Fault analysis and preventive measures of bearing damage caused by motor shaft current

A power plant 3# desulfurization island desulfurization fan drive motor model is YKK710-6, rated power 1 800kW, rated voltage 6kV, rated speed 995r/min, non-coupling end bearing model is FAG NU240EC. During the motor maintenance period, the electrical maintenance personnel replaced the rolling bearings on both sides of the motor. During the single-machine test run of the motor after the first replacement of the bearings, the non-coupling end bearing had obvious abnormal noise. Shut down and remove the bearing, and found that the bearing has signs of running out of the outer ring. After simply handling and adjusting the gap, continue to carry out the stand-alone test run, but there is still abnormal noise. After running for 1h, the machine stopped, and the bearing was removed, and it was found that the bearing raceway was seriously damaged. The workshop replaced the bearing again for a stand-alone test run, but the situation did not improve. To find out why, the motor was subjected to a vibration test.

1. On-site vibration test

During the on-site vibration test of the motor, there is a harsh abnormal sound at the non-coupling end of the motor, and the coupling end of the motor is normal. The test instruments are VM-63 portable vibration meter, DP1500 vibration data collector, and Smurfs shock pulse bearing detector. mm/s, stop standard is >11.2mm/s),

After analyzing the test data, it is found that the vibration intensity, total acceleration energy and impact value of the bearing position of the non-coupling end of the motor are significantly greater than that of the motor coupling end position, and the shock value has reached the red zone indicated by the shock pulse bearing detector. Although there is no obvious bearing fault frequency in the horizontal vibration spectrum of this measuring point, there is an obvious shock frequency band in the high frequency of 2 500-4 000 Hz (Figure 1, Figure 2), and the high frequency noise generated there can be clearly heard on site. Therefore, it is preliminarily determined that the bearing at the measuring point is faulty, and the bearing has reached the end of its life. It is recommended to replace the bearing.

The spectrogram shows that the radial and axial power frequency vibration amplitudes of the motor rotor are relatively prominent, and there are more prominent high-frequency components such as double frequency and triple frequency, and the radial vibration amplitude is greater than the axial vibration amplitude.

In order to further analyze the cause of the failure, it was found that the vibration amplitude increased significantly after increasing the load. The waveform shows a shock pulse of 40.13ms, the frequency is the power frequency of the motor rotor 24.91Hz, and the radial waveform is asymmetric.

The main fault of the preliminary diagnosis is that the bearing at the output end of the motor is loose, and the bearing is checked.

There are scratches on the outer ring, and the inner ring is also loose. Due to the larger radial clearance of the rolling bearing, there is a difference in the revolution speed of the balls, which leads to the wear of the cage holes, and the radial movement of the cage strikes the outer ring, which further aggravates the wear of the cage.

The fault was eliminated after replacing the bearing.

2. Failure analysis

In order to find out the root cause of the failure of the non-coupling end of the motor, the appearance inspection of the replaced rolling bearing was carried out, and it was found that there were many axial strip-shaped pits distributed irregularly on the raceway surface of the inner ring of the bearing, and the edge chamfer was the most severely damaged. And there are burn marks, obviously bearing damage caused by the motor shaft current.

  1. Usually the cause of the motor shaft current

(1) Due to the imbalance of the magnetic field of the magnetic circuit, there is a rotating magnetic flux that intersects with the rotating shaft.

(2) When the rotor winding has a ground fault, a ground current is generated.

(3) There is residual magnetic flux on the rotating shaft, which acts as a unipolar generator.

(4) The directionality of the iron core material causes uneven reluctance of the magnetic circuit.

(5) Caused by static electricity.

(6) The relationship between the number of sector slices and the number of motor pole pairs is incorrect during design. Although there are many reasons for the shaft current generated by the motor, this motor has been running normally before the overhaul. Moreover, the overhaul only replaced the bearings on both sides of the motor, and did not overhaul the motor windings, so it can be inferred that the problem occurred in the assembly. superior. Through further investigation, it was determined that the cause of the failure was that when the motor was overhauled and installed, a seemingly useless cable was not connected, and this cable was precisely to prevent the generation of shaft current, and the motor ran normally after treatment.

  1. Prevention of motor shaft current

The specific measures to prevent the shaft current from the motor from damaging the bearing are as follows.

(1) Strengthen the maintenance management and standardize the maintenance process. During the motor maintenance process, the structure and connection of the motor should not be changed at will.

(2) Insulate the rolling bearing at one end of the motor from its end cover. For this purpose, add insulating washers and insulate the fixing bolts of the rolling bearing to cut off the passage of the shaft current. When the motor is overhauled, the wear of the insulating pad should be checked.

(3) Install carbon brush diversion on the motor shaft to divert the shaft current in time to prevent damage to the bearing.

(4) Use insulated bearings for motors that may have shaft currents.

3. Summary

Maintenance personnel often think that bearing damage is a problem in assembly, but do not understand the seriousness of motor shaft current damage to rolling bearings, which not only causes great waste of manpower and material resources, but also seriously affects the normal progress of production. Therefore, understanding the impact of motor shaft current on bearings Injury hazards and preventive measures are very important.

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