Why some motors use insulated end shield?

One of the reasons for shaft current is that in the manufacture of a motor, due to the uneven magnetoresistance of the stator and rotor along the axial direction of the iron core circumference, the magnetic flux is generated and the rotating shaft is intersected, thus inducing the electromotive force. Because the shaft current or shaft voltage is not easy to measure, when the rolling bearing burning accident occurs, the harm of the shaft current is revealed.

There are two reasons for shaft current: first, the magnetic reluctance of the magnetic circuit is unbalanced, there is a rotating magnetic flux that intersects with the rotating shaft, and when the rotor winding fails to the ground, there is a ground current; Second, there is a residual magnetic flux on the rotating shaft, which acts as a unipolar generator.

For large motor, especially variable frequency motor, the probability of shaft current is larger, and the bearing electrical corrosion problem is also more. In order to fundamentally solve this problem, insulated bearings and insulated end caps came into being.

The conditions for the formation of axial current are: one is that there is axial voltage, and the other is to form a loop. In a motor with normal design and operating conditions, there is only a small difference in voltage between the two ends of the shaft, and due to the treatment of oil film or bearing insulation, it is not enough to cause harm.

If there is a problem in a certain link, the shaft voltage exceeds the limit value, it may break down the original insulation, and form a loop in the rotating shaft, bearing inner ring, bearing outer ring, bearing chamber, so the surface of the rotating shaft bearing position and bearing inner ring will produce small and deep circular corrosion points or strip arc scars due to arc discharge, which is enough to burn the journal and bearing shell.

According to the structure and working principle of the synchronous generator, due to the combination of the stator core joint, the stator silicon steel sheet joint, the air gap between the stator and the rotor is uneven, the shaft center is inconsistent with the magnetic field center, and so on, the main shaft of the unit will inevitably rotate in an incomplete symmetric magnetic field. In this way, an AC voltage is generated at both ends of the shaft.