Ten reasons for motor vibration

There are many reasons for motor vibration, and they are also very complicated. Motors with more than 8 poles will not cause vibration due to motor manufacturing quality problems. Vibration is common in 2-6 pole motors. GB10068-2000, "Vibration Limits and Test Methods for Rotating Motors", stipulates the vibration limits, measurement methods and rigidity judgment standards for motors with different center heights on a rigidity basis. Based on this standard, it can be judged whether the motor meets the standard.

The harm of motor vibration

The vibration of the motor will shorten the life of the winding insulation and bearings, affect the normal lubrication of the sliding bearings, and the vibration force will cause the insulation gap to expand, allowing external dust and moisture to invade, resulting in reduced insulation resistance and increased leakage current, and even insulation breakdown and other accidents. In addition, the vibration of the motor will easily cause the cooler water pipe to crack and the welding point to vibrate open, and at the same time cause damage to the load machinery, reduce the accuracy of the workpiece, cause fatigue of all mechanical parts that are vibrated, and loosen or break the anchor screws. The motor will also cause abnormal wear of the carbon brushes and slip rings, and even serious brush fire will occur and burn the collector ring insulation. The motor will generate a lot of noise, which usually occurs in DC motors.

Ten reasons why electric motors vibrate

1. Caused by imbalance of rotor, coupler, coupling and transmission wheel (brake wheel).

2. Loose core brackets, loose oblique keys and pins, and loose rotor binding can all cause imbalance in the rotating parts.

3. The axis system of the linkage part is not centered, the center line does not overlap, and the centering is incorrect. The main cause of this failure is poor alignment and improper installation during the installation process.

4. The center lines of the linkage parts are consistent when cold, but after running for a period of time, the center lines are destroyed due to deformation of the rotor fulcrum, foundation, etc., resulting in vibration.

5. The gears and couplings connected to the motor are faulty, the gears are not meshing well, the gear teeth are severely worn, the wheels are poorly lubricated, the couplings are skewed or misaligned, the tooth shape and pitch of the gear coupling are incorrect, the gap is too large or the wear is severe, all of which will cause certain vibrations.

6. Defects in the motor structure itself, such as oval journal, bent shaft, too large or too small gap between the shaft and the bearing, insufficient stiffness of the bearing seat, base plate, part of the foundation or even the entire motor installation foundation.

7. Installation problems: the motor and the base plate are not firmly fixed, the base bolts are loose, the bearing seat and the base plate are loose, etc.

8. If the gap between the shaft and the bearing is too large or too small, it will not only cause vibration but also cause abnormal lubrication and temperature of the bearing.

9. The load driven by the motor transmits vibration, such as the vibration of the fan or water pump driven by the motor, which causes the motor to vibrate.

10. Wrong stator wiring of AC motor, short circuit of rotor winding of wound asynchronous motor, short circuit between turns of excitation winding of synchronous motor, wrong connection of excitation coil of synchronous motor, broken rotor bars of cage asynchronous motor, deformation of rotor core causing uneven air gap between stator and rotor, resulting in unbalanced air gap magnetic flux and thus vibration.

Vibration causes and typical cases

There are three main reasons for vibration: electromagnetic reasons; mechanical reasons; and electromechanical mixed reasons.

1. Electromagnetic reasons

1. Power supply: three-phase voltage is unbalanced, and the three-phase motor runs with a missing phase.

2. Stator: the stator core becomes oval, eccentric, and loose; the stator winding is broken, grounded, short-circuited, and connected incorrectly, and the three-phase current of the stator is unbalanced.

For example: before the boiler room sealed fan motor was overhauled, red powder was found on the stator core, and the stator core was suspected to be loose, but it was not within the scope of the standard overhaul, so it was not handled. After the overhaul, the motor made a shrill scream during the test run. After replacing a stator, the fault was eliminated.

3. Rotor fault: the rotor core becomes oval, eccentric, and loose. The rotor cage bars and end rings are welded open, the rotor cage bars are broken, the winding is wrong, the brush contact is poor, etc.

For example: During the operation of the toothless saw motor in the sleeper section, it was found that the stator current of the motor swung back and forth and the vibration of the motor gradually increased. Based on the phenomenon, it was judged that the motor rotor cage bars were likely to be welded and broken. After the motor was dismantled, it was found that the rotor cage bars had 7 fractures, and 2 of the serious ones had both sides and the end rings completely broken. If it is not discovered in time, it may cause a serious accident of stator burning.

2. Mechanical reasons

1. The motor itself:

The rotor is unbalanced, the shaft is bent, the slip ring is deformed, the air gap between the stator and rotor is uneven, the magnetic center of the stator and rotor is inconsistent, the bearing is faulty, the foundation is poorly installed, the mechanical structure is not strong enough, there is resonance, the anchor screws are loose, and the motor fan is damaged.

Typical case: After the upper bearing of the condensate pump motor was replaced, the motor shaking increased, and the rotor and stator showed slight signs of bore sweeping. After careful inspection, it was found that the motor rotor was lifted to the wrong height, and the magnetic center of the rotor and stator was not aligned. After re-adjusting the thrust head screw cap, the motor vibration fault was eliminated. After the cross-line hoist motor was overhauled, the vibration was always large and showed signs of gradual increase. When the motor was hooked, it was found that the motor vibration was still large and there was a lot of axial stringing. Disassembly revealed that the rotor core was loose and the rotor balance was also problematic. After replacing the spare rotor, the fault was eliminated and the original rotor was returned to the factory for repair.

2. Cooperation with coupling:

The coupling is damaged, the coupling connection is poor, the coupling center is not accurate, the load is unbalanced, the system resonates, 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 of this failure is poor centering and improper installation during the installation process. There is also a situation that the center line of some linkage parts is consistent in the cold state, but after running for a period of time, due to the deformation of the rotor fulcrum, foundation, etc., the center line is destroyed again, thus generating vibration.

For example:

a. The circulating water pump motor has been vibrating too much during operation. The motor was checked and found to have no problems, and everything was normal under no-load. The pump team believed that the motor was operating normally. However, it was finally found that the motor alignment was too far off. After the pump team realigned it, the motor vibration was eliminated.

b. After the pulley of the boiler room induced draft fan was replaced, the motor vibrated during 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 returned to normal.

3. Motor Mixing Reasons

1. Motor vibration is often caused by uneven air gap, which causes unilateral electromagnetic pulling force, and the unilateral electromagnetic pulling force further increases the air gap. This electromechanical mixed effect manifests as motor vibration.

2. The axial movement of the motor is caused by the electromagnetic pull due to the rotor's own gravity or the installation level and the wrong magnetic center, which causes the motor's axial movement, resulting in increased motor vibration. In severe cases, the shaft wears out and the bearing temperature rises rapidly.

There are problems with the gears and couplings connected to the motor. This kind of fault is mainly manifested in poor gear engagement, serious wear of gear teeth, poor lubrication of the wheels, skewed and misaligned couplings, incorrect tooth shape and pitch of the gear coupling, excessive clearance or serious wear, which will cause certain vibrations.

The motor's own structural defects and installation problems. This kind of fault is mainly manifested in the oval of the shaft neck, the bending of the rotating shaft, the gap between the shaft and the bearing bush is too large or too small, the rigidity of the bearing seat, the base plate, a part of the foundation or even the entire motor installation foundation is insufficient, the motor and the base plate are not fixed firmly, the foot bolts are loose, the bearing seat and the base plate are loose, etc. The gap between the shaft and the bearing bush is too large or too small, which can not only cause vibration, but also cause abnormal lubrication and temperature of the bearing bush.

The load driven by the motor transmits vibration.

For example: the vibration of the turbine of the steam turbine generator, the vibration of the fan and water pump driven by the motor, cause the motor vibration.