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Causes and fault identification of various vibrations in electric motors

Date:2023-11-17   Author:XINDA MOTOR

Causes and fault identification of various vibrations in electric motors

There are many reasons for motor vibration, and the locations and vibration characteristics are different. If the vibration characteristics and related factors caused by various sources of the motor are analyzed, it will help to identify and detect abnormal motor vibration. 
Common abnormal vibrations in motors include the following.
1. Electromagnetic vibration caused by abnormality
(1) The vibration frequency is twice the power frequency ;
(2) Cut off the application source and the electromagnetic vibration disappears immediately ;
(3) Vibration can be measured on the stator frame and bearings ;
(4) Vibration is related to the stiffness of the machine base and the load of the motor.
The main reasons are :
(1} The three-phase magnetic field of the stator is asymmetric ;
(2) Looseness of the stator core and stator coil will increase the electromagnetic vibration and electro-alkali noise of the stator. In this case, in the vibration spectrum diagram, in addition to the basic components of 2f , the electromagnetic vibration may also appear 4f, 6f, and 8f. harmonic components ;
(3) The foot bolts of the motor base are loose, which results in a reduction in the stiffness of the base, causing the motor to resonate in a frequency range close to 2 f, thus increasing the vibration of the stator, resulting in abnormal vibration.
2. Electromagnetic vibration caused by uneven air gap
There are two situations one is static unevenness ; the other is dynamic unevenness, and the vibration characteristics of the two are not exactly the same .
1. Characteristics of electromagnetic vibration caused by static air gap eccentricity
(1) The electromagnetic vibration frequency is twice the power frequency 0 .
(2) Vibration increases as the eccentricity value increases, and the same is true with the motor load.
(3) It is difficult to distinguish the electromagnetic vibration generated by the air gap monk core from the electromagnetic vibration generated by the stator abnormality .    
2. Characteristics of electromagnetic vibration caused by dynamic air gap eccentricity
(1) Electromagnetic vibrations at the rotor rotation frequency and the synchronous speed frequency of the rotating magnetic field may occur.
(2) The electromagnetic vibration pulsates with a period of 1/2 sf 0. Therefore, when the motor load increases and s becomes large, its pulsation rhythm accelerates.
(3) Electric motors often produce electromagnetic noise consistent with the pulsation rhythm.
3. Electromagnetic vibration caused by abnormal rotor conductor
Characteristics of electromagnetic vibration caused by abnormal rotor windings :
(1) The electromagnetic force and vibration waveform of electromagnetic vibration caused by abnormal rotor winding and electromagnetic vibration caused by dynamic eccentricity of the rotor are similar. The phenomena are similar and difficult .
(2) When the motor load increases, this vibration increases, which is more significant when the load exceeds 50% .
(3) If a spectrum analysis is performed on the motor stator current waveform or vibration waveform, in the spectrum diagram, side frequencies of ±2 sf appear on both sides of the fundamental frequency. Based on the relationship between the side frequencies and the amplitude of the fundamental frequency , the degree of the fault can be judged. .
4. Vibration caused by unbalanced rotor
Reasons for motor rotor imbalance include rotor parts falling off and shifting, insulation shrinkage causing the rotor coil to shift and loosen, coupling imbalance, cooling fan and rotor surface scaling , etc. The above factors are particularly sensitive to high-speed motors. The vibration caused by rotor imbalance has the following characteristics :
(1) The vibration frequency and the rotational speed frequency are equal.
(2) The vibration amplitude increases as the rotational speed increases, but has nothing to do with the motor load ;
(3) The vibration is largest in the radial direction and smaller in the axial direction .
5. Mechanical vibration caused by abnormal rolling bearings
Mechanical vibrations can occur during operation due to damage to rolling bearings and errors in design and manufacturing .
1. Rolling bearing damage
Rolling bearings will suffer from wear, surface peeling, pitting, chipping, rust and other failures due to overload, poor lubrication , incorrect installation, shaft current, entry of foreign matter, etc. Bearing damage, processing and assembly errors, and the structure of the rolling bearing whether the bearing is operating normally.    
2. Vibration caused by poor processing and assembly
(1) The vibration amplitude is maximum in the axial direction ;
(2) The vibration frequency is the same as the rotation frequency.
3. Vibration caused by nonlinear characteristics of bearings
(1) The vibration frequency is an integer or fractional multiple of the rotation frequency :;
(2) The vibration is axial ;
(3) Vibration is closely related to rotation speed.
6. Vibration of sliding bearings
1. Oil film vortex causes abnormal vibration
(1) The vibration frequency is slightly lower than half of the rotation frequency, usually 0.42 0.48f ;
(2) The vibration of oil film whirling is radial ;
(3) Oil film whirl often appears suddenly . The diagnosis method is to change the viscosity and temperature of the lubricating oil after the oil film whirl.
degree, the vibration will be reduced or eliminated.
2. Oil film oscillation produces abnormal vibration
(1) The vibration frequency is equal to the first-order critical speed of the rotor, and the operating speed is close to twice the first-order critical speed of a large-scale high-speed flexible
Rotor motors are prone to oil film oscillation ;
(2) Oil film oscillation is a radial vibration ;
(3) Reduce the imbalance of the rotor, reduce the viscosity of the lubricating oil and increase the oil temperature, which can make the oil film oscillation disappear.
7. Mechanical vibration caused by poor installation and adjustment
1. Vibration caused by axis misalignment during installation    
(1) The greater the deviation of the axis ;
(2) The part of the vibration that is twice the rotation frequency increases ;
(3) When the motor runs alone, these vibrations will disappear
2. Vibration caused by poor coupling fit
(1) The vibration frequency is the same as the motor rotation frequency ;
(2) The vibration phases of the connecting machinery and motor ends are opposite ;
(3) The vibration will disappear when the motor runs alone.