Ten Causes and Solutions of Abnormal Vibration of Motor

Ten causes of motor vibration

1. Unbalanced rotor, coupler, coupling and transmission wheel (brake wheel).

2. Loose iron core support, failure of inclined key and pin, and loose binding of rotor will cause imbalance of rotating parts.

3. The shafting of the linkage part is not aligned, the center lines are not coincident, and the centering is incorrect. This kind of failure is mainly caused by poor alignment and improper installation during installation.

4. The center line of the linkage part is coincident in the cold state, but after running for a period of time, the center line is destroyed again due to the deformation of the rotor fulcrum and foundation, resulting in vibration.

5. Faulty gears and couplings connected with the motor, bad gear occlusion, serious wear of gear teeth, poor lubrication of the gears, skewed and misplaced coupling, incorrect tooth shape and pitch of the gear coupling, excessive clearance or serious wear will all cause certain vibration.

6. The structural defects of the motor itself, such as elliptical journal, bent shaft, too large or too small clearance between shaft and bearing bush, and insufficient rigidity of bearing seat, foundation plate and foundation of the whole motor installation foundation.

7. Installation problems, such as unstable fixation between the motor and the foundation plate, loose foot bolts, loose bearing seat and foundation plate, etc.

8. Too large or too small clearance between shaft and bearing bush can not only cause vibration, but also cause abnormal lubrication and temperature of bearing bush.

9. The load driven by the motor conducts vibration, such as the vibration of the fan and water pump driven by the motor, which causes the motor vibration.

10. AC motor stator wiring error, wound asynchronous motor rotor winding short circuit, synchronous motor excitation winding turn-to-turn short circuit, synchronous motor excitation coil connection error, cage asynchronous motor rotor bar broken, rotor core deformation causes uneven air gap between stator and rotor, resulting in air gap flux imbalance and vibration.

Harm of motor vibration

The vibration of the motor will shorten the winding insulation and bearing life, affect the normal lubrication of the sliding bearing, and the vibration force will cause the insulation gap to expand, so that the external dust and moisture will invade into it, resulting in the decrease of insulation resistance and the increase of leakage current, even resulting in insulation breakdown and other accidents. In addition, when the motor vibrates, it is easy to crack the water pipe of the cooler, and the welding point vibrates. At the same time, it will cause the damage of the load machine, reduce the precision of the workpiece, cause the fatigue of all the vibrated mechanical parts, loosen or break the anchor screw, and the motor will cause the abnormal wear of the carbon brush and slip ring, which may even cause serious brush fire and burn the collector ring insulation, and the motor will make a lot of noise. This kind of situation usually happens from time to time in DC motors.

Causes of vibration and typical cases

There are three main reasons for vibration: electromagnetic reasons; Mechanical reasons; Electromechanical reasons.

1. electromagnetic reasons

(1) Power supply: the three-phase voltage is unbalanced, and the three-phase motor runs out of phase.

(2) Stator: the stator core becomes elliptical, eccentric and loose; The stator has broken wire, ground breakdown, turn-to-turn short circuit and wrong wiring, and the three-phase current of the stator is unbalanced.

For example, red powder was found in the stator core of the sealed fan motor in boiler room before overhaul, and it was suspected that the stator core was loose, but it did not belong to the standard overhaul scope, so it was not treated. After overhaul, the motor screamed harshly during the trial run, and the fault was eliminated after replacing a stator.

(3) Rotor failure: the rotor core becomes elliptical, eccentric and loose. Open welding between rotor cage and end ring, broken rotor cage, wrong winding, poor brush contact, etc.

For example, during the operation of the toothless saw motor in the sleeper section, it is found that the motor stator current swings back and forth, and the vibration of the motor gradually increases. According to the phenomenon, it is judged that the rotor cage bars of the motor may be welded and broken. After the motor is disassembled, it is found that seven rotor cages are broken, and two serious two sides and end rings are all broken. If it is not found in time, it may cause a bad accident of stator burning.

2. Mechanical reasons

(1) the motor itself:

Unbalanced rotor, bent shaft, deformation of slip ring, uneven air gap between stator and rotor, inconsistent magnetic center between stator and rotor, bearing failure, poor foundation installation, insufficient strength and resonance of mechanical mechanism, loose anchor screws, and damage of motor fan.

Typical case: After replacing the upper bearing of the condensate pump motor in the factory, the motor shaking increased, and the rotor and stator showed slight signs of bore sweeping. After careful inspection, it was found that the lifting height of the motor rotor was wrong, and the magnetic centers of the rotor and stator were not aligned. After re-adjusting the thrust head screw cap, the motor vibration fault was eliminated. After overhaul, the vibration of the overhead hoist motor is always on the high side, and it shows signs of increasing gradually. When the motor falls off the hook, it is found that the motor vibration is still very large, and there is a great axial string motion. After disintegration, it is found that the rotor core is loose, and the rotor balance is also problematic. After replacing the spare rotor, the fault is eliminated, and the original rotor is returned to the factory for repair.

(2) Cooperation with coupling: coupling is damaged, coupling connection is poor, coupling alignment is inaccurate, load machinery is unbalanced, system resonance, etc.

Part of the linkage shafting is misaligned, the center lines are not coincident, and the centering is incorrect. This kind of failure is mainly caused by poor alignment and improper installation during installation. There is another case, that is, the centerlines of some linkage parts coincide in the cold state, but after running for a period of time, the centerlines are destroyed due to the deformation of the rotor fulcrum and foundation, which leads to vibration.

A, circulating water pump motor, vibration has been too large in operation, motor inspection without any problems, no-load and everything is normal, water pump class think that the motor is running normally, and finally check out the motor alignment center difference is too much, water pump class after a new alignment, motor vibration is eliminated.

B. After replacing the pulley of the induced draft fan in the boiler room, the motor vibrates during the trial operation and the three-phase current of the motor increases. After checking all circuits and electrical components, it is found that the pulley is unqualified. After replacement, the motor vibration is eliminated and the three-phase current of the motor also returns to normal.

3. Reasons for motor mixing

(1) Motor vibration is often uneven air gap, which leads to unilateral electromagnetic tension, and unilateral electromagnetic tension further increases the air gap. This electromechanical hybrid action is motor vibration.

(2) The axial movement of the motor, due to the electromagnetic tension caused by the gravity of the rotor itself or the installation level and the wrong magnetic center, causes the axial movement of the motor, causing the vibration of the motor to increase. In serious cases, the shaft wears the shoe root, which causes the temperature of the bearing bush to rise rapidly.

(3) There is something wrong with the gears and couplings associated with the motor. This kind of failure is mainly manifested in bad gear occlusion, serious wear of gear teeth, poor lubrication of gears, skew and dislocation of coupling, incorrect tooth shape and pitch of tooth coupling, excessive clearance or serious wear, which will all cause certain vibration.

(4) Structural defects and installation problems of the motor itself. This kind of failure is mainly manifested as elliptical journal, bent shaft, too large or too small clearance between shaft and bearing bush, insufficient rigidity of the whole motor installation foundation of bearing seat, foundation plate and foundation, unstable fixing between motor and foundation plate, loose foot bolts, loose between bearing seat and foundation plate, etc. However, too large or too small clearance between shaft and bearing bush can not only cause vibration, but also cause abnormal lubrication and temperature of bearing bush.

(5) The load dragged by the motor conducts vibration. For example, the vibration of steam turbine of turbogenerator, the vibration of fan and water pump driven by motor cause the vibration of motor.

How to find the cause of vibration

To eliminate motor vibration, we must first find out the causes of vibration. Only by finding out the causes of vibration can we take targeted measures to eliminate motor vibration.

1. Before the motor is stopped, check the vibration of each part with a vibration meter. For the parts with large vibration, test the vibration values in three directions vertically, horizontally and axially in detail. If the anchor screw is loose or the bearing end cap screw is loose, fasten it directly. After fastening, measure the vibration 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 burnt out. Single-phase operation of the motor can not only cause vibration, It will also cause the temperature of the motor to rise rapidly. Observe whether the ammeter pointer swings back and forth, and the current swings when the rotor bar is broken. Check whether the three-phase current of the motor is balanced, and contact the operating personnel to stop the motor when problems are found, so as to avoid burning the motor.

2. If the vibration of the motor is not solved after the surface phenomenon is treated, continue to disconnect the power supply, untie the coupling, and separate the load machine connected to the motor. 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 something wrong with the motor itself. In addition, the power-off method can be adopted to distinguish whether it is the electrical reason or the mechanical reason. When the power is cut off, the motor does not vibrate or the vibration is reduced immediately, then

Overhaul according to the cause of the failure.

1. Maintenance for electrical reasons:

First of all, determine whether the three-phase DC resistance of the stator is balanced. If it is unbalanced, it means that there is open welding at the welding part of the stator connection. Disconnect the winding phases to find out. In addition, whether there is turn-to-turn short circuit in the winding, if the fault is obvious, scorch marks can be seen from the insulation surface. Or, measure the stator winding with an instrument. After the turn-to-turn short circuit is confirmed, the motor winding will be offline again.

For example, water pump motor, in operation, the motor not only vibrates greatly, but also the bearing temperature is high. Minor repair tests show that the DC resistance of the motor is unqualified, and the stator winding of the motor is open-welded. After troubleshooting, the motor runs normally.

2. Maintenance for mechanical reasons:

Check whether the air gap is even, and if the measured value exceeds the standard, readjust the air gap. Check the bearing and measure the bearing clearance. If the bearing is unqualified, replace it with a new one. Check the deformation and looseness of the iron core. The loose iron core can be cemented with epoxy resin, check the rotating shaft, repair the bent rotating shaft, rework it or straighten it directly, and then do a balance test on the rotor. During the commissioning of the fan motor after overhaul, the motor not only vibrates greatly, but also the temperature of the bearing bush exceeds the standard. After several days of continuous treatment, the fault remains unsolved. My team members found that the air gap of the motor was very large and the tile seat level was not qualified. After the cause of the failure was found, the clearances of various parts were readjusted, and the motor had a successful trial run.

3. Check that the mechanical part of the load is normal:

There is no problem with the motor itself, and the fault is caused by the connecting part. At this time, check whether the foundation level, inclination, strength, center alignment of the motor are correct, whether the coupling is damaged, and whether the extension and winding degree of the motor shaft meet the requirements.