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.The IEC 60034-2 standard developed by the International Electrotechnical Commission (IEC) is a standard for rotating motor vibration measurement. This standard specifies the measurement method and evaluation criteria for motor vibration, including vibration limit values, measuring instruments and measurement methods. Based on this standard, it can be determined whether the motor vibration meets the standard.

The harm of motor vibration to the motor

The vibration generated by the motor will shorten the life of the winding insulation and bearings, affect the normal lubrication of the 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 causing accidents such as insulation breakdown.In addition, the vibration generated by the motor can easily cause the cooler water pipes to crack and the welding points to vibrate open. At the same time, it will 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 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. This situation generally occurs in DC motors.

Ten reasons why electric motors vibrate

1.The rotor, coupler, coupling, and drive wheel (brake wheel) are unbalanced.

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 bar of cage asynchronous motor, deformation of rotor core causing uneven air gap between stator and rotor, leading to 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: the three-phase voltage is unbalanced and the three-phase motor runs in a missing phase.

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

For example: Before the overhaul of the sealed fan motor in the boiler room, red powder was found on the stator core. It was suspected that the stator core was 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 screaming sound during the test run. The fault was eliminated after replacing a stator.

3. Rotor failure: The rotor core becomes elliptical, eccentric, and loose. The rotor cage bar and the end ring are welded open, the rotor cage bar is 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 motor stator current swung back and forth, and the motor vibration gradually increased. According to the phenomenon, it was judged that the motor rotor cage bar might be welded and broken. After the motor was disassembled, it was found that there were 7 fractures in the rotor cage bar, and the two serious ones were completely broken on both sides and the end ring. If it is not discovered in time, it may cause a serious accident of stator burning.

2.Mechanical reasons

1.The motor :

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

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 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 dropped the hook, it was found that the motor vibration was still large and there was a large axial string. After disassembly, it was found 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 is poorly connected, the coupling is not centered, the load is mechanically unbalanced, and the system resonates. The shaft system of the linkage part is not centered, the center line does not overlap, and the centering is incorrect. The main reason for this fault is poor centering and improper installation during the installation process. There is another situation, that is, the center line of some linkage parts is consistent when cold, but after running for a period of time, the center line is destroyed due to deformation of the rotor fulcrum, foundation, etc., resulting in vibration.

For example:

a. The vibration of the circulating water pump motor has always been large during operation. The motor inspection has no problems and everything is normal when it is unloaded. The pump class believes that the motor is running normally. Finally, it is found that the motor alignment center is too different. After the pump class re-aligns, the motor vibration is eliminated.

b. After the pulley of the boiler room induced draft fan is replaced, the motor generates vibration during the trial operation and the three-phase current of the motor increases. All circuits and electrical components are checked and there are no problems. Finally, it is found that the pulley is unqualified. After replacement, the motor vibration is eliminated and the three-phase current of the motor returns to normal.

3. Electromechanical mixed reasons:

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

2. The motor axial string movement, due to the rotor’s own gravity or installation level and the wrong magnetic center, causes the electromagnetic tension to cause the motor axial string movement, causing the motor vibration to increase. In severe cases, the shaft wears the bearing root, causing the bearing temperature to rise rapidly.

3. The gears and couplings connected to the motor are faulty. This fault is mainly manifested in 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.

4. Defects in the motor’s own structure and installation problems. This fault is mainly manifested as an elliptical shaft neck, a bent shaft, too large or too small gap between the shaft and the bearing, insufficient rigidity of the bearing seat, base plate, part of the foundation, or even the entire motor installation foundation, loose fixation between the motor and the base plate, loose foot bolts, looseness between the bearing seat and the base plate, etc. Too large or too small gap between the shaft and the bearing can not only cause vibration, but also abnormal lubrication and temperature of the bearing.

5. The load driven by the motor conducts vibration.

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

How to find the cause of vibration?

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

1. Before the motor is shut down, use a vibration meter to check the vibration of each part. For the parts with large 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 size to observe whether 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 burned out. The single-phase operation of the motor can not only cause vibration, but also cause the temperature of the motor to rise rapidly. Observe whether the ammeter pointer swings back and forth. When the rotor is broken, the current swings. Finally, check whether the three-phase current of the motor is balanced. If any problems are found, contact the operator in time to stop the motor to avoid burning the motor.

2. If the motor vibration is not resolved after the surface phenomenon is dealt with, continue to disconnect the power supply, loosen the coupling, separate the load machinery 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 machinery. 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 cause or a mechanical cause. When the power is cut off, the motor stops vibrating or the vibration is reduced immediately, which means that it is an electrical cause, otherwise it is a mechanical failure.

Troubleshooting

1. Inspection of electrical reasons:

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 stator connection welding part. Disconnect the winding phases for search. In addition, 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: water pump motor, the motor not only vibrates violently during operation, but also has a high bearing temperature. The 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 elimination method, the motor ran normally.

2. Repair of mechanical reasons:

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 the new bearings. Check the deformation and looseness of the iron core. The loose iron 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. During the trial run after the overhaul of the fan motor, the motor not only vibrated violently, but also the bearing temperature exceeded the standard. After several days of continuous processing, the fault was still not solved. When helping to deal with it, my team members found that the air gap of the motor was very large and the level of the bearing seat was unqualified. After the cause of the fault was found, the gaps of each part were readjusted, and the motor was successfully tested once.

3. Check the load mechanical part:

The cause of the fault was caused by the connection part. At this time, it is necessary to check the foundation level of the motor, the inclination, strength, whether the center alignment is correct, whether the coupling is damaged, and whether the motor shaft extension winding meets the requirements.

Steps to Deal with Motor Vibration

1. Disconnect the motor from the load, test the motor without any load, and check the vibration value.

2. Check the vibration value of the motor foot according to the IEC 60034-2 standard.

3. If only one of the four foot or two diagonal foot vibrations exceed the standard, loosen the anchor bolts, and the vibration will be qualified, indicating that the foot pad is not solid, and the anchor bolts cause the base to deform and vibrate after tightening. Pad the foot firmly, re-align and tighten the anchor bolts.

4. Tighten all four anchor bolts on the foundation, and the vibration value of the motor still exceeds the standard. At this time, check whether the coupling installed on the shaft extension is flush with the shaft shoulder. If not, the exciting force generated by the extra key on the shaft extension will cause the horizontal vibration of the motor to exceed the standard. In this case, the vibration value will not exceed too much, and the vibration value can often decrease after docking with the host, so the user should be persuaded to use it.

5. If the vibration of the motor does not exceed the standard during the no-load test, but exceeds the standard when loaded, there are two reasons: one is that the alignment deviation is large; the other is that the residual unbalance of the rotating parts (rotor) of the main engine 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 generated excitation force 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.

6. The vibration velocity (intensity) does not exceed the standard, but the vibration acceleration exceeds the standard, and the bearing can only be replaced.

7. The rotor of the two-pole high-power motor has poor rigidity. If it is not used for a long time, the rotor will deform and may vibrate when it is turned again. This is due to poor storage of the motor. Under normal circumstances, the two-pole motor is stored during storage. The motor should be cranked every 15 days, and each cranking should be rotated at least 8 times.

8. The motor vibration of the sliding bearing is related to the assembly quality of the bearing. Check whether the bearing has high points, whether the oil inlet of the bearing is sufficient, the bearing tightening force, the bearing clearance, and the magnetic center line are appropriate.

9. In general, the cause of motor vibration can be simply judged from the vibration values ​​in three directions. If the horizontal vibration is large, the rotor is unbalanced; if the vertical vibration is large, the installation foundation is uneven and bad; if the axial vibration is large, the bearing assembly quality is poor. This is just a simple judgment. It is necessary to consider the actual cause of the vibration based on the on-site conditions and the above-mentioned factors.

10. After the rotor is dynamically balanced, the residual unbalance of the rotor has been solidified on the rotor and will not change. The vibration of the motor itself will not change with the change of location and working conditions. The vibration problem can be handled well at the user’s site. 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., on-site dynamic balancing or return to the factory for processing is required.

Anhui Mingteng Permanent Magnetic Electromechanical Equipment Co., Ltd.’s(https://www.mingtengmotor.com/) production technology and quality assurance capabilities

Production technology

1.Our company has a maximum swing diameter of 4m, height of 3.2 meters and below CNC vertical lathe, mainly used for motor base processing, in order to ensure the concentricity of the base, all motor base processing is equipped with corresponding processing tooling, low-voltage motor adopts “one knife drop” processing technology.

Shaft forgings usually use 35CrMo, 42CrMo, 45CrMo alloy steel shaft forgings, and each batch of shafts is in accordance with the requirements of “Technical Conditions for Forging Shafts” for tensile test, impact test, hardness test and other tests. Bearings can be selected according to the needs of SKF or NSK and other imported bearings.

2.Our company’s permanent magnet motor rotor permanent magnet material adopts high magnetic energy product and high internal coercivity sintered NdFeB, conventional grades are N38SH, N38UH, N40UH, N42UH, etc., and the maximum working temperature is not less than 150 °C. We have designed professional tooling and guide fixtures for magnetic steel assembly, and qualitatively analyzed the polarity of the assembled magnet by reasonable means, so that the relative magnetic flux value of each slot magnet is close, which ensures the symmetry of the magnetic circuit and the quality of the magnetic steel assembly

3.The rotor punching blade adopts high-specification punching materials such as 50W470, 50W270, 35W270, etc., the stator core of the forming coil adopts the tangential chute punching process, and the rotor punching blade adopts the punching process of the double die to ensure the consistency of the product.

4.Our company adopts a self-designed special lifting tool in the stator external pressing process, which can safely and smoothly lift the compact external pressure stator into the machine base; In the assembly of the stator and rotor, the permanent magnet motor assembly machine is designed and commissioned by itself, which avoids the damage of the magnet and bearing due to the suction of the magnet and the rotor due to the suction of the magnet during assembly.

Quality assurance capability

1.Our test center can complete the full-performance type test of voltage level 10kV motor 8000kW permanent magnent motors. The test system adopts computer control and energy feedback mode, which is currently a test system with leading technology and strong ability in the field of ultra-efficient permanent magnet synchronous motor industry in China.

2.We have established a sound management system and passed ISO9001 quality management system certification and ISO14001 environmental management system certification. Quality management pays attention to continuous improvement of processes, reduces unnecessary links, increases the ability to control five factors such as “man, machine, material, method, and environment”, and must achieve “people make the best use of their talents, make the best use of their opportunities, make the best use of their materials, make the best use of their skills, and make the best of their environment”.

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https://mp.weixin.qq.com/s/BoUJgXnms5PQsOniAAJS4A

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