Generally, motors with 8 or more poles do not experience vibration due to manufacturing defects. Vibration is more common in 2-6 pole motors, according to GB10068-2000.
The standard "Vibration Limits and Test Methods for Rotating Electrical Machines " specifies the vibration limits, measurement methods, and criteria for determining the rigidity of the foundation for motors with different center heights on a rigid foundation. Based on this standard, it can be determined whether the motor meets the standard.
The hazards of electric motor vibration
Vibration in an electric motor can shorten the lifespan of winding insulation and bearings, affect the normal lubrication of sliding bearings, and cause insulation gaps to widen, allowing external dust and moisture to intrude, resulting in reduced insulation resistance and increased leakage current, and even insulation breakdown and other accidents.
In addition, the vibration generated by the motor can easily cause the cooler water pipes to crack, the weld joints to break, and it can also damage the load machinery, reduce the precision of the workpiece, cause fatigue in all the mechanical parts that are subjected to vibration, and cause the anchor bolts to loosen or break.
Motor vibration can cause abnormal wear of carbon brushes and slip rings, and may even lead to severe sparking that burns the slip ring insulation. This will cause the motor to generate a lot of noise, a situation that also occurs frequently in DC motors.
Vibration cause
There are three main situations:
Electromagnetic reasons; mechanical reasons; electromechanical hybrid reasons.
I. Electromagnetic reasons
1. Power supply: Three-phase voltage is unbalanced, and the three-phase motor is operating with a single phase.
2. Stator: The stator core becomes elliptical, eccentric, or loose; the stator winding has open circuits, grounding breakdowns, inter-turn short circuits, wiring errors, and unbalanced three-phase currents.
Typical Case:
Before overhauling the boiler room sealing fan motor, red powder was found on the stator core, suggesting that the stator core might be loose. However, this was not within the scope of the standard overhaul and was therefore not addressed. After the overhaul, the motor emitted a piercing scream during test run. The fault was resolved after replacing the stator.
3. Rotor faults: The rotor core becomes elliptical, eccentric, or loose. Other issues include: weld failure between the rotor cage bars and end rings, broken rotor cage bars, incorrect winding, and poor brush contact.
Typical Case:
During operation of the toothed saw motor in the sleeper section, it was found that the stator current of the motor was swinging back and forth, and the motor vibration was gradually increasing. Based on the phenomenon, it was judged that the rotor cage bars of the motor may have been welded open and broken. After the motor was disassembled, it was found that there were 7 broken rotor cage bars, and the two most serious bars were completely broken on both sides and end rings. If it had not been discovered in time, it could have caused a serious accident of stator burnout.
II. Mechanical reasons
1. The motor itself
Rotor imbalance, bent shaft, deformed slip ring, uneven air gap between stator and rotor, inconsistent magnetic centers between stator and rotor, bearing failure, poor foundation installation, insufficient strength of mechanical mechanism, resonance, loose foundation bolts, and damaged motor fan.
Typical Case:
After the upper bearing of the condensate pump motor was replaced, the motor vibration increased, and there were slight signs of rotor and stator rubbing. After careful inspection, it was found that the motor rotor was not lifted at the correct height and the magnetic center of the rotor and stator was not aligned. After readjusting the thrust head screw cap, the motor vibration fault was eliminated.
The vibration of the cross-line hoist motor remained excessive after maintenance and showed signs of gradually increasing. When the motor was lowered by the hook, it was found that the motor vibration was still very large and there was a lot of axial movement. Upon disassembly, it was found that the rotor core was loose and the rotor balance was also problematic. After replacing it with a spare rotor, the fault was eliminated and the original rotor was returned to the factory for repair.
2. Compatibility with couplings
Damaged couplings, poor coupling connections, inaccurate coupling alignment, unbalanced load machinery, system resonance, etc. Misalignment of the shafts in the linkage components, non-coincidence of center lines, and incorrect centering are also possible causes. These faults are primarily caused by improper alignment or installation during the installation process.
Another situation is that the center lines of some linkage parts are aligned when cold, but after running for a period of time, due to the deformation of the rotor support, foundation, etc., the center lines are broken again, thus causing vibration.
Typical Case:
a. The motor of the circulating water pump had been vibrating excessively during operation. The motor was inspected and found to be working properly, and it was also running normally under no-load conditions. The pump crew believed that the motor was operating normally. However, the final inspection revealed that the motor was not aligned properly. After the pump crew realigned the motor, the vibration was eliminated.
b. After replacing the pulley of the boiler room induced draft fan, the motor vibrated during the test run, and the three-phase current of the motor increased. After checking all circuits and electrical components, no problems were found. Finally, it was found that the pulley was unqualified. After replacing it, the motor vibration was eliminated, and the three-phase current of the motor also returned to normal.
III. Reasons for Motor Mixing
1. Motor vibration is often caused by uneven air gap, which leads to unilateral electromagnetic pull. This unilateral electromagnetic pull further increases the air gap. This electromechanical hybrid effect manifests as motor vibration.
2. Axial movement of the motor: Due to the rotor's own weight, improper installation level, or misalignment of the magnetic center, electromagnetic pull is generated, causing axial movement of the motor and increasing motor vibration. In severe cases, shaft wear occurs at the bearing root, causing the bearing temperature to rise rapidly. Faults in the gears and couplings connected to the motor can also cause this. This type of fault mainly manifests as poor gear meshing, severe tooth wear, poor lubrication of the couplings, misalignment or misalignment of the couplings, and incorrect tooth profile, pitch, excessive clearance, or severe wear of gear couplings, all of which can cause vibration.
Defects in the motor's structure and installation issues can cause these problems. These faults mainly manifest as elliptical journals, bent shafts, excessively large or small clearances between the shaft and bearing bush, insufficient rigidity of the bearing housing, base plate, foundation, or even the entire motor mounting base, loose fixing between the motor and the base plate, loose foundation bolts, and looseness between the bearing housing and the base plate. Excessively large or small clearances between the shaft and bearing bush can not only cause vibration but also lead to abnormal lubrication and temperature of the bearing bush.
3. Vibration transmitted by the load driven by the motor.
Typical Case:
The vibration of the steam turbine in the steam generator, and the vibration of the fan and water pump driven by the motor, cause the motor to vibrate.
How to find the cause of vibration
1. Before stopping the motor, use a vibration meter to check the vibration of each part. For areas with large vibrations, measure the vibration values in detail in three directions: vertical, horizontal, and axial. If the foundation bolts or bearing end cover bolts are loose, tighten them directly. After tightening, measure the vibration again and observe whether it has been eliminated or reduced.
Secondly, check if the three-phase voltage of the power supply is balanced and if any of the three-phase fuses are blown. Single-phase operation of the motor can not only cause vibration but also cause the motor temperature to rise rapidly. Observe whether the ammeter pointer swings back and forth. When the rotor bar is broken, the current swings will occur.
Finally, check whether the three-phase current of the motor is balanced. If any problems are found, contact the operators immediately to stop the motor from running, so as to avoid burning out the motor.
2. If the motor vibration is not resolved after addressing the surface issues, continue to disconnect the power supply, uncouple the coupling, and mechanically separate the motor from the load connected to it, allowing the motor to rotate independently.
If the motor itself does not vibrate, the vibration source is likely due to misalignment of the coupling or the load mechanism. If the motor vibrates, the problem lies with the motor itself.
Another method is to use the power outage method to distinguish between electrical and mechanical causes. If the motor stops vibrating or the vibration decreases immediately upon power failure, it indicates an electrical cause; otherwise, it is a mechanical failure.
Repair the fault based on its cause.
1. Troubleshooting electrical issues:
First, check if the three-phase DC resistance of the stator is balanced. If it is unbalanced, it indicates that there is a weld break at the stator connection welding point. Disconnect the winding phases to check. Also, check if there is a short circuit between turns in the winding. If the fault is obvious, you can see burn marks on the insulation surface, or use an instrument to measure the stator winding. After confirming the short circuit between turns, rewire the motor winding.
Typical Case:
The water pump motor was vibrating excessively and the bearing temperature was too high during operation. A minor repair test revealed that the motor's DC resistance was not up to standard and there was a weld breakage in the motor stator winding. After the fault was found and eliminated by the elimination method, the motor operated normally.
2. Troubleshooting mechanical issues:
Check if the air gap is uniform. If the measured value exceeds the standard, readjust the air gap. Inspect the bearings and measure the bearing clearance. If it is not up to standard, replace the bearing. Check the core for deformation and looseness. Loose cores can be glued and filled with epoxy resin. Inspect the shaft. For bent shafts, repair by welding and remachining or straighten the shaft directly. Then perform a balance test on the rotor.
3. Overhaul of the load-bearing mechanical parts:
If the motor itself is not faulty, then the cause of the malfunction is the connection. In this case, check the motor's foundation level, tilt, strength, whether the center alignment is correct, whether the coupling is damaged, and whether the motor shaft extension deflection meets the requirements.
Steps to handle motor vibration
1. Disconnect the motor from the load, test the motor without load, and check the vibration value.
2. Check the vibration value of the motor base. According to the national standard GB10068-2006, the vibration value at the base plate shall not exceed 25% of the corresponding position of the bearing. If it exceeds this value, it means that the motor foundation is not a rigid foundation.
3. If only one or two diagonally opposite feet exceed the vibration standard, loosen the anchor bolts and the vibration will be within acceptable limits. This indicates that the foot is not properly shimmed . Tightening the anchor bolts caused the machine base to deform and generate vibration . Shim the foot properly, realign it, and tighten the anchor bolts.
4. If the vibration value of the motor still exceeds the standard after tightening all four anchor bolts on the foundation, check whether the coupling installed on the shaft extension is flush with the shaft shoulder. If not, the excitation force generated by the excess key on the shaft extension will cause the horizontal vibration of the motor to exceed the standard.
In this situation, the vibration value will not exceed the limit by much, and it often decreases after docking with the host machine. Users should be persuaded to use it. During factory testing of the two-pole motor, a half-key is installed in the keyway of the shaft extension according to GB10068-2006. Excess keys will not add extra excitation force. If necessary, simply trim off half of the excess key length.
5. If the vibration of the motor is within the standard during no-load testing but exceeds the standard under load, there are two possible reasons: one is that the alignment deviation is too large;
Another cause is that the residual imbalance of the rotating parts (rotor) of the host machine and the residual imbalance of the motor rotor are in phase overlap. After docking, the residual imbalance of the entire shaft system at the same position is large, resulting in a large excitation force and causing vibration. In this case, the coupling can be disengaged, either of the two couplings can be rotated 180°, and then docked again for testing. The vibration will decrease.
6. If the vibration velocity (intensity) is within the standard, but the vibration acceleration is within the standard, the bearing must be replaced.
7. Due to their poor rigidity, the rotor of a two-pole high-power motor will deform if it is not used for a long time, and may vibrate when it is turned on again. This is caused by improper storage of the motor. Under normal circumstances, during the storage period of a two-pole motor, the motor should be rotated every 15 days, and each rotation should be at least 8 revolutions.
8. The vibration of the motor in the sliding bearing is related to the assembly quality of the bearing bush. Check whether there are high points in the bearing bush, whether the oil supply to the bearing bush is sufficient, the bearing bush tightness, the bearing bush clearance, and whether the magnetic center line is appropriate.
9. Generally, the cause of motor vibration can be easily judged by the magnitude of vibration in three directions: large horizontal vibration indicates rotor imbalance; large vertical vibration indicates an uneven or poor installation foundation; and large axial vibration indicates poor bearing assembly quality.
This is just a simple judgment. The real cause of the vibration needs to be found by considering the situation on site and the factors mentioned above.
10. Special attention should be paid to axial vibration in Y-series box motors. If the axial vibration is greater than the radial vibration, it will cause great damage to the motor bearings and may lead to bearing seizure.
Pay close attention to the bearing temperature. If the locating bearing heats up faster than the non-locating bearing, stop the machine immediately. This is due to axial vibration caused by insufficient axial stiffness of the machine base, and the machine base should be reinforced.
11. After dynamic balancing, the residual imbalance of the rotor is fixed on the rotor and will not change. The vibration of the motor itself will not change with the location or working conditions. Vibration problems can be handled well at the user's site.
Under normal circumstances, it is not necessary to perform dynamic balancing on the motor during maintenance, except in very special cases, such as flexible foundations or rotor deformation, in which case on-site dynamic balancing or returning to the factory is required.
