Common Bearing Problems
Bearing issues in three-phase motors are one of the primary causes of motor failures. Statistics have shown that over 50 percent of motor failures are related to bearing problems. Besides increasing equipment downtime, this also significantly raises maintenance costs. The following are some common bearing problems in three-phase motors and their causes:
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Electrical Discharge Damage (EDM): Electrical discharge damage mainly occurs in motors equipped with variable frequency drives (VFDs). High-frequency pulse signals generated by VFDs can create shaft currents inside the motor. When such currents pass through the bearings, they cause pitting on the bearing raceways and result in “fluting” patterns—regular wave-like patterns on the raceway surface. This phenomenon is generally known as “electric corrosion.” Statistics show that at least 30% of bearing damage in VFD-driven motors is directly related to electrical discharge damage.
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Mechanical Misalignment and Shaft Movement: Misalignment between the motor shaft centerline and the load shaft centerline causes additional radial and axial forces on the bearings. This situation often arises when the motor is improperly installed or the coupling is incorrectly adjusted. In cases of misalignment, the bearings undergo abnormal wear along with high-frequency vibration. Prolonged operation under these conditions leads to further damage to the bearings. Records indicate that over 20% of bearing failures result from mechanical misalignment and shaft movement issues.
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Poor Lubrication: Lubrication reduces friction and limits wear in bearings. Poor lubrication can occur due to insufficient lubrication, excessive lubrication, or using the wrong type of lubricant. This leads to temperature rises in the bearing due to excessive wear and eventual failure. Based on experience, lubrication problems account for more than 40% of total bearing failures. Additionally, the contamination of lubricants should not be overlooked. Entrapped air, moisture, or other contaminants within the lubricant can result in the buildup of sludge or corrosive substances, compromising the bearing rolling elements and raceways.
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Vibration and Resonance: Vibration is inevitable during operation. Slight vibration is normal, but excessive vibration or resonance due to structural design can greatly affect the life of the bearing. Excessive vibration usually occurs under conditions such as rotor imbalance, mechanical looseness, and external shocks. This leads to early fatigue points on the bearing raceway surface, the appearance of microcracks that develop over time, and ultimately results in bearing breakdown.
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High Temperature and Thermal Overload: If the operating temperature of motor bearings exceeds the normal range—usually between 70°C to 90°C—the lubricating oil will oxidize or decompose, losing its lubricating effect. When bearings operate under high-temperature conditions for a long period, the internal clearance increases, causing the contact area between the bearing rolling elements and raceways to expand. This leads to further temperature rise and increased wear.
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Corrosion: In the presence of moisture, acidic, or alkaline substances, chemical reactions can occur on the bearing surface, resulting in rust. Such corrosive substances roughen the bearing surface, increasing friction. In severe cases, the bearing may seize or even fracture. The failure rate of motor bearings operating under humid conditions is about 15% to 20% higher compared to those operating under normal dry conditions.
Bearing Fault Diagnosis Techniques
Accurate diagnostic techniques help technicians predict bearing problems early and take appropriate preventive maintenance measures. The most commonly used bearing fault diagnostic techniques are listed here:
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Vibration Analysis: Vibration analysis is one of the most frequent bearing fault diagnostic methods. By performing vibration spectrum analysis during motor operation, technicians can identify characteristic frequencies of internal bearing components, such as ball pass frequency, outer race frequency, and inner race frequency. This enables the detection of defects like pitting, spalling, or wear. Research indicates that through vibration analysis, more than 85% of early-stage bearing failures can be detected.
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Infrared Thermography: Thermography uses a thermal imaging camera to detect the temperature distribution on the motor surface. When there is friction or poor lubrication inside the bearings, the temperature at the bearing location rises considerably higher than other areas, forming hot spots in the thermal image. This technique is ideal for the quick identification of thermal overload and poor lubrication issues in motor bearings, especially when the bearings are very hot and inaccessible.
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Acoustic Emission Analysis: Acoustic emission tests use special sensors to detect high-frequency sound waves generated during bearing operation. Each time the internal bearing elements interact with the raceways, acoustic signals are generated. By processing these signals, technicians can identify micro-fatigue cracks long before macroscopic cracks appear in the bearings.
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Lubricant Analysis: By sampling the lubricant and analyzing wear particles, contaminants, and moisture content, technicians can determine the condition of the bearings regarding wear and contamination levels. This inspection directly reflects the physical and chemical changes inside the bearing and provides a basis for further diagnosis.
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Electrical Discharge Detection: This technique detects discharge phenomena caused by currents passing through motor bearings. Bearing degradation caused by electrical discharge can be identified in time by monitoring the bearing discharge with sensors. This is a very effective method for diagnosing bearing faults in VFD-driven motors.
Strategies for Solving Bearing Problems
Based on the common bearing problems in three-phase motors, here are some effective strategies for addressing these issues:
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Prevent Electrical Discharge Damage: In the case of VFD motors, insulated bearings may be installed. These are bearings manufactured from ceramic materials and bearings with coatings installed on them for insulation in order to impede the shaft’s current path. Alternatively, shaft grounding rings or brushes can be installed, which may guide excess current out of the motor housing and hence will not allow it to pass through the bearings. In addition, choose a VFD with shaft current-reduction features like common mode current suppression technology. This effectively reduces the risk of electrical discharge damage.
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Ensure Mechanical Alignment: Employ a laser alignment tool at the time of installation for both the motor and the load to achieve perfect alignment. In so doing, there will be a significant elimination of additional loads on the bearings and a reduction in the levels of vibration. On matters of maintenance, periodic checks will be able to ascertain that the motor and the load are always in a position of perfect alignment.
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Optimize Lubrication Management: An enterprise can avoid the main reasons for poor lubrication by planning routine maintenance of lubrication. It is necessary to choose the right type and ideal quantity of lubricant in order to avoid loss or excessive lubrication. For those that cannot be lubricated frequently by hand, some automatic lubrication devices can be installed on them to ensure the amount of lubricant supply always reaches the ideal quantity.
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Control Vibration and Resonance: Regularly run dynamic balance tests on the motor and load to eliminate a vibration imbalance. Mount anti-vibration mounts or apply damping material to reduce the transmission of vibrations whenever necessary. In resonance-type problems, remedies may range from structural adjustments to the addition of damping methods to eliminate resonance effects.
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Thermal Management and Temperature Control Measures: It is found that with highly improved motor temperatures, especially for those working under high temperatures, additional installation of cooling fans or improvement in the design of heat dissipation can reduce the temperature of the bearing. Besides, real-time temperature fluctuation detection can be realized by installing temperature sensors on the motor casing or bearings to avoid excessive temperature that causes bearing failure.
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Corrosion Protection: Anti-corrosion coating bearings should be used in working environments where the atmosphere is humid or chemically corrosive or install bearings with better sealing. The seals should be changed regularly to avoid moisture or corrosive substances entering the bearing cavity.
Advanced Bearing Protection and Monitoring Technology
With the development of modern technology, new methods for bearing protection and condition monitoring in three-phase motors have become available:
Smart bearings have been one of the major breakthroughs in bearing technology in recent years. Various sensors—such as temperature, vibration, and humidity sensors—are integrated into these bearings, enabling real-time access to bearing status data. By uploading sensor data to the cloud or an IoT platform, technicians can remotely monitor motor operating status and take immediate action if abnormalities occur.
Meanwhile, condition monitoring systems have become more intelligent. By processing various data from bearings with artificial intelligence algorithms, fault identification and prediction can be automatically accomplished. Research shows that with AI technology, the precision of bearing fault prediction can exceed 90%, significantly reducing unplanned downtime.