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This article discusses the causes of motor vibration and the methods for troubleshooting it; it presents very practical experience summaries, which I hope will be helpful for your work and studies. Generally, motors with more than 8 poles do not experience vibration due to manufacturing defects. Vibration is common in 2–6 pole motors, GB10068-2000. \"Vibration Limits and Testing Methods for Rotating Motors\" specifies the vibration limits, measurement methods for motors with different center heights on a rigid foundation, as well as the criteria for determining the rigidity of such a foundation. Based on these standards, it is possible to determine whether a motor meets the requirements. I. Hazards of motor vibration: Vibration in motors can shorten the insulation life of the windings as well as the lifespan of the bearings. It also affects the proper lubrication of sliding bearings. The vibrating forces cause the insulation gaps to widen, allowing external dust and moisture to penetrate therein, which leads to a decrease in insulation resistance and an increase in leakage current; in severe cases, this can even result in insulation breakdown. Furthermore, the vibration generated by the motor can easily cause the cooling water pipes to crack, as well as the welding points to separate. It also leads to damage to the machinery under load, reduces the precision of the workpieces, causes fatigue in all mechanical components that are subjected to vibration, and may result in the anchor screws becoming loose or breaking. Motor vibration can also cause abnormal wear of the carbon brushes and slip rings; in severe cases, arcing may occur, leading to the destruction of the insulation around the slip rings. The motor will then produce significant noise. This situation often occurs in DC motors as well. II. Reasons related to vibration: There are mainly three situations: reasons of electromagnetic nature ； Mechanical reasons ； Reasons related to the mechanical-electrical hybrid aspect. 1. Reasons related to electromagnetism: 1) Power supply issues: Unbalanced three-phase voltage, operation of three-phase motors with one phase missing. 2) Stator aspect: The stator core becomes elliptical, eccentric, or loose; the stator windings suffer from breaks, ground faults, inter-turn short circuits, wiring errors, and unbalanced three-phase currents. Typical case: Before repairing the motor of the sealing fan in the boiler room, red powder was found on the stator core; it was suspected that the stator core was loose. However, this issue fell outside the scope of standard overhauls, so no action was taken. During testing after the overhaul, the motor emitted a loud screeching sound, and the problem was resolved after replacing the stator. 3) Rotor failure: The rotor core becomes elliptical, eccentric, or loose. Weld failure between the rotor cage bars and end rings, broken rotor cage bars, incorrect winding, poor brush contact, etc. Typical case: During the operation of the toothed saw motor in the sleeper production section, it was observed that the stator current fluctuated back and forth, and the vibration of the motor increased gradually. Based on these symptoms, it was suspected that there might be welding defects or breaks in the rotor bars. Upon disassembling the motor, it was found that 7 of the rotor bars were broken; in two particularly severe cases, both sides of those bars as well as their connections to the end rings were completely broken. If this issue had not been detected in time, it could have led to serious damage to the stator. 2 Mechanical reasons: 1) Issues with the motor itself such as rotor imbalance, bent shafts, deformed slip rings, uneven air gaps between the stator and rotor, misalignment of the magnetic centers of the stator and rotor, bearing failures, poor foundation installation, insufficient strength of the mechanical structure, resonance, loose anchor bolts, and damaged motor fans. Typical case: After replacing the upper bearing of the motor for the plant’s condensate pump, the motor began to vibrate more severely, and there were signs of slight rubbing between the rotor and stator. Upon closer inspection, it was found that the lift height of the motor rotor was incorrect, and the magnetic centers of the rotor and stator were not aligned. By readjusting the screws used to secure the thrust head, the vibration problem of the motor was resolved. After maintenance, the vibration of the cross-line suspension motor remained high, with a tendency to increase further. When the motor was lowered, its vibration was still very high, and there was significant axial movement. Upon disassembly, it was found that the rotor core was loose and the rotor’s balance was also problematic. The issue was resolved after replacing it with a spare rotor, while the original rotor was sent back to the factory for repair. 2) Issues related to the coupling: damaged coupling, poor coupling connection, inaccurate centering of the coupling, unbalanced load machinery, system resonance, etc. The shafting in the linked portion is out of alignment; the center lines do not coincide, and the centering is incorrect. The main cause of this fault is poor alignment and improper installation during the setup process. There is another scenario in which, at rest, the centerlines of the connected components coincide, but after operating for a while, deformations in the rotor supports and foundation cause these centerlines to diverge again, resulting in vibration. Typical case: a. The motor of the circulating water pump exhibited high vibration levels during operation; no issues were found upon inspection of the motor, and it worked normally when unloaded. The water pump team believed that the motor was operating properly, but it was later determined that there was a significant deviation in the motor’s alignment. After the water pump team realigned the motor, the vibration problems were resolved. b. After replacing the pulley in the boiler room’s exhaust fan, the motor experienced vibration during testing, and the current in its three phases increased. All circuits and electrical components were checked and found to be fine; it was later discovered that the pulley was defective. After replacing it, the vibration in the motor disappeared, and the current in its three phases returned to normal levels. 3 Reasons for electromechanical coupling: 1) Motor vibration is often caused by uneven air gaps, which result in unilateral electromagnetic forces; these unilateral electromagnetic forces in turn cause the air gaps to increase further. This electromechanical coupling manifests as motor vibration. 2) Axial movement of the motor occurs due to electromagnetic forces resulting from the rotor’s own weight, improper installation level, or misalignment of the magnetic center; this leads to axial movement of the motor, increased vibration, and in severe cases, wear of the bearing shells, causing their temperature to rise rapidly. There is a problem with the gears and couplings connected to the motor. This type of fault is primarily manifested by poor gear meshing, severe tooth wear, inadequate lubrication of the gears, misalignment or skewing of the couplings. Incorrect tooth shapes and pitches in gear couplings, as well as excessive gaps or severe wear, can all cause vibration. Defects in the motor’s structure itself and issues with installation. Such faults are mainly manifested as an elliptical shape of the journal, bending of the rotating shaft, excessive or insufficient clearance between the shaft and the bearing shells, insufficient stiffness in certain parts of the bearing housing, base plate, or foundation – or even in the entire motor mounting foundation. The motor may not be securely fixed to the base plate, the foot bolts may be loose, and there may be looseness between the bearing housing and the base plate. An excessive or insufficient gap between the shaft and the bearing bush can not only cause vibration but also lead to abnormalities in the lubrication and temperature of the bearing bush. 3) Typical cases of vibration transmission from motor-driven loads: vibration of the turbine in steam turbine generators, and vibration of fans and water pumps driven by motors, which in turn causes vibration in the motors. III. How to identify the cause of vibration 1. Before shutting down the motor, use a vibration meter to check the vibration levels in various parts. For those areas with high vibration levels, measure the vibration values in the vertical, horizontal, and axial directions. If the foundation screws or the screws on the bearing covers are loose, they can be tightened directly; after tightening, measure the vibration levels again to see if the vibration has decreased or been reduced. Next, it is necessary to check whether the three-phase voltage of the power supply is balanced and whether the fuses in these three phases are blown. Operating the motor in single phase can not only cause vibration but also lead to a rapid increase in the motor’s temperature. Observe whether the pointer of the ammeter swings back and forth; current fluctuations occur when there are broken bars in the rotor. Finally, check whether the three-phase currents of the motor are balanced; if any issues are detected, contact the operators immediately to stop the motor from operating, in order to prevent damage to the motor. 2. If the motor vibration persists despite addressing the surface phenomena, continue to disconnect the power supply, release the coupling, and separate the motor from the load machinery it is connected to, allowing the motor to rotate on its own. If the motor itself does not vibrate, it indicates that the source of vibration is due to an improperly aligned coupling or the load mechanism. If the motor vibrates, then there is a problem with the motor itself. Additionally, the power-off method can be used to determine whether the issue is electrical or mechanical; if the vibration stops immediately or decreases when the power is turned off, it indicates an electrical problem, whereas otherwise it is a mechanical fault. IV. Troubleshooting based on the causes of the fault 1. For electrical-related issues, the first step is to check whether the DC resistance of the stator’s three phases is balanced. If it is not balanced, this indicates that there are welding defects in the stator connections; the windings need to be disconnected and checked phase by phase. It is also necessary to determine whether there are any inter-turn short circuits. If the fault is obvious, signs of burning can be seen on the surface of the insulation, or instruments can be used to examine the stator windings. Once an inter-turn short circuit is identified, the motor windings must be reconnected. Typical case: Water pump motor. During operation, the motor experienced significant vibration and high bearing temperatures. A minor repair inspection revealed that the motor’s DC resistance was not within acceptable limits, and there were weld cracks in the motor’s stator windings. By using the method of elimination, the fault was identified and corrected, after which the motor operated normally. 2. For mechanical reasons, check whether the air gap is even; if the measured value exceeds the standard, adjust the air gap again. Inspect the bearings and measure the bearing clearance; if they do not meet the requirements, replace them with new ones. Check for any deformation or looseness in the core – a loose core can be repaired by bonding it with epoxy resin. Inspect the rotating shaft as well: bend in the shaft should be corrected through welding and reshaping, or the shaft can be straightened directly, after which a balance test should be conducted on the rotor. 3. Inspection of the mechanical components under load: If there is no problem with the motor itself, then the cause of the fault lies in the connection parts. In such cases, it is necessary to check the levelness of the motor’s foundation, its tilt and strength, whether the centering is correct, whether the coupling is damaged, and whether the deflection of the motor shaft meets the required standards. V. Steps to address motor vibration: 1. Disconnect the motor from the load, run the motor without a load, and measure the vibration levels. 2. Check the vibration level at the motor’s footings. According to the national standard GB10068-2006, the vibration level at the footing area shall not be greater than 25% of the corresponding vibration level at the bearing location; if this value is exceeded, it indicates that the motor foundation is not a rigid one. 3. If only one of the four feet, or two diagonally opposite feet, exhibit excessive vibration, loosen the foot bolts; the vibration levels will then become normal. This indicates that the padding under that foot is not adequate, and the tightening of the foot bolts causes deformation of the base, resulting in vibration. Ensure the padding under the foot is proper, realign it properly, and then tighten the foot bolts. 4. Tighten all four foot bolts on the base completely; yet the motor’s vibration levels remain above the acceptable range. In this case, check whether the coupling installed on the shaft extension is level with the shaft shoulder. If it isn’t level, the excitation force generated by the excess keys on the shaft extension can cause the motor to vibrate excessively horizontally. In such cases, the vibration level does not increase significantly; often, the vibration level decreases after connection to the main unit. The user should be persuaded to use it. During factory testing, bipolar motors are equipped with half-keys in the shaft keyways in accordance with GB10068--2006. Extra keys will not add additional excitation force. To handle it, simply trim off the excess length of the unnecessary keys. 5. If the vibration of the motor during no-load operation is within acceptable limits, but it exceeds the limits when under load, there are two possible reasons: one is a large alignment deviation. In another case, the phase of the residual unbalance in the rotating component (rotor) of the host machine overlaps with that of the residual unbalance in the motor rotor; as a result, the residual unbalance at the same position across the entire shaft system is high, generating large excitation forces that cause vibration. At this point, the couplings can be disconnected; rotating either of the two couplings by 180° and then reconnecting them will reduce the vibration. 6. If the vibration velocity (intensity) is within the specified limits but the vibration acceleration is excessive, the bearings must be replaced. 7. Due to their poor rigidity, the rotors of two-pole high-power motors can deform if not used for an extended period of time; this may lead to vibration when the motor is started again. This is a result of improper storage of the motors. Under normal circumstances, during storage, two-pole motors… The motor should be turned manually every 15 days, with at least 8 full rotations per turn. 8. The vibration of the motor with sliding bearings is related to the quality of the bearing bush assembly; it is necessary to check whether there are any high points on the bearing bushes, whether there is sufficient oil supply to them, the tightening force on the bearing bushes, the clearance between them, and whether the magnetic centerline is appropriate. 9. Under normal circumstances, the causes of motor vibration can be simply determined by examining the levels of vibration in three directions: high horizontal vibration indicates rotor imbalance; high vertical vibration suggests that the installation foundation is not level or of poor quality; high axial vibration points to poor quality in the bearing assembly. This is merely a simple assessment; the true cause of the vibration must be determined by considering the actual conditions on site and taking all the factors mentioned above into account. 10. For Y-series box motors, special attention should be paid to axial vibration. If axial vibration is greater than radial vibration, it can cause severe damage to the motor bearings and lead to shaft seizure. Pay attention to monitoring the bearing temperature; if the positioning bearings heat up faster than the non-positioning bearings, the machine should be stopped immediately. This is axial vibration caused by insufficient axial stiffness of the base; the base should be reinforced. 11. After dynamic balancing of the rotor, the residual imbalance in the rotor is fixed there and does not change; moreover, the vibration of the motor itself does not vary depending on location or operating conditions, so the vibration problem can be resolved at the user’s site. Under normal circumstances, there is no need to perform dynamic balancing on the motor again after maintenance, unless in very special cases such as flexible foundations or rotor deformation, in which case on-site dynamic balancing or return to the factory for treatment is required.