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According to the structure and working principle of synchronous generator, the main shaft of the unit will inevitably rotate in an incomplete symmetrical magnetic field due to the combined joint of stator core and stator silicon steel sheet, the uneven air gap between stator and rotor, and the inconsistency between shaft center and magnetic field center.
In this way, an AC voltage will be generated at both ends of the shaft.Under normal conditions, the insulation resistance of the rotating part of the unit to the ground is required to be greater than 0.5m Ω. If both ends of the main shaft are grounded at the same time, shaft current may be generated.
Motors with current carrying wires and magnetic circuit structures usually cause shaft magnetization or pulsating flux. The pulsating magnetic flux induces voltage in the circuit formed by the shaft, bearing and casing, so the shaft current flows through the circuit, and the surface of the shaft and sliding bearing bush, or the surface of the rolling bearing and bearing sleeve are damaged, which appears as increased friction and heating, deterioration of the operating performance of the rolling bearing, etc.
The shaft voltage is the voltage generated between the two bearing ends of the motor or between the motor shaft and the bearing. The AC asynchronous motor operates under the sinusoidal alternating voltage, and the rotor of the motor operates in the sinusoidal alternating magnetic field.
When the magnetic resistance in the circumferential direction of the stator core of the motor is unbalanced, the alternating magnetic flux intersecting the shaft will be generated, resulting in the alternating potential. When the rotating magnetic pole of the motor rotates, the magnetic flux passing through each magnetic pole changes, resulting in the magnetic flux intersecting the shaft.
With the rotation of the magnetic pole, the magnetic flux of the chain intersecting the shaft changes alternately, and the shaft voltage is generated. This voltage is generated along the axial direction. If it is in direct contact with the bearings on both sides of the shaft to form a closed circuit, shaft current will be generated.
On the other hand, during the operation of the motor, the load fluid rubs with the rotating body and generates static charge on the rotating body. The gradual accumulation of charge can also generate shaft voltage. The shaft voltage generated by this situation is different from that generated by magnetic alternation in mechanism.
The shaft voltage generated by electrostatic charge is intermittent and aperiodic, and its size is closely related to the operating state, fluid state and other factors. Generally, the shaft voltage is only about 0.5 ~ 2V, but because the current loop impedance is very small, there will be a large shaft current, which will do great harm to the motor bearing.
Under normal conditions, there is a lubricating oil film between the rotating shaft and the bearing, which plays the role of insulation. For low shaft voltage, this lubricating oil film can still protect its insulation performance and will not produce shaft current.
When the shaft voltage increases to a certain value, especially when the motor is started, the lubricating oil film in the bearing has not been stably formed, and the shaft voltage will break through the oil film and discharge, forming a circuit, resulting in various hazardous accidents. It can be seen that the shaft voltage is an internal factor. The key to analyze the generation of shaft current is to understand the shaft voltage.
During the operation of the motor, if there is shaft current between the bearing rolling elements and bearing rings at both ends, or between the motor shaft and the bearing, the service life of the motor bearing will be greatly shortened. Slightly, it can run for thousands of hours, and seriously, it can only run for a few hours or less, which has a great impact on on-site production.
The shaft current will pass through the metal contact point between the bearing and the rotating shaft. Because the metal contact point is very small, the current density of these points is large, and the high temperature will be generated in an instant, causing the local burning of the bearing. The melted bearing alloy splashes under the action of grinding pressure, so small pits are burned on the inner surface of the bearing or strip arc scars are pressed out on the inner surface of the bearing.
Due to the ablation of the shaft current, the surface of the rolling element and the raceway of the bearing ring are heated and the temperature is abnormal, and the heavy ones are locked or scattered to trigger the overcurrent protection shutdown, and even lead to the burning of the motor.
Therefore, in case of bearing damage accident in large and medium-sized motors using rolling bearings, special attention should be paid to checking the bearing surface traces during maintenance. For burns caused by shaft current, strip burn marks like washboard on the runway of the inner and outer rings of the bearing will be found when the bearing is removed for inspection, which is a common feature of the damage of shaft current to the rolling bearing.
For the root cause of shaft current, the following preventive measures are generally taken on site:
In order to prevent shaft current caused by magnetic imbalance and other reasons, an insulating diaphragm can be added at the bearing seat and bearing support at the non shaft extension end to cut off the circuit of shaft current.
For the shaft voltage caused by static charge, a grounding carbon brush can be added on the shaft at the load side of the motor, and the carbon brush must be reliably grounded. In this way, we can lead the static charge on the motor shaft to the earth at any time, so that it can not form shaft voltage and avoid the instantaneous shaft current formed by charge discharge.
In order to avoid shaft current caused by insulation damage of wires of other motor accessories, conduct regular and detailed inspection and strengthen the insulation of wires or gaskets to eliminate unnecessary hidden dangers of shaft current.
For the lubricating insulating medium oil between the shaft and bearing bush, the purity of the lubricating oil must be checked in time. If it is found that there is water in the oil, it must be filtered, otherwise the insulation strength of the oil film cannot meet the requirements and is easy to be broken down by low voltage. In addition, we also need to keep the insulating pad clean and dry, so as to make the insulating pad play an insulating role.
Sometimes the shaft current acts on the motor bearing and the bearing burning accident will not attract people’s attention. In case of bearing burning accident, we often only pay attention to mechanical coordination. After replacing the new bearing, because the shaft current of the motor was not eliminated, the bearing burning accident was caused, resulting in unnecessary losses.
In fact, the shaft voltage generated by large AC motor is inevitable, and the shaft current formed by the shaft voltage does serious harm to the equipment. Therefore, practical measures must be taken to prevent the formation of shaft current and reduce the damage of shaft current to the equipment to a great extent. Generally, through the above treatment, the shaft current of most motors is very small, which has not caused substantial harm to the motor structure.