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Bearing Failures on VFD Controlled Electric Motors

January 3, 2017

Shaft currents can have a destructive effect on electric motor bearings. This article, with information from our friends at AEGIS Shaft Grounding Rings <INSERT LINK TO: https://www.hecoinc.com/new-equipment/aegis-shaft-grounding-ring>, is intended to inform professionals about these voltages and their effects on bearings in electric motors.

Because of the high-speed switching frequencies in PWM inverters, variable frequency drives induce shaft currents in AC motors. The switching frequencies of insulated-gate bipolar transistors (IGBT) used in these drives produce voltages on the motor shaft during normal operation through parasitic capacitance between the stator and rotor. These voltages, which can register 10-40 volts peak, are easily measured by touching an oscilloscope probe to the shaft while the motor is running. (Reference: NEMA MG1 Section 31.4.4.3)

Image of an EDM Pit
Once these voltages reach a level sufficient to overcome the dielectric properties of the bearing grease, they discharge along the path of least resistance — typically the motor bearings — to the motor housing. During virtually every VFD switching cycle, induced shaft voltage discharges from the motor shaft to the frame via the bearings, leaving a small fusion crater (fret) in the bearing race. When this event happens, temperatures are hot enough to melt bearing steel and severely damage the bearing lubrication.

Image of a frosted bearing race

These discharges are so frequent (millions per hour) that before long the entire bearing race becomes marked with countless pits known as frosting. A phenomenon known as fluting may occur as well, producing washboard-like ridges across the frosted bearing race. Fluting causes excessive noise and vibration, and in heating, ventilation, and air-conditioning systems, it is magnified and transmitted by the ducting. Regardless of the type of bearing or race damage that occurs, the resulting motor failure often costs many thousands or even tens of thousands of dollars in downtime and lost production.

Image of a fluted outer bearing race

Failure rates vary widely depending on many factors, but evidence suggests that a significant portion of failures occur only 3 to 12 months after system startup. Because many of today’s AC motors have sealed bearings to keep out dirt and other contaminants, electrical damage has become the most common cause of bearing failure in AC motors with VFDs.

Here is a video showing you more detail on these issues:

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Justin Hatfield

HECO – All Systems Go

269-381-7200

jhatfield@hecoinc.com

 

About the author:

Justin Hatfield is Vice President of Operations of HECO. He is responsible for Electric Motor & Drive Sales, Electric Motor & Generator Repairs, Spare Solutions, and Predictive Services. Justin was instrumental in developing HECO MAPPS (Motor and Powertrain Performance Systems) <INSERT LINK TO: https://www.hecoinc.com/heco-system/electric-motor-repair-heco-system> which focuses on “why” you have a motor problem instead of simply “What” product or service should be recommended. HECO is an EASA Accredited Service Center.

Posted in Equipment Management, Predictive

Chart of voltages from the shaft of a running motor