Motor vibration reasons and typical cases

Motor vibration reasons and typical cases

Vibration of the motor will shorten the insulation and bearing life of the winding, affect the normal lubrication of the sliding bearing, and the vibration force will promote the expansion of the insulation gap, so that the external dust and moisture can intrude, resulting in lower insulation resistance and increased leakage current, or even the formation of insulation breakdown and other accidents.In addition, vibration of the motor easily make the cooling water pipe cracked, the welding points cracked, at the same time can cause loaded machine damaged, reduce the precision of workpiece, can cause all the vibrating mechanical parts fatigue, can make the anchor bolt is loose or broken, motor and can lead to abnormal wear of carbon brush and slip ring, can appear even serious brush fire and burned collector ring, insulation motor will make a big noise, this kind of situation usually also have occurred in the dc motor.

There are three main reasons for motor vibration: electromagnetic, mechanical, electromechanical mixed.

Electromagnetic reasons

1. Power supply: three-phase voltage imbalance, three-phase motor phase operation.

2. Stator: the stator core becomes elliptic, eccentric and loose;Stator windings are broken, ground breakdown, inter-turn short circuit, wiring error, stator three-phase current imbalance.

3. Rotor fault: the rotor core becomes elliptic, eccentric and loose.Rotor cage and end ring open welding, broken rotor cage, winding error, poor brush contact.

Example: the operation of the motor stator current is found in one motor, the motor vibration increases gradually, according to the phenomenon of judging the motor rotor cage bar has the possibility of the welding and cracking, found that there are 7 rotor cage bar fracture after the disassembly of motor , both sides of the root and end ring fracture on the serious 2 pcs. And if has not been found  in time, it is likely to cause loss of the stator of bad accidents.

Mechanical reasons

1. Motor itself:

The rotor is unbalanced, the shaft is bent, the slip ring is deformed, the air gap between stator and rotor is uneven, the magnetic centers of stator and rotor are inconsistent, the bearing is out of order, the foundation is not properly installed, the mechanical mechanism is not strong enough and resonant, the anchor screw is loose, and the motor fan is damaged.

Typical case: after replacing the upper bearing of the condensate pump motor in the factory, the motor's shaking increases, and the rotor and stator have slight signs of scratching. After careful inspection, it is found that the motor's rotor is not lifted at the right height, and the stator's magnetic center is not aligned. After adjusting the thrust head screw and the cap, the motor's vibration fault is eliminated.

2. Coupling: damaged coupling, poor coupling connection, inaccurate coupling alignment, load mechanical imbalance, system resonance, etc.

Linkage part of the shaft system is not in the center, the center line does not coincide, the centering is not correct.The main cause of this kind of failure is the installation process, poor and improper installation. Another case is that some linkage parts of the Central Line in the cold state is coincident and consistent, but after running for a period of time due to the rotor fulcrum, foundation and other deformation, the Central Line is destroyed, and thus produce vibration.

For example, the circulating pump motor, the vibration has been too big during the operation, motor inspection find no problems, no load is also normal. In the final check, find that the motor alignment center is not correct. After the pump maintenance engineer went on the re-alignment, motor vibration was eliminated.

B. After replacing the pulley, the induction fan in the boiler room will vibrate during the test operation of the motor and the three-phase current of the motor will increase. Check all the circuits and electrical components and find no problems.

Motor mixing reasons

1. Motor vibration is ofter caused by uneven air gap, causing unilateral electromagnetic tension, and unilateral electromagnetic tension and further increase the air gap imbalance. this electromechanical mixed action lead to motor vibration.

2. Motor axial endplay, due to the gravity of the rotor itself or the installation level and the wrong magnetic center, caused by the electromagnetic tension, resulting in the motor axial endplay, resulting in increased vibration of the motor, in serious case the shaft root is grinding, so that the temperature of the bearing bush rapidly increased.

There is something wrong with the gear and coupling connected with the motor.This kind of fault is mainly manifested as bad gear occlusion, serious gear wear, bad lubrication, coupling skew, dislocation, bad gear type of coupling, bad coupling tooth shape, bad tooth distance, clearance is too large or serious wear, will cause a certain vibration.

Defects in motor structure and installation problems.Such faults are mainly manifested as ellipse of shaft neck, bending of rotating shaft, excessive or too small clearance between shaft and bearing bush, insufficient stiffness of the motor installation foundation, loose fixation between motor and foundation plate, loose bolts of foundation, loose between bearing seat and foundation plate, etc.Too large or too small clearance between shaft and bearing bush can not only cause vibration but also abnormal lubrication and temperature of bearing bush.

Motor driven load will transfer vibration.For example: steam turbine vibration of the turbogenerator, motor drag fan, pump vibration,  will cause the motor vibration.

How to find the cause of vibration

In order to eliminate the motor vibration, first of all to find out the cause of vibration, only after find the cause of vibration, can take targeted measures to eliminate the motor vibration.

1. The motor did not stop, use vibration measurement tool to check each vibration, to measure the vibration of the larger vibration parts according to the detailed test to three directions: perpendicular , horizontal, and axis. If bolts or bearing end cover screw is loose, can directly tighten them. After tighten, measure the vibration again, and after observing whether it is eliminated or reduced. Secondly to check whether three-phase voltage is balanced , whether there is burned out phenomenon in three-phase fuse . The motor running at single phase can not only cause vibration, but also cause the motor temperature rise quickly.

2. If after the above normal treatment , motor vibration is unresolved, should disconnect the power supply, loose coupling, seperate the motor with the load of mechanical,  then only run the motor.If there isn't vibration in motor itself, then the source of vibration is caused by the coupling is not find or load machine.If the motor vibration still exist, then there is something wrong with the motor itself. It also can take action to distinguish whether it is electrical problem, or mechanical reasons. After the power outage, the motor stops the vibration or vibration reduced immediately, then is electrical reason, otherwise it is a mechanical failure.