There are many reasons for motor vibration, and it is also very complex. Motors with more than 8 poles will not experience vibration due to motor manufacturing quality issues. Vibration is common in 2-6 pole motors.
GB10068-2000, "Vibration Limits and Test Methods for Rotating Motors," specifies the vibration limits, measurement methods, and criteria for determining rigid foundations for motors with different center heights. Based on this standard, it can be determined whether the motor meets the requirements.
The hazards of motor vibration
The motor generates vibrations, which can shorten the insulation and bearing life of the winding, affect the normal lubrication of the sliding bearing, and cause the insulation gaps to expand due to vibration forces.
This allows external dust and moisture to invade, resulting in reduced insulation resistance, increased leakage current, and even insulation breakdown accidents.
In addition, motor vibrations can easily cause water pipes in the cooling system to crack, weld points to break, and damage the load machinery, reducing workpiece accuracy.
It can also cause fatigue in all mechanically vibrating parts, loosen or break anchor bolts, and result in abnormal wear of carbon brushes and slip rings.
In severe cases, it can lead to brush fires and burn the insulation of the collector rings. The motor will generate a lot of noise, and this situation generally occurs in DC motors as well.
10 reasons for motor vibration
1. Caused by the imbalance of the rotor, coupling, coupling, and transmission wheel (brake wheel).
2. The iron support bracket is loose, the key and pin are loose and ineffective, and the rotor binding is not tight, all of which can cause imbalance in the rotating parts.
3. The linkage part of the shaft system is not aligned properly, the center lines do not coincide, and the alignment is incorrect. The main cause of this fault is poor alignment and improper installation during the installation process.
4. The central axis of the linkage section is aligned and consistent when in a cold state, but after running for a period of time, it is disrupted due to deformation of the rotor pivot, foundation, etc., resulting in vibration.
5. The gears and couplings connected to the motor are faulty, with poor gear engagement, severe gear tooth wear, inadequate lubrication of the gears, misalignment or displacement of the couplings, incorrect tooth shape or tooth spacing of the gear couplings, excessive clearance or severe wear, all of which can cause certain vibrations.
6. Defects in the structure of the motor itself, such as elliptical shaft neck, bent shaft, excessive or insufficient clearance between the shaft and the bearing, insufficient rigidity of the bearing seat, base plate, foundation, or even the entire motor installation foundation.
7. Installation issues, such as loose fixation between the motor and the base board, loose bottom foot bolts, and loose connection between the bearing seat and the base board.
8. The excessive or insufficient clearance between the shaft and the bearing can not only cause vibration but also lead to abnormal lubrication and temperature of the bearing.
9. The load driven by the motor conducts vibrations, such as the vibrations caused by the motor-driven fan or water pump.
10. AC motor stator wiring error, short circuit in the winding of the squirrel-cage induction motor rotor, inter-turn short circuit in the excitation winding of the synchronous motor, incorrect connection of the excitation coil of the synchronous motor, broken bars in the rotor of the squirrel-cage induction motor, deformation of the rotor iron core causing uneven air gap between the stator and rotor, resulting in unbalanced magnetic flux in the air gap and causing vibration.
Causes of vibration and typical cases
There are three main reasons for vibration: electromagnetic reasons, mechanical reasons, and electromechanical reasons.
1. Electromagnetic reasons
Power supply: Three-phase voltage imbalance, three-phase motor running with missing phase.
Stator aspect: Stator iron core becomes elliptical, eccentric, loose; stator winding breaks. Line, ground breakdown, inter-turn short circuit, wiring error, stator three-phase current imbalance.
Line, ground breakdown, inter-turn short circuit, wiring error, stator three-phase current imbalance.
Example: Before the maintenance of the sealed fan motor in the boiler room, red powder was found on the stator core, suspecting that there was a loose phenomenon in the stator core.
However, it did not belong to the scope of standard overhaul projects, so it was not dealt with. During the test run after the overhaul, the motor made a piercing scream. The fault was resolved after replacing the stator.
Rotor failure: The rotor core becomes elliptical, eccentric, or loose. The rotor cage and end ring are welded open, the rotor cage breaks, there are wiring errors, and poor contact with the brushes.
Example: During the operation of the sleeper section, it was found that the motor stator current of the toothless saw motor oscillated back and forth, and the motor vibration gradually increased.
Based on the phenomenon, it was judged that there might be open welding and breakage of the motor rotor cage. After dismantling the motor, it was found that there were 7 fractures in the rotor cage, and 2 of them were severely fractured on both sides and the end ring. If not detected in a timely manner, it could potentially cause severe accidents such as stator burnout.
2. Mechanical reasons
In terms of the motor itself:
Rotor imbalance, shaft bending, slip ring deformation, uneven air gap between stator and rotor, inconsistent magnetic center of stator and rotor, bearing failure, poor foundation installation, insufficient mechanical structure strength, resonance, loose anchor bolts, damaged motor fan.
Typical case: After replacing the upper bearing of the condensate pump motor in the factory, the motor's shaking increased and there were slight signs of rubbing between the rotor and stator. After careful inspection, it was found that the lifting height of the motor rotor was incorrect and the magnetic center of the rotor and stator did not align.
After readjusting the thrust head screw cap, the motor vibration problem was eliminated. After the motor was lifted with a cross-line lifting ring for maintenance, the vibration remained significantly high and showed a gradual increase.
When the motor was unhooked, it was found that the motor still had significant vibration and axial movement. Upon disassembly, it was discovered that the rotor core was loose and there were balance issues with the rotor. After replacing the spare rotor, the problem was resolved, and the original rotor was sent back to the factory for repair.
In terms of coupling coordination:
Coupling damage, poor coupling connection, inaccurate coupling alignment, unbalanced load machinery, system resonance, etc. The misalignment of the linkage shaft system, non-coincidence of centerlines, and incorrect alignment are the main causes of these faults, mainly due to poor alignment and improper installation during the installation process. Another situation is that some centerlines of the linkage parts are aligned in a consistent manner when in a cold state, but after running for a period of time, vibration occurs due to deformation of the rotor support points, foundations, etc., which destroys the centerlines.
For example: a. The motor of the circulating water pump has been vibrating excessively during operation. There are no issues found during motor inspection, and it runs normally under no load. The pump team believes that the motor is operating normally. However, it was eventually discovered that the motor was significantly off-center. After realigning the motor, the vibration was eliminated.
b. After replacing the pulley of the boiler room's induced draft fan, the motor generated vibrations during the trial run, and the three-phase current of the motor increased. After checking all circuits and electrical components without any issues, it was finally discovered that the pulley was not qualified. After replacement, the motor vibrations were eliminated, and the three-phase current of the motor returned to normal.
Motor hybrid reasons:
Motor vibration is often caused by uneven air gaps, resulting in unbalanced electromagnetic forces. The unbalanced electromagnetic forces further increase the air gaps, and this combined mechanical and electrical effect manifests as motor vibration
Motor axial movement, caused by the gravity of the rotor itself or improper installation level and magnetic center, resulting in electromagnetic force, causing motor axial movement and increasing motor vibration. In severe cases, shaft bearing wear occurs, leading to a rapid increase in bearing temperature.
The gears and couplings connected to the motor have problems. This type of malfunction is mainly characterized by poor gear engagement, severe gear tooth wear, poor lubrication of the gears, misalignment or displacement of the couplings, incorrect tooth shape or tooth spacing of the gear couplings, excessive clearance or severe wear, all of which can cause certain vibrations.
Defects in the motor's own structure and installation issues.
This type of failure is mainly manifested as elliptical shaft neck, bent shaft, excessive or insufficient clearance between the shaft and the bearing, insufficient rigidity of the bearing seat, base plate, or foundation, inadequate fixation between the motor and the base plate, loose anchor bolts, and looseness between the bearing seat and the base plate. Excessive or insufficient clearance between the shaft and the bearing can not only cause vibration but also lead to abnormal lubrication and temperature of the bearing.
The load driven by the motor conducts vibration.
For example: The turbine vibration of a steam turbine generator, the fan and water pump vibration driven by the motor, causing motor vibration.
How to find the cause of vibration?
To eliminate motor vibration, it is necessary to first identify the cause of the vibration. Only by finding the cause of the vibration can targeted measures be taken to eliminate motor vibration.
1. Before the motor stops, use a vibration meter to check the vibration of each part. For parts with larger vibrations, test the vibration values in three directions: vertical, horizontal, and axial. If the anchor bolts or bearing end cover screws are loose, tighten them directly.
After tightening, measure the vibration again to observe if it has been eliminated or reduced. Next, check if the three-phase voltage of the power supply is balanced and if there are any blown fuses. Single-phase operation of the motor can not only cause vibration but also rapidly increase the motor's temperature. Observe if the ammeter pointer swings back and forth, as this phenomenon occurs when the rotor is broken.
Finally, check if the three-phase current of the motor is balanced. If any problems are found, promptly contact the operating personnel to stop the motor to prevent it from being damaged by burning.
2. If the vibration of the motor is not resolved after dealing with surface phenomena, continue to disconnect the power supply, loosen the coupling, and separate the motor from the load machinery connected to it. Run the motor alone. If the motor itself does not vibrate, it indicates that the source of vibration is due to the misalignment of the coupling or the load machinery. If the motor vibrates, it indicates that the motor itself has a problem. Additionally, the power-off method can be used to differentiate between electrical and mechanical causes. If the motor immediately stops vibrating or the vibration is reduced when the power is cut off, it indicates an electrical cause; otherwise, it is a mechanical failure.
Carry out maintenance for the cause of the malfunction
1. Electrical troubleshooting due to electrical reasons.
First, determine whether the three-phase DC resistance of the stator is balanced. If it is unbalanced, it indicates that there is an open welding phenomenon in the stator connection welding area. Disconnect the winding phase to search for it. In addition, check if there is an inter-turn short circuit in the winding. If the fault is obvious, burnt marks can be seen on the insulation surface, or use instruments to measure the stator winding. After confirming the inter-turn short circuit, disconnect the motor winding again.
For example: Water pump motor, during operation, the motor not only vibrates but also has a high temperature in the main bearing. After conducting a minor repair test, it was found that the DC resistance of the motor was not up to standard and there were open welds in the stator winding. By using the process of elimination, the fault was identified and eliminated, and the motor is now running normally.
2. Mechanical troubleshooting:
Check if the air gap is uniform. If the measurement exceeds the standard, readjust the air gap. Check the bearings and measure the clearance between them. If it is not up to standard, replace the bearings. Check for deformation and looseness of the iron core. Loose iron cores can be filled with epoxy resin adhesive. Check the rotating shaft. Repair or straighten the bent shaft, then perform a balance test on the rotor. During the trial run after the major overhaul of the fan motor, not only did the motor vibrate greatly, but the temperature of the bearing bush exceeded the standard. After several days of continuous troubleshooting, the problem remained unresolved. Our team members discovered that the motor had a very large air gap and the bearing seat was also not up to standard. After identifying the cause of the problem, the gaps between various parts were readjusted, and the motor successfully completed one trial rotation.
3. The load machinery part is checked normal, and the motor itself has no problem.
The cause of the malfunction is due to the connection part. At this time, it is necessary to check the motor's base level, inclination, strength, correct center alignment, whether the coupling is damaged, and whether the motor shaft deflection meets the requirements.
Steps to handle motor vibration
1. Disconnect the motor from the load, run a no-load test on the motor, and measure the vibration level.
2. Check the vibration value of the motor base according to the national standard GB10068-2006. The vibration value at the base plate should not exceed 25% of the corresponding position of the bearing. If it exceeds this value, it indicates that the motor foundation is not rigid.
3. If only one of the four base feet or two diagonal feet vibrate excessively, loosen the ground foot bolts, and the vibration will become acceptable. This indicates that the base feet are not properly cushioned, and tightening the ground foot bolts causes deformation of the machine base, resulting in vibration. Make sure the base feet are properly cushioned, realign them, and tighten the ground foot bolts.
4. Tighten all four anchor bolts on the base. Even after doing so, if the vibration value of the motor still exceeds the standard, check whether the coupling installed on the shaft extension is aligned with the shaft shoulder.
If it is not aligned, the excessive excitation force generated by the extra key on the shaft extension will cause the motor to vibrate horizontally beyond the standard. In this case, the vibration value does not exceed by much and often decreases after the motor is connected to the main unit. It should be explained to the user and convince them to use it.
During the factory test of the two-pole motor, according to GB10068-2006, a half key is installed in the keyway of the shaft extension. The extra key will not add additional excitation force. If necessary, simply cut off the extra key to the general excess length.
5. If the motor has acceptable vibration during a no-load test but exceeds the vibration limit under load, there are two possible reasons: one is a significant positive deviation; the other is the overlap of residual unbalance of the rotating parts (rotor) of the main unit and the residual unbalance of the motor rotor, resulting in a large residual unbalance at the same position of the entire shaft system after coupling, causing significant vibration.
In this case, the coupling can be disconnected, and either one of the couplings can be rotated 180 degrees before re-coupling for testing, which will reduce the vibration.
6. The vibration velocity (intensity) does not exceed the standard, but the vibration acceleration exceeds the standard, so only the bearings need to be replaced.
7. Due to poor rigidity, the rotor of a high-power motor may deform if left unused for a long time, which may cause vibrations when it is turned on again. This is due to improper storage of the motor. Under normal circumstances, during the storage period of the motor, it is necessary to rotate the motor at least 8 turns every 15 days.
8. The motor vibration of the sliding bearing is related to the assembly quality of the bearing shell. It is necessary to check if there are high points on the bearing shell, if the oil supply to the bearing shell is sufficient, the tightness of the bearing shell, the clearance between the bearing shell, and if the magnetic centerline is appropriate.
9. In general, the causes of motor vibration can be simply determined by the magnitude of vibration in three directions: if the horizontal vibration is large, it indicates rotor imbalance; if the vertical vibration is large, it suggests poor installation foundation; if the axial vibration is large, it indicates poor bearing assembly quality. However, this is just a simple judgment.
To find the true cause of the vibration, it is necessary to consider the on-site situation and comprehensively analyze the factors mentioned above.
10. The vibration of the Y-series box-type motor should pay special attention to axial vibration.
If the axial vibration is greater than the radial vibration, it will cause great harm to the motor bearings and may lead to shaft-holding accidents. Pay attention to observe the bearing temperature. If the temperature of the locating bearing rises faster than that of the non-locating bearing, the motor should be stopped immediately. This is because the insufficient axial stiffness of the motor base causes axial vibration, and the motor base should be reinforced.
11. After dynamic balancing, the residual unbalance of the rotor has been fixed on the rotor and will not change. The vibration of the motor itself will also not change with the variation of location or operating conditions.
It can handle vibration issues at the user's site. In general, it is not necessary to perform dynamic balancing verification on the motor during maintenance, except in very special cases such as flexible foundations or rotor deformation, where on-site dynamic balancing or factory processing is required.
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