Tom Bishop
Technical Support Specialist
Electrical Apparatus Service Association
St. Louis, MO
The paper "Dealing with Shaft and Bearing Currents" by Tom Bishop, presented at the EASA Convention 2007, addresses the critical issues related to shaft and bearing currents in electric motors and generators. It begins by explaining the symptoms of bearing currents, which often manifest as audible noise from the bearings, indicating advanced stages of failure. Visual inspection of failed bearings may reveal fluting of the races, frosting of the balls or rollers, and a dull grey or dark "smoky" finish on the bearing surfaces.
The paper categorizes current damage into three types: electric pitting, fluting, and micro-cratering. Electric pitting is characterized by single crater damage, typically seen in DC applications like railway traction motors. Fluting appears as multiple lines across the bearing races, caused by mechanical resonance vibration. Micro-cratering, the most common type of damage in motors powered by variable frequency drives (VFDs), results in a dull surface with molten pit marks.
Bishop explains that shaft voltage becomes problematic when it leads to bearing current, which can discharge through the lubricant film on the bearings, causing damage. He describes methods for determining if damaging current levels are present, such as measuring the voltage from the shaft to the motor frame. If the shaft to frame voltage exceeds certain thresholds, it indicates potentially harmful bearing currents.
The paper identifies several causes of damaging currents, including magnetic dissymmetry, electrostatic discharges, and capacitive coupling between the stator windings and rotor. Magnetic dissymmetry is often associated with larger motors that have segmented laminations, leading to asymmetric flux and circulating currents. Electrostatic discharges can occur in applications like belt drives and fans, while capacitive coupling is common in motors supplied by VFDs.
Bishop outlines various solutions to eliminate or control shaft and bearing currents. These include insulating bearings, using shaft grounding brushes, and installing filters or reactors between the drive and motor. Insulating bearings can be achieved through methods such as insulated housings, insulated bearing journals, and ceramic rolling elements. Shaft grounding brushes provide a low resistance path to divert current away from the bearings. Filters and reactors help reduce the magnitude of bearing currents by modifying the VFD output waveform.
The paper also discusses the importance of grounding and the use of stranded, low-impedance ground cables to establish a dedicated common ground path between the motor and drive. Proper grounding helps minimize the common mode voltage and reduce the risk of damaging bearing currents.
In conclusion, Bishop emphasizes the need for condition monitoring to detect early signs of bearing current damage. Techniques such as vibration analysis, lubricant analysis, and microscopic analysis can help identify and address issues before they lead to complete bearing failure.
Key Points Covered:
- Symptoms of bearing currents and types of current damage
- Methods for determining if damaging current levels are present
- Causes of damaging currents: magnetic dissymmetry, electrostatic discharges, capacitive coupling
- Solutions to eliminate or control shaft and bearing currents: insulating bearings, shaft grounding brushes, filters, reactors
- Importance of grounding and using stranded, low-impedance ground cables
- Condition monitoring techniques: vibration analysis, lubricant analysis, microscopic analysis
Key Takeaways:
- Bearing currents can cause significant damage to motor bearings, often indicated by audible noise and visual signs like fluting and frosting.
- Determining the presence of damaging currents involves measuring shaft to frame voltage and identifying thresholds.
- Various causes of damaging currents include magnetic dissymmetry, electrostatic discharges, and capacitive coupling.
- Effective solutions include insulating bearings, using shaft grounding brushes, and installing filters or reactors.
- Proper grounding and the use of stranded ground cables are crucial for minimizing common mode voltage.
- Condition monitoring techniques help detect early signs of bearing current damage and prevent complete bearing failure.
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