1. Causes of motor axial compression

Axis voltage of three-phase AC motor is usually caused by magnetic circuit imbalance of the motor, which mainly includes the following situations:

(1) In the production process of the motor, the poor stowing process of the stator core magnetic steel sheet of the motor leads to the uneven reluctance of the stator and rotor along the circumference of the core, and the magnetic flux of the cross-link with the rotating shaft is generated, so as to generate the induced electromotive force.

(2) In the process of using the motor, due to the use environment and the motor itself manufacturing (rigidity) problems, resulting in the motor stator and rotor air gap changes, resulting in the change of reluctance, uneven flux leakage, and the electromotive force induced with the shaft.

(3) Electrostatic induction will also cause the motor to produce shaft voltage.If there is a large number of high-voltage equipment around the motor, the motor shaft may produce an induced voltage.Or in the case of electrification of various signal devices used with the motor, if the device is weak insulation and contact with the rotating shaft, the rotating shaft of the motor will produce shaft voltage.

2. Causes of motor shaft current generation

Although the motor shaft voltage is very low, only 0.5~2 V, but it will form a voltage path between the motor shaft, the frame and the housing, because the impedance of the current loop is very small, so it will produce a large shaft current P.

Under normal circumstances, there is a lubricating oil film between the shaft and the bearing, which can play the role of insulation.For lower shaft voltages, the oil film acts as an insulator and does not generate shaft current.However, when the shaft voltage increases to a certain value, especially when the motor starts, because the lubricating oil film in the bearing has not been established stably, the shaft voltage will break through the oil film to discharge and form a charged circuit.

When the motor bearing oil film breaks down, with the increase of bearing temperature, the number of bearing breakdown points increases, and the equivalent resistance of the bearing will decrease. The shaft current will channel the metal contact points between the bearing and the rotating shaft. Because the contact point is small and the current density is large, high temperature will be generated instantly and the bearing will be partially burned.The fused bearing alloy flying under the action of grinding pressure is lost, so small pits are produced on the inner surface of the bearing.Usually because the shaft hardness and mechanical strength is higher than the bearing alloy, so the bearing inner ring raceway appears washboard like dents.

3. Treatment measures

(1) Reinforce motor stator housing and foundation

After the motor disassembly inspection, it is found that due to the insufficient strength of the motor body, the poor fastness between the stator core and the shell, the vibration and noise of the motor appear.As the motor shell is generally welded with thin steel plate, it will vibrate obviously when the motor load is large.The vibration wave makes the motor stator housing tremble, accompanied by a large noise.In order to solve the vibration and noise problem, the motor housing is strengthened to change the natural frequency of the housing.In the process of reinforcement, considering the detachable property of the original motor stator core and casing, the reinforcement plate is made into a detachable structure, which is not only conducive to installing the stator core, but also convenient for replacement.

At the same time, the foundation of the motor is reinforced again to strengthen its rigidity and improve the running stability of the motor.The above measures can eliminate the adverse effect of induction current caused by the unbalance of the magnetic resistance caused by the continuous change of the stator and rotor magnetic circuit.Before modification.

(2) Modification of motor end cover

The original motor bearing sleeve and end cover are integrated design, and the non-transmission end is not insulated. Therefore, if the motor produces shaft voltage during operation, a path will be formed and shaft current will damage the bearing.In view of this situation, the end cover of the non-transmission end was modified, and the original bearing sleeve was replaced with a new bearing sleeve wrapped by an insulating epoxy resin layer with good insulation performance.The insulation layer can completely cut off the shaft current loop and eliminate the influence of the shaft current on the bearing.

(3) Efficiency measures

In order to completely eliminate the shaft voltage, a carbon brush can also be installed between the motor shaft extension end and the end cover. The carbon brush is connected with the end cover and the rotating shaft to eliminate the induced potential and bearing potential difference on the shaft.

4. Protective measures to prevent bearing damage

Test the voltage of the rotating shaft of the field motor to the ground. If the voltage of the shaft appears, it will be processed according to the following scheme.

(1) Consider reducing axial potential.The grounding carbon brush is installed at the shaft end to ensure its reliable grounding and reliable contact with the rotating shaft. The rotating fine potential is kept at zero potential to eliminate the shaft current.

(2) Cut off the shaft current loop.Insulation isolation is added at the non-transmission end bearing seat and bearing bracket to prevent shaft current generation.

(3) Check the damage degree of the insulation of the motor accessory wire.Use the maintenance opportunity to check and strengthen the wire or gasket insulation, to avoid the motor accessory wire insulation damage caused by shaft current generation.

(4) often check the insulation strength of the bearing seat.For motors that have taken bearing bearing insulation isolation measures, the insulation strength of bearing pedestal should be measured regularly with 500 V mehmmeter, which should not be less than 0.5MQ.If the test value exceeds the standard, the insulation isolation should be processed again until the test is qualified.

(5) Grasping the bearing temperature rise at any time to maintain the purity of lubricating oil insulation medium.Frequently check the motor bearing temperature and bearing lubricating oil quality, timely analyze the bearing temperature rise according to the load and ambient temperature, find out the cause of temperature rise and deal with it in time.For the bearing motor, if water is found in the lubricating oil, it must be filtered, otherwise the insulation strength of the oil film will not meet the requirements, and it is easy to be broken down by low voltage.