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I. Hazards of shaft current During the operation of explosion-proof motors, if there is a shaft current at either end of the bearings or between the motor shaft and the bearings, it will **reduce the service life of the motor bearings**. Mild cases can operate for thousands of hours, while severe cases can only operate for a few hours, posing a significant impact on safe production in the field. At the same time, the direct and indirect economic losses resulting from bearing damage and replacement cannot be underestimated. II. Generation of shaft voltage and shaft current Shaft voltage is the voltage that appears at the two bearing ends of a motor or between the motor’s rotating shaft and the bearings. The reasons for its occurrence generally include the following: 1) Magnetic imbalance leads to shaft voltage. Due to factors such as the stacking of fan-shaped laminations and silicon steel sheets, along with the presence of core slots and ventilation holes, there is an uneven magnetic resistance in the magnetic circuit. Additionally, alternating magnetic fluxes passing around the rotating shaft induce shaft voltage at both ends of the shaft. 2) Shaft voltage generated by inverter-powered supply: When an explosion-proof motor operates with inverter-powered supply, since the supply voltage contains high-order harmonic components, electromagnetic induction occurs at the ends of the stator winding coils, at the connection points, and between the rotating shafts as a result of these voltage pulse components. This leads to changes in the potential of the rotating shaft, thereby generating shaft voltage. 3) Axial voltage generated by electrostatic induction: There are many high-voltage devices in the area surrounding where the explosion-proof vibration motor operates; under the influence of strong electric fields, axial voltage is induced at both ends of the rotating shaft. 4) The presence of an external power source leads to the generation of shaft voltage. Due to the complex wiring in the operating area, especially with numerous connections for large motor protection and measurement components, whenever a wire end is connected to the shaft, shaft voltage is generated. 5) Other reasons: Factors such as the accumulation of static charge and insulation damage in the temperature sensing elements can all lead to the generation of shaft voltage. Once the shaft voltage is established, an axis current is generated as soon as a path is formed between the rotating shaft and the frame or housing. https://www.zuolidianji.com/uploads/allimg/20191126/1-191126162P9233.jpg III. Damage to bearings caused by shaft current Under normal conditions, there is a lubricating oil film between the rotating shaft and the bearings, which serves as an insulating layer. At lower shaft voltages, this layer of lubricating oil film can still maintain its insulating properties, preventing the generation of shaft current. However, when the shaft voltage increases to a certain level, especially during motor startup when the lubricating oil film in the bearings has not yet stabilized, the shaft voltage will break down this oil film and cause discharge, thus forming a circuit. The shaft current will flow through the metal contact points between the bearings and the shaft. Since these metal contact points are very small, the current density at these points is high, which generates high temperatures instantly and causes localized melting of the bearings. The melted bearing alloy is splashed under the force of compression, resulting in small pits on the inner surface of the bearings. Generally, because the hardness and mechanical strength of the shaft are higher than those of the bearing’s molten alloy, the typical symptom is the appearance of strip-shaped arc scars on the inner surface of the bearing. IV. Prevention of shaft current To address the root causes of shaft current, the following preventive measures are generally adopted on-site: (1) Install grounding carbon brushes at the shaft ends to reduce the shaft potential. These carbon brushes must be in reliable contact with the ground as well as with the rotating shaft, ensuring that the shaft potential remains at zero, thereby eliminating shaft current. (2) To prevent shaft currents from arising due to reasons such as magnetic imbalance, insulating partitions are often installed at the bearing housings and bearing supports on the non-shaft-end side in order to break the circuit of the shaft current. (3) To avoid shaft current caused by damaged insulation of the wires connected to other electric motors, it is often necessary for maintenance personnel to conduct thorough inspections and strengthen the insulation of the wires or gaskets, in order to eliminate potential sources of unnecessary shaft current. https://www.zuolidianji.com/