Analysis of Bearing Failure and Causes of Shaft Current

According to the bearing failure phenomenon, China Bearing Network analyzed that the main reason for the rapid failure of the motor bearing may be caused by shaft current. Therefore, technicians were invited to carry shaft current detection equipment to accurately diagnose the operation of the motor. After on-site measurement, it was indeed found that there was shaft current passing through the bearing.

Bearing

Judging from the inner sleeve of the free-end bearing that was damaged twice, in addition to the traces of the axial stepped deep groove on the outer ring raceway, there are more point-like micro holes on closer inspection. These dot-shaped micro holes should be caused by shaft current damage to the bearing. The main cause of the shaft current is the large shaft voltage in the motor rotor. Under the action of the shaft voltage, when the motor shaft-bearing seat-base insulation breaks to form a path, the motor generates a very large shaft current. When the current passes through the rotor and the bearing, a discharge occurs, causing electric burns to the bearing inner sleeve. Dotted micropores.

Bearing

As a result, the running performance of the bearing deteriorates, the friction loss and heat generation increase, and the bearing temperature rises rapidly, causing the bearing grease to melt and overflow from the bearing, and the amount of insulating grease is further reduced, forming a vicious circle, which eventually leads to rapid bearing damage. Due to severe bearing wear and displacement of the motor rotor, the drive end and the non-drive end of the motor rotor are not concentric, and the radial force of the bearing is uneven, causing the bearing outer ring raceway to grind out an axial stepped deep groove similar to a washboard pattern. Groove scratches.

Analysis of the reasons for the shaft current generated by the high-voltage motor. There are many reasons for the shaft current generated by the high-voltage motor. One of them is that when the equipment factory manufactures the motor, due to the purity of the raw materials and other objective factors, the stator and rotor of the motor are along the iron core. The magnetic resistance in the circumferential direction is not uniform, and the magnetic flux interlinked with the rotating shaft is generated, thereby inducing the electromotive force. Because the bearings of large motors are not insulated, when the shaft voltage is generated, it is very easy to form a shaft current, and the shaft voltage will quickly decrease after the shaft current is formed. If the shaft voltage generated by the motor is generally not easy to detect, even if a bearing damage accident occurs, most People can't find the reason for a while. However, when we replace the original bearing with an insulated bearing, we will measure the shaft voltage, and naturally find that the motor has shaft current.

Bearing

        Prevention of motor shaft current, through analysis, it is not difficult to find that during maintenance work, if the motor is improperly installed or assembled, the transmission system will not be able to run smoothly for a long time, or the motor assembly process is not sophisticated enough, resulting in the stator and rotor magnetism of the motor. The axial force generated by the non-coincidence of the center will cause the vibration of the motor bearing, the increase of noise, and the increase of shaft voltage. If the measures to prevent shaft current are not complete, it will cause the motor to malfunction soon after operation, the bearing sound will be abnormal, the vibration will increase, the temperature will rise, and the bearing will be damaged due to electric corrosion.

There are generally several reasons for the shaft current generated by the high-voltage motor: poor manufacturing and installation, uneven air gap caused by the eccentricity of the rotor, caused by static electricity, but generally the static current is small and the effect will not be too great. Although the shaft voltage generated by the motor is very low, only about 1V~2V, because the impedance of the current loop is very small, a large shaft current will be generated when the motor is running, and the harm to the rolling bearing of the motor is self-evident . The magnetic field is asymmetrical, there are harmonics in the power supply current, there is a gap between the two semicircles of the detachable stator core, and the number of pieces of the stator core stacked in a sector is inappropriate.

The above reasons can be improved by scientific methods, but they are difficult to completely overcome. High-voltage motors with larger capacity have measures to prevent shaft currents. One method is to add insulating pads at the free end bearing of the motor, and insulate the fixing bolts of the bearing, or replace the free end bearing with an insulated bearing to block the path of shaft current. Another method is to use carbon brushes to ground the bearings at both ends of the large motor so that the shaft current does not pass through the bearings.