In the operation of the motor, the potential difference between the two ends of the shaft will appear, that is, the voltage of the shaft. In the small motor, the action of shaft current is slight and generally undetectable. Large motors, on the other hand, often have high shaft voltages high enough to break through the insulating oil film in the bearings and produce shaft currents. Shaft current, flowing through the loop composed of rotating shaft, bearing and bottom plate, produces electric erosion points or rub-plate burning marks on the journal and bearing bush or causes the rolling bearing to overheat and burn out.
Measures to prevent shaft current:
Improve the insulation of the bearing seat to the ground and cut off the current loop of the shaft.
Agrounding brush is installed on the shaft to discharge the shaft current toward the earth along the brush, thus reducing the damage caused by the shaft current to the bearing.
Therefore, the high voltage motor with large capacity has anti-shaft current measures: In general, the insulation pad is added to the non-load end to the black the shaft current. Another method is to use a carbon brush to connect the two ends of a large motor so that the shaft current does not pass through the bearing.