The basic conditions for the formation of current are voltage and a closed circuit. The premise for the generation of shaft current is the existence of shaft voltage and a closed circuit. Why does a motor running at industrial frequency have shaft voltage? There are two reasons for the generation of shaft voltage in the operation of rotating motors: one is alternating magnetic flux, and the other is the accumulation of static charge.
The shaft voltage generated by the former is continuous and periodic. Normally, the rotor of the motor runs in a symmetrical, sinusoidal alternating magnetic field. The alternating potential induced by the motor rotor cutting the magnetic field, and the alternating current generated is also symmetrical. Therefore, there will be no asymmetric voltage between the two ends of the rotor under normal circumstances. However, when the magnetic resistance of the stator core of the motor is unbalanced in the circumferential direction, an asymmetric alternating potential will be generated, which will also generate a shaft voltage. This voltage is generated along the axial direction. The shaft voltage generated by static charge is intermittent and non-periodic. During the operation of the motor, the fluid on the load side will rub against the running rotating body and generate static charge on the rotating body. The charge gradually accumulates and generates shaft voltage.
The harm of shaft current to motors
During the operation of large and medium-sized AC motors, once the rotor shaft voltage forms a loop, shaft current will be generated, which is a typical low-voltage and high-current mode. Oil lubrication is used between the shaft and the bearing, and the motor bearing is pressed on the oil film. Due to the low amplitude of the shaft voltage, the insulation of the oil film is generally not broken down. During the high-speed operation of the rotor, if the lubricating oil quality does not meet the requirements, or there is a lack of oil, the oil film will be broken and broken down, resulting in metallic contact between the shaft and the bearing. At the moment of contact, the shaft voltage will form a closed loop, resulting in low-voltage breakdown. At this time, the shaft current generated is quite large, reaching hundreds of amperes or even thousands of amperes in an instant, which is enough to burn the shaft neck and the bearing.
The gradual accumulation of static charge generated on the shaft AC motors by running friction causes the potential of the shaft to continue to rise due to charging. When the running shaft contacts any part outside the rotating body, it will discharge through the part. If the running shaft does not contact the parts outside the rotating body, the charge will continue to accumulate and eventually generate an excessively high voltage. If the voltage exceeds the insulation strength of the bearing oil film, the charge will be discharged in a very short time, forming a shaft current.
The shaft current will flow through the loop composed of the rotating shaft, the bearing inner ring, the bearing outer ring, and the bearing chamber. The most obvious phenomenon is the small and deep circular corrosion spots produced by arc discharge at the rotating shaft bearing position and the bearing inner ring surface. The shaft current not only destroys the stability of the oil film and the conditions for the formation of the oil film, but also causes many corrosion spots on the surface of the rotating shaft and the inner ring of the bearing due to discharge, destroying the good cooperation between the rotating shaft and the bearing, causing the bearing to fail to work. Under special circumstances, strong shaft current will produce a strong arc on the contact surface between the journal and the bearing bush, causing damage to the journal and bearing bush, causing vibration and noise of the motor, and finally preventing the motor from operating normally.
