1. Ten Causes of Motor Vibration

1. Imbalance of the rotor, coupler, coupling, and drive pulley.

2. Loose core supports, loose or ineffective bevel keys and pins, and loose rotor strapping can all cause imbalance in the rotating components.

3. Misalignment of the linkage shaft system, misalignment of centerlines, and incorrect centering. This failure is primarily caused by poor alignment or improper installation during installation.

4. The centerlines of the linkage components align when cold, but after a period of operation, deformation of the rotor fulcrum, foundation, and other factors can disrupt this centerline, causing vibration.

5. Faults in the gears and couplings connected to the motor. Poor gear engagement, severe tooth wear, poor lubrication of the gears, skew or misalignment of the couplings, incorrect tooth profile and pitch, excessive clearance, or severe wear can all cause vibration.

6. Defects in the motor’s structure, such as an elliptical journal, a bent shaft, excessive or insufficient clearance between the shaft and bearing, insufficient rigidity of the bearing seat, base plate, part of the foundation, or even the entire motor mounting base.

7. Installation problems, such as insecure fixings between the motor and base plate, loose anchor bolts, looseness between the bearing seat and base plate, etc.

8. Excessive or insufficient clearance between the shaft and bearing can not only cause vibration but also lead to abnormal lubrication and temperature of the bearing.

9. Vibration transmitted by the load driven by the motor, such as vibration of a fan or water pump driven by the motor, can cause motor vibration.

10. Incorrect stator wiring in AC motors, short circuits in the rotor windings of wound-rotor asynchronous motors, inter-turn short circuits in the excitation windings of synchronous motors, incorrect field coil connections in synchronous motors, broken rotor bars in cage-type asynchronous motors, and deformation of the rotor core can lead to uneven stator and rotor air gaps, resulting in air gap magnetic flux imbalance and thus vibration.

2. Causes of vibration

1. Electromagnetic Causes

Power Supply: Three-phase voltage imbalance, three-phase motors running with a missing phase.

Stator: Stator core becomes oval, eccentric, or loose; stator windings suffer from wire breakage, ground breakdown, interturn short circuits, wiring errors, and unbalanced stator three-phase current.

For example: Before overhauling a boiler room sealed fan motor, red powder was found on the stator core. Looseness was suspected, but this was not covered by a standard overhaul and was not addressed. After the overhaul, the motor produced a sharp squealing noise during a test run. Replacing the stator eliminated the problem.

Rotor Fault: Rotor core becomes oval, eccentric, or loose. Rotor cage bar welds may be open, rotor cage bar breakage, winding errors, poor brush contact, etc.

2. Mechanical Causes

Motor-related issues: rotor imbalance, bent shaft, deformed slip rings, uneven air gaps between the stator and rotor, misaligned magnetic centers between the stator and rotor, bearing failure, poor foundation installation, insufficient mechanical strength, resonance, loose anchor screws, and motor fan damage.

Coupling-related issues: coupling damage, poor coupling connection, inaccurate coupling centering, load mechanical imbalance, system resonance, etc. Linkage shaft misalignment, misaligned centerlines, and incorrect centering. This type of fault is primarily caused by poor alignment or improper installation. Alternatively, the centerlines of some linkages may be aligned when cold, but after a period of operation, deformation of the rotor pivot and foundation can cause this centerline to become disrupted, resulting in vibration.

3. Electromechanical Mixed Causes

1. Motor vibration is often caused by uneven air gaps, which induce unilateral electromagnetic pull. This unilateral electromagnetic pull further increases the air gap, resulting in motor vibration.

2. Motor axial play. Electromagnetic pull caused by the rotor’s own gravity, misaligned mounting, or misaligned magnetic center causes axial play, increasing motor vibration. In severe cases, the shaft may wear the bearing root, rapidly increasing bearing temperature.

3. Faults in the gears or couplings connected to the motor. This fault is primarily manifested by poor gear engagement, severe tooth wear, poor gear lubrication, skew or misalignment of the coupling, incorrect tooth profile and pitch, excessive clearance, or severe wear in the gear coupling, all of which can cause some vibration.

4. Structural defects and installation issues in the motor itself. This fault typically manifests as an oval journal, a bent shaft, excessive or insufficient clearance between the shaft and bearing shells, insufficient rigidity of the bearing seat, base plate, part of the foundation, or even the entire motor mounting foundation, a loose connection between the motor and base plate, loose anchor bolts, or looseness between the bearing seat and base plate. Excessive or insufficient clearance between the shaft and bearing shells can not only cause vibration but also lead to abnormal lubrication and temperature of the bearing shells.

5. Vibration transmitted by the load driven by the motor. For example, vibration from the turbine of a steam turbine generator or vibration from the fan or water pump driven by the motor can cause motor vibration.

3. How to Identify the Cause of Vibration?

To eliminate motor vibration, you must first identify the cause. Only by identifying the cause can you take targeted measures to eliminate the vibration.

Before shutting down the motor, use a vibration meter to check the vibration of each component. For areas with high vibration, measure the vibration levels in three directions: vertical, horizontal, and axial. If the anchor screws or bearing cover screws are loose, tighten them immediately. After tightening, measure the vibration again to see if the vibration is eliminated or reduced. Next, check the three-phase power supply voltage for balance and any blown fuses. Single-phase operation of the motor can not only cause vibration but also rapidly increase the motor temperature. Observe the ammeter needle for oscillation; current oscillation occurs when a rotor bar is broken. Finally, check the three-phase current balance of the motor. If any problems are found, contact the operator immediately and stop the motor to prevent burnout.

If the motor vibration is not resolved after the surface phenomenon is treated, 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 coupling not being aligned 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, it means that it is an electrical cause, otherwise it is a mechanical failure.

4. Troubleshooting the Cause

Electrical Troubleshooting: First, check the three-phase DC resistance of the stator to see if it’s balanced. If it’s unbalanced, it indicates a loose weld at the stator wiring connection. Disconnect the windings and check for interturn shorts. If the problem is obvious, see scorch marks on the insulation surface. Alternatively, use an instrument to measure the stator windings. Once interturn shorts are confirmed, reinstall the motor windings.

Mechanical Troubleshooting: Check the air gap for uniformity. If the measured value exceeds the standard, readjust the air gap. Inspect the bearings and measure the bearing clearance. If unsatisfactory, replace them with new bearings. Check the iron core for deformation and looseness. Loose cores can be glued and sealed with epoxy glue. Inspect the shaft. Any bent shafts should be re-welded or straightened. Then, perform a rotor balance test. During the trial run after the fan motor overhaul, the motor not only vibrated excessively but also experienced excessive bearing temperatures. After several days of continuous troubleshooting, the problem remained unresolved. While assisting with the issue, my team discovered that the motor’s air gap was excessively large and the bearing mount was not level. After identifying the cause and readjusting the clearances, the motor successfully ran on test.

The load mechanism checked normally, and the motor itself was fine. The cause of the problem lay in the connection. We should check the motor’s foundation level, tilt, strength, center alignment, coupling damage, and motor shaft extension to ensure it meets requirements.