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Causes and prevention of motor vibration

source：未知   time：2024-10-31 09:18nbsp;  click：

Introduction
Motor vibration will shorten the life of winding insulation materials and bearings, affect the normal lubrication of bearings, expand the gap between insulation materials and electromagnetic wires, and easily invade external dust and moisture, resulting in reduced insulation resistance, increased leakage current, and even insulation breakdown failure. In addition, motor vibration can easily cause equipment welding points to open, damage load machinery, reduce workpiece accuracy, loosen or break anchor screws, and generate noise.
1. Analysis of motor vibration causes There are three main reasons for motor vibration: electromagnetic, mechanical and electromechanical mixed. The main electromagnetic reasons are:
1) The three-phase voltage of the power supply is unbalanced, and the motor runs in a phase-missing manner.
2) The stator core is elliptical, eccentric, and loose; the stator winding is broken, grounded, short-circuited, and the wiring is wrong, and the stator three-phase current is unbalanced.
3) The rotor core is elliptical, eccentric, and loose. The cage bars and end rings of the rotor are welded, broken, and the winding is wrong, etc.
 
The main mechanical reasons are:
1) The rotor of the motor itself is unbalanced, the shaft is bent, the air gap between the stator and the rotor is uneven, the magnetic center of the stator and the rotor is inconsistent, the bearing is faulty, the foundation is poorly installed, the mechanical structure is not strong enough, resonance, the anchor screws are loose, and the motor fan is damaged.
2) The motor and the dragged machinery have problems, the coupling is damaged, the coupling connection is poor, the coupling center is inaccurate, the load machinery is unbalanced, the system resonance, etc. The shaft system of the linkage part is not centered, the center line does not overlap, and the centering is incorrect. The main cause is poor alignment and improper installation during the installation process. Another situation is that the center line of the linkage part coincides when it is cold. After running for a period of time, the rotor fulcrum and the foundation are deformed, and the center line is destroyed, causing vibration. For example, a. The vibration of the circulating water pump motor has been too large during operation. There is no problem in the inspection, and everything is normal when it is unloaded. Check the motor alignment and find that the center difference is too large. After alignment, the motor vibration is eliminated. b. After the pulley of the boiler room induced draft fan was replaced, the motor vibrated during the trial operation, and the three-phase current of the motor increased. After checking all circuits and electrical components, it was found that the pulley was unqualified. After replacement, the motor vibration was eliminated, and the three-phase current of the motor also returned to normal.
The main reasons for electromagnetic mixing are:
1) The uneven air gap of the motor causes unilateral electromagnetic tension, which further increases the air gap. This electromechanical mixing effect is manifested as motor vibration.
2) The axial movement of the motor. Due to the gravity of the rotor itself or the installation level and the misalignment of the magnetic center, the electromagnetic tension caused by the electromagnetic tension causes the axial movement of the motor, causing the motor vibration to increase. There are problems with the gears and couplings connected to the motor, which are manifested as poor gear engagement, severe wear of the gear teeth, poor lubrication of the wheels, skewed and misaligned couplings, incorrect tooth shape and pitch of the gear coupling, excessive gap or severe wear, which will cause certain vibrations. The structural defects and installation problems of the motor itself are mainly manifested as the oval of the shaft neck, the bending of the rotating shaft, the bearing seat, the foundation plate, a part of the foundation, and even the entire motor installation foundation. The rigidity is not enough, the motor and the foundation plate are not fixed firmly, the foot bolts are loose, the bearing seat and the foundation plate are loose, etc. There are two reasons why the motor does not exceed the standard in the no-load test, but the load vibration exceeds the standard: one is that the alignment deviation is large; the other is that the residual unbalance of the rotating parts (rotor) of the main machine and the residual unbalance of the motor rotor overlap in phase. After docking, the residual unbalance of the entire shaft system at the same position is large, and the excitation force generated is large, causing vibration. At this time, the coupling can be disengaged, and either of the two couplings can be rotated 180°, and then docked for testing, and the vibration will decrease.
2) The vibration intensity does not exceed the standard, but the vibration acceleration exceeds the standard, and the bearing can only be replaced. Due to the poor rigidity of the rotor of the two-pole high-power motor, the rotor will be deformed if it is not used for a long time, and it may vibrate when it is rotated again. This is due to poor storage of the motor. Under normal circumstances, the two-pole motor is stored during. The motor should be cranked every 15 days, and each time it should be cranked for at least 8 turns.
3) In general, the cause of motor vibration can be simply judged from the vibration size in three directions. If the horizontal vibration is large, the rotor is unbalanced; if the vertical vibration is large, the installation foundation is uneven; if the axial vibration is large, the bearing assembly quality is poor. If the axial vibration of the motor is greater than the radial vibration, it will cause great harm to the motor bearing and cause a shaft sticking accident. Pay attention to the bearing temperature. If the locating bearing heats up faster than the non-locating bearing, the machine should be shut down immediately. This is because the axial vibration caused by the insufficient axial stiffness of the machine base should be reinforced. After the rotor is dynamically balanced, the residual imbalance of the rotor has been solidified on the rotor and will not change, and the vibration of the motor itself will not change with the change of location and working conditions. In general, it is not necessary to perform dynamic balancing on the motor when repairing it, except for extremely special cases, such as flexible foundation, rotor deformation, etc., which require on-site dynamic balancing or return to the factory for processing
 
2. Measures to eliminate motor vibration When the motor vibrates, first use a vibration meter to check the vibration of each part. For parts with larger vibration, test the vibration values ​​in detail in the vertical, horizontal and axial directions. If the anchor screws or bearing end cover screws are loose, they can be tightened directly. After tightening, measure the vibration again to see if it is eliminated or reduced. Secondly, check whether the three-phase voltage of the power supply is balanced and whether the three-phase fuse is blown. Single-phase operation of the motor can not only cause vibration, but also make the temperature of the motor rise rapidly. Observe whether the ammeter pointer swings back and forth. Current swing will occur when the rotor bar is broken. Finally, check whether the three-phase current of the motor is balanced. If any problem is found, stop the motor in time to avoid burning the motor. If the motor still vibrates after the surface phenomenon is treated, it is necessary to disconnect the coupling, separate the load machine connected to the motor, and turn the motor alone. If the motor itself does not vibrate, it means that the vibration source is caused by the misalignment of the coupling or the load machine. If the motor vibrates, it means that there is a problem with the motor itself. In addition, the power-off method can be used to distinguish whether it is an electrical reason or a mechanical reason. When the power is off, the motor does not vibrate or the vibration is reduced, which means that it is an electrical reason, otherwise it is a mechanical failure. To repair the cause of the vibration failure, first determine whether the three-phase DC resistance of the stator is balanced. If it is unbalanced, it means that there is an open weld at the welding part of the stator connection. Disconnect the winding phases to find whether there is a short circuit between turns in the winding. If the fault is obvious, you can see the burn marks on the insulation surface, or use an instrument to measure the stator winding. After confirming the short circuit between turns, the motor winding is taken offline again. For example: a water pump motor, the motor not only vibrates greatly during operation, but also has a high bearing temperature. A minor repair test found that the motor DC resistance was unqualified and the motor stator winding had an open weld. After the fault was found and eliminated by the elimination method, the motor ran normally. Secondly, check whether the air gap is uniform. If the measured value exceeds the standard, readjust the air gap. Check the bearings and measure the bearing clearance. If it is unqualified, replace it with a new bearing. Check the deformation and looseness of the core. The loose core can be glued and filled with epoxy resin glue. Check the shaft, re-weld the bent shaft or directly straighten the shaft, and then do a balance test on the rotor. If the load mechanical part of the motor is normal and there is no problem with the motor itself, the cause of the failure may be caused by the connection part. At this time, check the horizontal plane of the motor foundation, inclination, strength, center alignment, whether the coupling is damaged, and whether the shaft body deflection meets the requirements. First, disconnect the motor and the load, idle the motor, and detect the vibration value. Check the vibration value of the motor foot. According to the national standard GB10068-2006, the vibration at the foot plate shall not be greater than 25% of the corresponding position of the bearing. If it exceeds this value, it means that the motor foundation is not a rigid foundation. If only one of the four feet or two of the diagonal feet have excessive vibration, loosen the anchor bolts and the vibration will be acceptable, indicating that the foot pad is not solid, and the anchor bolts cause the base to deform and vibrate after tightening. Pad the foot, realign, and tighten the anchor bolts. If all four anchor bolts on the foundation are fully tightened, and the vibration of the motor still exceeds the standard, check whether the coupling is flush with the shaft shoulder. If not, the exciting force generated by the extra key on the shaft body will cause the horizontal vibration of the motor to exceed the standard. In this case, the vibration will not exceed the standard too much, and the vibration energy will often decrease after docking with the host. During the factory test, the motor is installed with a half key in the keyway of the shaft body, that is, the full key is cut off half of the length or height, and the test is performed after installation.
 
3. Conclusion Vibration is an important indicator of the dynamic mechanical performance of the motor, which is guaranteed by the balance and assembly quality of the rotor. The motors produced in batches are judged by human perception and regularly sampled and tested with vibration meters. For motors that are matched with precision equipment, a vibration meter is used for full inspection during the factory test to ensure that the motor's no-load vibration is acceptable. For motors that vibrate suddenly and abnormally during use, it is usually necessary to first distinguish whether it is caused by mechanical or electromagnetic reasons, and analyze whether it is caused by equipment or installation problems. Then, according to the specific situation, take appropriate measures to eliminate the fault.

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