Explosion-proof motors are a type of motor that can be used in flammable and explosive plants. They isolate or do not generate sparks during operation. They are mainly used in coal mines, oil and gas, petrochemicals and chemical industries. In addition, they are also widely used in textile, metallurgy, city gas, transportation, grain and oil processing, papermaking, medicine and other sectors.
Vibration of explosion-proof motors can reduce winding insulation, shorten bearing service life, and loosen welding points; they may also damage the load machinery and reduce accuracy; they can loosen or crack the anchor bolts; and they can also cause abnormal wear of brushes and collector rings.
What are the causes of explosion-proof motor vibration?
There are several factors that cause explosion-proof motor vibration.
(1) The large vibration of the mechanical load is transmitted to the explosion-proof motor, forcing the explosion-proof motor to vibrate.
(2) The rubber on the coupling bolt between the explosion-proof motor and the mechanical load is severely worn, or there is a tightening force on one side.
(3) The base between the explosion-proof motor and the mechanical load is not well calibrated, and the shaft of the explosion-proof motor is bent.
(4) The foundation is broken and the foundation bolts are not tightened.
(5) The three-phase power supply is unbalanced.
(6) The bearing is worn and the gap of the bearing exceeds the specified value.
(7) The coaxiality of the rotor and the stator is poor, and the air gap is uneven; the end cover bolts are not tightened; the rotor of the explosion-proof motor is unbalanced.
(8) The stator core is not installed tightly, which is accompanied by electromagnetic sound.
(9) There are many cracks or welds on the cage rotor copper bars.
When the explosion-proof motor vibrates, you should first check the impact of the surrounding components on the explosion-proof motor, then untie the coupling and let the explosion-proof motor run idle. If there is no vibration when idling, it means that the vibration is caused by the misalignment of the base line between the explosion-proof motor shaft and the mechanical load shaft, or the fault of the mechanical load. If the explosion-proof motor still vibrates when idling, it is caused by the explosion-proof motor itself. The power can be cut off for judgment. If the vibration disappears immediately after the power is cut off, it is electromagnetic vibration, which may be caused by a broken wire in the parallel branch of the winding, damaged bearings, uneven air gap, etc. If the motor still oscillates after the power is cut off, it is a mechanical oscillation, such as an unbalanced rotor or a damaged bearing.
What are the common causes of motor vibration?
From the perspective of the motor itself, the large motor vibration value is mainly related to the dynamic balance of the rotor.
From the perspective of the motor’s external conditions, the installation quality of the motor and the host, the alignment, the lack of rigidity of the foundation, the close natural frequencies of the motor and the foundation, and other problems can also cause the motor’s vibration value to exceed the standard.
Note: When the vibration value of the motor is qualified during the no-load test but exceeds the standard after the motor is loaded, the problems outside the motor should be checked first.
(1) Motor bearing reasons
Some equipment manufacturers use the cold-beating method (not the method of heating the coupling and then hot-fitting) when installing the motor coupling, which causes damage to the bearing raceway and retainer, causing the motor vibration value to exceed the standard.
Incorrect hoisting methods during the user’s on-site hoisting process (such as the motor falling to the ground quickly, collision, etc.) can also cause damage to the bearing raceway and retainer, causing the motor vibration value to exceed the standard.
The motor is placed at the project site for a long time, the oil injection nozzle and oil injection pipe are missing, the motor bearing is rusted due to water ingress, and the grease deteriorates. The above factors will cause the motor vibration value to exceed the standard.
(2) Insufficient rigidity of the equipment foundation
The rigidity of the common base of the equipment is insufficient, or the rigidity of the concrete foundation under the common base is insufficient. After the motor is powered on, the vibration value measured exceeds the standard. At this time, it is necessary to solve the problem of excessive vibration value by reinforcing the foundation.
The national standard GB10068-2008 “Mechanical vibration of motors with a shaft center height of 56mm or more – Vibration measurement and assessment limit values” stipulates that the rigid installation should meet the following requirements: The maximum vibration speed measured in the horizontal and vertical directions on the motor foot (or on the base near the seat bearing or stator foot) should not exceed 25% of the maximum vibration speed measured in the horizontal or vertical direction on the adjacent bearing.
(3) Excessive flatness of the base
The common base surface is uneven, the motor is placed on the base or the motor is tightly attached to the base, and the motor vibration value exceeds the standard after power is turned on. The vibration value is normal after loosening all or part of the motor foot bolts, but exceeds the standard when tightened again.
a. During the on-site installation of the motor, the motor foot is hit hard with a sledgehammer when centering and aligning.
b. The motor foot is not leveled or solid during alignment, and the motor foot bolts are tightened during installation.
The above situations will cause the motor foot to deform, the bearing to be abnormally stressed, and the motor vibration value to exceed the standard after power is turned on.
When the motor base is processed in the factory, it is processed once on the milling machine, so there should be no doubt that the motor foot is uneven when it leaves the factory.
When the motor foot is found to be deformed (visually observed and checked with a feeler gauge, and placed on the platform for inspection), the solution is to re-mill the motor foot on the flat surface.
(4) Changes in the deflection of the motor shaft
This usually occurs in high-voltage 2-pole motors and sliding bearing motors. After being placed for a long time or out of use at the user’s site (warehouse), the motor shaft is not rotated 180° regularly, causing the motor shaft to deflect (bend), and the motor vibration value exceeds the standard after power is turned on.
To solve the problem of excessive vibration value of such motors, it is generally necessary to re-balance the rotor to reduce the vibration value of the motor.
(5) The natural frequency of the motor and the foundation is close
The overall natural frequency of the motor and the foundation is close to the 1st or 2nd frequency of the motor (the avoidance rate is not enough), and the vibration value will also exceed the standard after the motor is powered on; it can be determined through vibration detection (spectrum analysis). At this time, it is necessary to strengthen the foundation to solve the problem of excessive vibration value.
(6) The vibration value of the motor is qualified in a single test, but the vibration value exceeds the standard after loading.
After the rotating part (rotor) of the host and the motor rotor are connected, the alignment meets the standard requirements, but due to the large residual imbalance of the entire shaft system, the excitation force generated causes the vibration value of the motor to exceed the standard.
At this time, the coupling can be disengaged, either of the two couplings can be rotated 180 degrees, and then the two couplings can be connected and tested. The vibration value will decrease.
(7) Alignment problem
There is no problem with the coupling of the motor and the host, but the alignment deviation is large, causing the motor vibration value to exceed the standard.
Installation method for motor vibration measurement
The vibration of the motor is closely related to its installation. In terms of evaluating the balance and vibration of rotating motors, in order to ensure the repeatability of the test and provide comparable measurement data, it is necessary to measure a single motor under appropriate specified test conditions.
1. Free suspension
The motor is suspended on a spring or installed on an elastic support (spring, rubber pad, etc.). The maximum natural vibration frequency (fno) of the motor and its free suspension system should be less than one-third of the corresponding test motor speed frequency (f1). For motors with speeds below 600 r/min, it is not practical to use the free suspension measurement method; for motors with speeds above 3600 r/min, the static displacement Z should not be less than the value at a speed of 3600 r/min.
In order to reduce the influence of the mass and moment of inertia of the suspension system on the vibration intensity level, the effective mass of the elastic support should not be greater than one-tenth of the test motor.
2. Rigid installation
2.1 Overview
During the workshop operation test after the motor is assembled, the motor should be firmly and safely installed on a heavy and massive foundation or test foundation during vibration measurement. Elastic installation of the motor is not allowed.
The natural frequencies in the horizontal and vertical directions of all tests should not appear within the following ranges:
-±10% of the motor rotation frequency;
-±5% of twice the rotation frequency; or
-±5% of one and two times the grid frequency.
The manufacturer can choose one of the following two installation conditions.
2.2 Rigid installation on a heavy and massive foundation
One of the characteristics of a heavy and massive foundation is that the maximum vibration velocity measured in the horizontal and vertical directions on the motor foot (or on the base near the pedestal bearing or stator foot) does not exceed 30% of the maximum vibration velocity measured in the horizontal or vertical direction on the adjacent bearing. The ratio of the vibration velocity of the foot to the bearing is valid for the grid frequency component or the twice grid frequency component (if the latter needs to be measured).
Note 1: The rigidity of the foundation is a relative quantity, which is compared with the bearing system of the motor. The ratio of the bearing housing vibration to the foundation vibration is used as a characteristic quantity for evaluating the foundation elasticity;
Note 2: If the motor is mounted on a structure other than a heavy foundation, it may be necessary to perform a system dynamic analysis and, if necessary, change the dynamic stiffness of the structure.
2.3 Rigid installation on the ground foundation
The motor is mounted on a rigid ground foundation and its resonant frequency must meet the conditions of the forced frequency in 2.1.
Note: This installation is mostly used in the manufacturer’s laboratory.
2.4 Horizontally mounted motor
During the test, the motor should be mounted on the foundation with bolts or clamps at all screw holes according to the requirements of 2.1 or 2.2. When the structure or equipment cannot meet the above fixing conditions, such as single-bearing motors, in this case, the user and the supplier should negotiate
2.5 Vertically mounted motor
The vertical motor should be placed on a solid rectangular or circular steel plate, which is drilled in the center of the motor shaft extension, with a machined flat surface to match the flange of the motor under test, and tapped with threaded holes to connect the flange bolts. The thickness of the steel plate should be at least 3 times the thickness of the flange, and 5 times is recommended. The side length of the steel plate relative to the diameter should be at least equal to the height of the top bearing from the steel plate.
The steel plate foundation should be clamped and firmly installed on a solid foundation to meet the requirements of 2.1 or 2.2. The flange connection should use fasteners of appropriate number and diameter. If the above installation method is not suitable, consultation should be carried out between the user and the supplier.
Post time: Aug-06-2024
