bearing currents

Bearing Currents: A Silent Threat to Electrical Machines

Bearing currents, a phenomenon often overlooked, can silently erode the lifespan of electrical machines. While seemingly innocuous, these currents can cause significant damage to bearings, leading to premature failure and costly downtime. This article delves into the root causes of bearing currents, their detrimental effects, and methods to mitigate them.

Understanding Bearing Currents:

Bearing currents arise due to the flow of electrical current through the bearing surfaces of an electric machine. This current flow is primarily caused by two main factors:

1. Electromagnetic Unbalance: In an ideal rotating machine, the magnetic field generated by the stator is perfectly balanced. However, in reality, factors such as rotor eccentricity, magnetic saturation, or winding imbalances can create an uneven magnetic field. This imbalance induces circulating currents within the rotor, which can flow through the bearing surfaces, creating "bearing currents".

2. High dv/dt Inverters: Modern variable frequency drives (VFDs) employ high-speed switching devices that can generate steep voltage gradients (dv/dt). These rapid voltage changes can charge up stray capacitances present within the machine, particularly between the stator and rotor, and between the rotor and shaft. This "capacitive coupling" allows current to flow through the bearing path, creating bearing currents.

The Silent Destruction:

Bearing currents, while often small in magnitude, can have detrimental effects on bearings:

• Electrolytic Corrosion: Bearing currents can lead to electrochemical reactions at the bearing surfaces, causing corrosion and pitting. This degradation weakens the bearing structure and reduces its load-carrying capacity.
• Electrical Discharge Machining (EDM): When the bearing currents are sufficiently high, electrical discharges can occur across the bearing surfaces. These discharges can create micro-cracks and pits, leading to rapid wear and failure.
• Increased Friction and Heat: Bearing currents can also generate heat within the bearings due to the resistance of the current path. This heat can lead to accelerated lubrication breakdown and premature bearing failure.

Mitigating Bearing Currents:

Several methods can be employed to minimize or eliminate bearing currents:

• Proper Machine Design: Careful design considerations, such as minimizing eccentricity and using balanced rotor configurations, can reduce electromagnetic unbalance and the resulting bearing currents.
• Shielding and Grounding: Installing conductive shields between the stator and rotor, and grounding the rotor shaft, can help divert bearing currents away from the bearing surfaces.
• VFD Settings: Properly configuring the VFD, including setting appropriate voltage ramp rates and using filtering techniques, can reduce the dv/dt and minimize capacitive coupling.
• Bearing Grease Selection: Using specially formulated bearing greases with high electrical resistivity can help minimize the flow of bearing currents.
• Bearing Monitoring: Regularly monitoring bearing temperature and vibration levels can provide early warning signs of bearing damage caused by bearing currents.

Conclusion:

Bearing currents are a significant concern for the reliability of electrical machines. Understanding the root causes, the detrimental effects, and the mitigation techniques is crucial for ensuring long-term machine performance. By implementing appropriate measures, we can prevent these silent threats and extend the lifespan of our valuable electrical assets.