Identification of Abnormal Motor Vibration Phenomena and Cause Analysis

1. Electromagnetic Vibration Caused by Abnormalities in the Stator of a Three-Phase AC Motor

Causes of Abnormal Stator Electromagnetic Vibration

1. Asymmetry of the Stator Three-Phase Magnetic Field: Issues such as unbalanced three-phase voltages in the power grid, single-phase operation caused by poor contact or broken wires, and asymmetry of the stator winding three phases can all lead to an asymmetric stator magnetic field, resulting in abnormal vibrations. For example, in a power grid with unbalanced three-phase voltages, the magnetic forces acting on different parts of the stator will be uneven, causing the motor to vibrate abnormally.
2. Looseness of the Stator Core and Stator Coils: This will increase the stator electromagnetic vibration and electromagnetic noise. When the stator core and coils are loose, they cannot maintain a stable position during the operation of the motor, and the electromagnetic forces acting on them will cause them to vibrate more severely, generating additional noise.
3. Looseness of the Electromagnetic Foot Lines: It is equivalent to a decrease in the stiffness of the machine base, which increases the stator vibration. The electromagnetic foot lines play an important role in fixing the stator and transmitting the electromagnetic forces. If they are loose, the stability of the stator will be affected, and the vibration will increase accordingly.

Characteristics of Stator Electromagnetic Vibration

1. Vibration Frequency: The vibration frequency is twice the power supply frequency, i.e., F = 2f. This characteristic can be used as an important basis for identifying stator electromagnetic vibration. By measuring the vibration frequency, if it is found to be twice the power supply frequency, it is likely that there is a problem with the stator electromagnetic field.
2. Disappearance of Vibration after Power Off: When the power is cut off, the electromagnetic vibration disappears immediately. This is because the electromagnetic forces that cause the vibration are generated by the current in the stator windings. Once the power is cut off, the current disappears, and so does the electromagnetic vibration.
3. Measurable Vibration Location: The vibration can be measured on the stator machine base and the bearings. This is because the electromagnetic forces generated by the stator are transmitted to the machine base and the bearings, causing them to vibrate.
4. Relationship with Machine Base Stiffness and Load: The vibration intensity is related to the stiffness of the machine base and the load. A stiffer machine base can better resist the electromagnetic forces and reduce the vibration, while a heavier load will increase the electromagnetic forces and thus the vibration.

2. Electromagnetic Force Caused by Static Eccentricity of the Air Gap

Causes of Static Eccentricity of the Air Gap

1. Manufacturing and Installation Errors: During the manufacturing and installation process of the motor, if there are errors in the positioning of the stator and rotor, it may lead to static eccentricity of the air gap. For example, improper assembly of the stator and rotor, or errors in the machining of the stator and rotor cores, can cause the center of the stator and the axis of the rotor to be misaligned.
2. Deformation of the Motor Components: Over time, due to factors such as thermal expansion and contraction, mechanical stress, and vibration, the motor components may deform, resulting in static eccentricity of the air gap. For example, the stator core may deform under the action of electromagnetic forces and mechanical stress, causing the air gap to become eccentric.

Characteristics of Electromagnetic Vibration Caused by Static Eccentricity of the Air Gap

1. Vibration Frequency: The electromagnetic vibration frequency is twice the power supply frequency, F = 2f, which is similar to the vibration frequency caused by stator abnormalities.
2. Relationship with Eccentricity and Load: The vibration increases with the increase of the eccentricity value and the load. As the eccentricity increases, the unilateral magnetic pull becomes larger, resulting in more severe vibration. And when the load increases, the electromagnetic forces in the motor also increase, further exacerbating the vibration.
3. Disappearance of Vibration after Power Off: Like stator electromagnetic vibration, the electromagnetic vibration caused by static eccentricity of the air gap disappears after the power is cut off.
4. Difficulty in Differentiation: The electromagnetic vibration caused by static eccentricity is very similar to that caused by stator abnormalities, making it difficult to distinguish between them. This requires more detailed inspection and analysis to accurately determine the cause of the vibration.

3. Electromagnetic Vibration Caused by Dynamic Eccentricity of the Air Gap

Causes of Dynamic Eccentricity of the Air Gap

1. Rotor Shaft Bending: If the rotor shaft is bent during manufacturing, transportation, or operation, it will cause the axis of the rotor to deviate from the center of the stator, resulting in dynamic eccentricity of the air gap. For example, improper handling during transportation or excessive mechanical stress during operation may cause the rotor shaft to bend.
2. Misalignment of the Rotor Core with the Shaft or Bearing: When the rotor core is not concentric with the shaft or the bearing, it will also lead to dynamic eccentricity. This may be due to manufacturing errors or problems during the assembly process.
3. Non-Circular Rotor Core: If the rotor core is not round, it will cause uneven air gap distribution during the rotation of the rotor, resulting in dynamic eccentricity.

Characteristics of Electromagnetic Vibration Caused by Dynamic Eccentricity of the Air Gap

1. Multiple Vibration Frequencies: Both the electromagnetic vibration frequencies of the rotor rotation frequency and the stator magnetic field rotation frequency may appear. This is because the dynamic eccentricity causes complex interactions between the stator and the rotor magnetic fields, resulting in the generation of multiple vibration frequencies.
2. Pulsating Amplitude: The amplitude of the electromagnetic vibration pulsates (vibrates) with time, and the pulsation frequency is 2sf, and the period is 1/2sf. When the motor load increases, the slip s increases, and the pulsation rhythm accelerates. This characteristic can be used to identify dynamic eccentricity of the air gap.
3. Electromagnetic Noise: The motor often generates electromagnetic noise consistent with the pulsation rhythm. The pulsating electromagnetic forces cause the motor components to vibrate, generating noise.
4. Disappearance of Vibration and Noise after Power Off: After the power is cut off, both the electromagnetic vibration and the electromagnetic noise disappear, as the electromagnetic forces that cause them are no longer present.

4. Electromagnetic Vibration Caused by Rotor Winding Faults

Causes of Rotor Winding Faults

1. Poor Casting Quality of Cage Bars: If the casting quality of the cage bars is poor, it may lead to broken bars and high resistance. For example, insufficient filling during the casting process or the presence of defects in the casting material can cause the cage bars to break or have high resistance.
2. Frequent Starting and Heavy Load: For cage rotors, frequent starting and heavy motor loads can cause broken bars or high resistance. Each start of the motor will generate a large starting current, which will cause stress on the cage bars. If the starting is too frequent or the load is too heavy, the cage bars may break or their resistance may increase.
3. Electrical Imbalance in the Rotor Winding Circuit of Wound Asynchronous Motors: This will generate unbalanced electromagnetic forces. Factors such as uneven winding resistance or incorrect connection of the rotor windings can lead to electrical imbalance in the rotor circuit.
4. Turn-to-Turn Short Circuit in the Magnetic Winding of Synchronous Motors: This is also a common cause of rotor winding faults, which can affect the normal operation of the motor and generate electromagnetic vibration.

Characteristics of Electromagnetic Vibration Caused by Rotor Winding Faults

1. Similarity to Dynamic Eccentricity Vibration: The electromagnetic vibration caused by rotor winding faults is similar in waveform and phenomenon to the electromagnetic vibration caused by rotor dynamic eccentricity, making it difficult to distinguish. The vibration frequency is f/p, and the amplitude pulsates at a frequency of 2sf. The motor generates electromagnetic noise consistent with the pulsation rhythm.
2. Relationship with Load: In no-load or light-load conditions, the vibration and rhythm noise are not obvious. When the load increases, especially when the load exceeds 50%, the vibration and noise become more obvious. This is because the unbalanced electromagnetic forces generated by the rotor winding faults are more pronounced under heavy loads.
3. Pulsation in Stator Current: There is also a pulsation change in the stator primary current, and the pulsation rhythm frequency is 2sf. This can be detected by measuring the stator current to help identify rotor winding faults.
4. Spectrum Analysis Characteristics: When performing spectrum analysis on the stator current waveform, side frequencies appear on both sides of the fundamental frequency in the frequency diagram. This is a characteristic manifestation of rotor winding faults in the frequency domain.
5. Characteristics of Synchronous Motor Excitation Winding Faults: In synchronous motors, a turn-to-turn short circuit in the excitation winding can cause electromagnetic vibration and noise with a frequency of f/p (rotational frequency), and there is no rhythmic pulsation vibration phenomenon, which is similar to the mechanical vibration caused by rotor imbalance.
6. Disappearance of Vibration and Noise after Power Off: After the power is cut off, both the electromagnetic vibration and the electromagnetic noise disappear, indicating that they are caused by electromagnetic factors.

5. Mechanical Vibration Caused by Rotor Unbalance

Causes of Rotor Unbalance

1. Uneven Mass Distribution of the Rotor: If the mass distribution of the motor rotor is uneven, it will cause the center of gravity to shift and be different from the center of the rotor. This may be due to manufacturing errors, such as uneven material density or improper shaping of the rotor components.
2. Detachment and Displacement of Rotor Components: The detachment and displacement of rotor components, as well as the displacement and looseness of the winding caused by insulation shrinkage, can also lead to rotor unbalance. For example, if a part of the rotor falls off during operation, it will change the mass distribution of the rotor and cause unbalance.
3. Unbalance of Couplings, Cooling Fans, and Pulleys: The unbalance of couplings, cooling fans, and pulleys connected to the rotor will also be transmitted to the rotor, causing it to vibrate. These components need to be balanced during manufacturing and installation to ensure the stability of the motor operation.
4. Uneven Dirt Accumulation on the Cooling Fan and Rotor Surface: The uneven accumulation of dirt on the cooling fan and the rotor surface can also change the mass distribution of the rotor, resulting in unbalance. Regular cleaning of these components is necessary to prevent this problem.

Characteristics of Mechanical Vibration Caused by Rotor Unbalance

1. Vibration Frequency: The vibration frequency is equal to the rotational frequency. This is a key characteristic of rotor unbalance vibration. By measuring the vibration frequency, if it is found to be equal to the rotational frequency of the rotor, it is likely that there is a rotor unbalance problem.
2. Relationship with Rotational Speed and Load: The vibration value increases with the increase of the rotational speed and is independent of the motor load. This is because the centrifugal force generated by the rotor unbalance is proportional to the square of the rotational speed. As the rotational speed increases, the centrifugal force increases, and the vibration becomes more severe.
3. Vibration Direction: The vibration value is the largest in the radial direction and very small in the axial direction. The centrifugal force caused by the rotor unbalance acts in the radial direction, so the vibration is mainly manifested in the radial direction.

6. Vibration Caused by Oil Film Whirl in Sleeve Bearings

Causes of Oil Film Whirl

1. Bearing Operating Conditions: After long-term operation of the bearing, the clearance may increase, or factors such as high lubricating oil viscosity, low oil temperature, and light bearing load can cause the oil film to thicken. When the oil film dynamic pressure is unstable, oil film whirl will occur. For example, if the lubricating oil viscosity is too high, the oil film will be too thick, and its stability will be affected.
2. Design and Manufacturing Factors: The design and manufacturing quality of the bearing itself can also affect the occurrence of oil film whirl. A bearing with an improper design or poor manufacturing accuracy may be more prone to oil film instability.

Characteristics of Oil Film Whirl in Sleeve Bearings

1. Vibration Frequency: The vibration frequency is slightly lower than half of the rotor rotation frequency Fr, approximately 0.42 – 0.48Fr. This characteristic frequency can be used to identify oil film whirl.
2. Vibration Direction: The vibration of oil film whirl is radial. The oil film forces act in the radial direction, causing the journal to vibrate radially.
3. Sudden Occurrence and Diagnosis Method: Oil film whirl often occurs suddenly. A diagnostic method is that when oil film whirl occurs, changing the viscosity and temperature of the oil can reduce and eliminate the vibration. This is because changing the oil properties can adjust the stability of the oil film.

7. Vibration Caused by Oil Film Oscillation in Sleeve Bearings

Causes of Oil Film Oscillation

Characteristics of Oil Film Oscillation

1. Oscillation Frequency: The oscillation frequency is equal to the first-order critical speed of the rotor. Large high-speed flexible rotor motors with a working speed close to twice the first-order critical speed are very prone to oil film oscillation.
2. Vibration Direction: Oil film oscillation is a radial vibration, similar to the vibration direction of oil film whirl.
3. Measures to Reduce Vibration: Reducing the rotor unbalance, decreasing the lubricating oil viscosity, and increasing the oil temperature can make the oil film oscillation disappear and be alleviated. By adjusting these factors, the stability of the oil film can be improved, and the vibration can be reduced.

8. Vibration Caused by Poor Processing and Assembly

Causes of Vibration Caused by Poor Processing and Assembly