIG. 4).
FIG. 4. (From left to right) An MDE end bearing with black grease due to stray current; an electrical fluting marking on an MDE outer race; a microscopic image showing electric arc pitting; balls with a stray current effect; and a black mark on a shaft MDE bearing seating area.
A survey conducted using the electrical discharge detector pena (FIG. 5) detected a reading of 558 (558 spikes of stray current in 30 sec) at the MNDE on the faulty motor. Additionally, all other VFD motors running at full speed were checked, revealing high electrical discharge values. The maintenance team was advised to plan the replacement of all VFD motor bearings with insulated bearings. Until the replacements were completed, it was recommended to run the VFD motor fan at a lower speed and manage the operational load with other non-VFD motor-operated fans to prevent bearing failure.
FIG. 5. An electrical discharge detector pena is a handheld tool for detecting electrical discharges in motor bearings.
VFD motors, replace standard bearings with insulated or ceramic bearings and install shaft grounding rings or brushes. Use VFD-rated shielded cables with proper grounding and optimize VFD settings to reduce voltage spikes. Applying common mode chokes, line reactors or dV/dt filters minimizes voltage imbalance. Ensure proper system grounding and regularly monitor bearings with electrical discharge detectorsa with motors running on full speed. These measures help prevent damage and extend motor lifespan.
NOTE
a SKF TKED-1
LITERATURE CITED
1 International Organization for Standardization (ISO) 15243:2017, “Rolling bearings — Damage and failures — Terms, characteristics and causes,” Ed. 2, 2017.
The Author
Sanjay Kumar is a condition monitoring specialist with > 14 yrs of experience in refinery, combined-cycle power plants, thermal power plants and liquefied natural gas (LNG) process plants, focusing on the condition monitoring and asset reliability of rotating equipment. He presently works at a fertilizer and petrochemical plant in Indorama, West Africa. His expertise includes vibration analysis, ultrasound, and root cause analysis of rotating equipment, aiming to optimize performance and reduce downtime. Kumar is passionate about industrial innovation and actively sharing insights on best practices in maintenance and reliability engineering.
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