Frequent Motor Bearing Problems Caused by Shaft Current

Motor bearings are essential components in the operation of electric motors. However, in recent years, a growing number of motor failures have been linked to bearing damage caused by shaft current. This issue is not only detrimental to the reliability of motors but also leads to costly repairs and downtime. Let’s explore how shaft currents cause bearing problems and how these issues can be prevented.

What is Shaft Current?

Shaft current is an unwanted electric current that can flow along the motor shaft and through the bearings to the ground. It is typically induced by variable frequency drives (VFDs), which are commonly used in modern electric motor systems. VFDs control motor speed and torque by converting AC to DC and then back to AC, which can induce high-frequency voltage in the motor shaft. These voltages can create electric currents that travel through the bearings.

How Shaft Current Affects Motor Bearings

Motor bearings are not designed to carry electrical current. When shaft current passes through a bearing, it can cause electrical discharges that result in pitting, erosion, and wear on the bearing surfaces. This electrical erosion, often referred to as "fluting," leads to surface damage that increases friction, heat, and wear, significantly reducing the lifespan of the bearing. Over time, this damage can lead to bearing failure and motor breakdown.

Common Bearing Damage Caused by Shaft Current

• Electrical Erosion: Electrical discharges from shaft current cause microscopic pitting or cratering on the bearing surface.

• Fluting: Repeated electrical discharges can lead to the development of grooves or "flutes" on the bearing raceway.

• Increased Friction and Heat: Damaged bearings experience higher friction and produce excess heat, which can affect the overall performance and efficiency of the motor.

Prevention and Mitigation Measures

Fortunately, there are several strategies to prevent or mitigate the damage caused by shaft currents:

1. Proper Grounding: Ensuring that the motor and its components are properly grounded can provide a low-resistance path for the electrical current, preventing it from traveling through the bearings. Grounding the motor’s frame and other parts can redirect shaft currents safely to the ground.

2. Shaft Grounding Devices: Installing shaft grounding rings or brushes on the motor shaft can provide a direct path for the shaft current to dissipate safely, preventing it from flowing through the bearings. These devices divert the electrical discharge to the ground, protecting the bearings.

3. Insulated Bearings: In some cases, insulated bearings are used to prevent shaft current from passing through the bearing. These bearings have electrical insulation built into them, but other measures may be required to complete the protection system.

4. Motor Design with Reduced Shaft Current: Some motor designs come with built-in features to reduce shaft currents. These designs can include electrical shielding or other components that help reduce the generation of shaft currents in the first place.

5. Use of High-Quality Bearings: Choosing bearings with better electrical properties and more resistance to electrical erosion can help mitigate damage. Additionally, using bearings with a protective coating can improve their resistance to electrical discharge.

Conclusion

Shaft current-induced bearing damage is an increasingly common problem in motors, especially those operating with variable frequency drives. The damage it causes can lead to motor failure, increased downtime, and expensive repairs. However, by employing proper grounding techniques, shaft grounding devices, insulated bearings, and quality motor designs, these issues can be minimized, extending the lifespan of the motor and improving its performance. Understanding and addressing shaft current problems is crucial for the reliability of modern electric motors and the industries that rely on them.