First, the shaft voltage, shaft current causes

 

1, magnetic imbalance generated shaft voltage

AC induction motor operating at sinusoidal alternating voltage, the rotor in a sinusoidal alternating magnetic field. As a result of motor stator rotor fan-shaped pieces, silicon steel and other components of the stack, coupled with the iron core slot, the presence of vents, resulting in an imbalance in the magnetic circuit yin yin. When the motor stator iron core in the circumferential direction of the yin and yang imbalance occurs, and the axis of alternating alternating magnetic flux, resulting in alternating potential. When the motor rotates, the magnetic pole rotates, the magnetic flux passing through each magnetic pole changes, and the shaft voltage is induced at both ends of the shaft, resulting in a magnetic flux intersecting with the shaft. With the rotation of the magnetic poles, the magnetic flux intersecting with the shaft alternates. This voltage is produced in the axial direction. If a closed loop is formed with the bearings on both sides of the shaft, a shaft current is produced. Under normal circumstances, this shaft voltage is about 1 ~ 2V.

 

2, inverter power supply shaft voltage

When the motor is operated with inverter power supply, the supply voltage contains higher harmonic components, so that the shaft voltage is generated by electromagnetic induction between the ends of the stator windings, the wiring part and the rotating shaft.

 

Asynchronous motor stator winding embedded in the stator core slot, the stator windings and stator windings and between the motor frame are distributed between the capacitance distribution, when the common inverter running at high carrier frequency, the inverter The common-mode voltage produces a drastic change in the leakage current through the distributed capacitance of the motor windings from the housing of the motor to the ground. The leakage current may form two types of radioactive and conductive electromagnetic interference. Due to the imbalance of the magnetic circuit of the motor, the static induction and the common-mode voltage are the causes of the shaft voltage and the shaft current. When the steep voltage suddenly changes at the input of the stator winding, due to the influence of the distributed capacitance, the voltage at each winding of the winding is unevenly distributed and the voltage near the port winding of the input winding is concentrated and the insulation is damaged or aged. Part of the general destruction of this phenomenon is the stator winding, the voltage is often concentrated in the invasion of the endpoint. In addition, due to the large reactance of the windings, the high-frequency component of the input voltage will be concentrated on the distributed capacitance near the input terminals, forming a LC series resonant circuit through the distribution capacitance between distribution lines, windings, and chassis to the ground line , When which produces high-frequency resonant current, it will have a wide range of failures. When a general-purpose inverter drives an induction motor with a small capacity, the problem of shaft voltage may not be taken into consideration. However, when a motor with a capacity of more than 200 kW is used, especially when existing fans, compressors and the like are retrofitted with variable frequency speed control, Check shaft voltage for early warning.

 

3, electrostatic induction shaft voltage

In the motor running site, due to the strong electric field of high voltage equipment, the shaft voltage is induced at both ends of the shaft.

 

4, static charge

During the operation of the motor, the fluid in the load and the rotating body run friction to generate electrostatic charge on the rotating body, and the electric charge gradually accumulates to generate the shaft voltage. The shaft voltage generated by this situation and the shaft voltage generated by the magnetic exchange vary in principle. Shaft voltage generated by electrostatic charge is intermittent and non-periodic, and its size and operating conditions, fluid conditions and other factors. Such as the accumulation of electrostatic charge, temperature measurement insulation damage and other factors are likely to lead to shaft voltage. Once the shaft voltage is established, the shaft current is generated once the passage is formed between the shaft and the base and the housing.

 

5, external reasons

The intervention of the external power supply generates the shaft voltage. As the operation of the site wiring is more complicated, especially large motor protection, measuring device wiring more, which one with a wire head lap connected to the shaft, it will produce shaft voltage down.

 

From the above analysis, the motor shaft voltage, shaft current is due to asymmetric magnetic circuit around the motor shaft, the rotor is not working heart, induced by pulsating magnetic flux and other reasons. It will make the shaft-bearing-shaft seat circuit current flow through the shaft in the motor rotor, shaft and bearing between the shaft and the bearing to form a shaft voltage. According to the different types of bearings, the pressure level is different, if more than the value of the bearings will be discharged through the oil film or conductive, producing point-like microporous bearings and bearings, and at the bottom of the black phenomenon. Serious when the shaft and bearings will be damaged, accompanied by a strong noise in the operation and the device housing live and so on.

 

Second, the motor shaft current hazards

 

Large and medium-sized AC motor with thin oil-lubricated sliding bearings, the motor shaft is immersed in the film. Under normal circumstances, the lubricating oil film between the shaft and the bearing play the role of insulation. For lower shaft voltages, no shaft current is generated. When the shaft voltage increases to a certain value, especially when the motor is started, the lubricating oil film has not been formed steadily. The shaft voltage will break the oil film and form a loop, resulting in a considerable axial current, which can reach hundreds or even thousands of amperes. Because the metal contact surface is very small, the current density, the bearing local melting, the bearing alloy is burned under the action of rolling force splashing, so the inner surface of the bearing burn small pits. The usual symptoms are that the inner surface of the bearing is pushed out of the strip arc scars and, in severe cases, enough to burn out the journals and bearings.

 

Electrostatic charges are generated on the shaft due to running friction, causing the potential of the shaft to rise as it is charged. When the operating shaft comes into contact with any part other than the rotating body, the part is allowed to discharge. Otherwise, we must continue to accumulate charge, and finally produce excessive voltage, if the bearing oil film exceeds the dielectric strength, the charge discharge in a very short period of time. The repetition of this phenomenon results in the shaft being damaged.

 

Third, to prevent the shaft current generated measures

1. Install a grounding brush on the shaft end to make the grounding brush reliably grounded and come into reliable contact with the shaft to ensure that the shaft potential is zero potential and the static charge on the shaft of the motor is always led to the earth to eliminate the shaft current.

 

2, in order to prevent magnetic imbalance and other reasons to produce shaft current, non-shaft end of the bearing seat and bearing bracket plus insulation partition, cut off the shaft current loop.

 

3, requiring maintenance personnel to carefully check and strengthen the wire or gasket insulation.

 

4, in addition to a seat in the base, the rest of the bearing housing and includes all the instruments installed on the shell and other metal parts are insulated to the ground, not insulated bearings should be connected to the brush by anti-static charge.

 

5. For the shaft voltage generated by the flux linkage of the shaft, an insulation pad can be added under the bearing seat on the motor side to cut off the circuit formed between the shaft and the bearing bush so that the shaft current can not be generated. However, in actual work, the role of the insulating mat is not clear. The method of installing from the insulating mat and the connection between the bearing and the oil pipe are all different to some degree, and finally the insulating mat can not reach the insulating effect and form the insulating layer Shaft current. By we often check the bearing housing insulation strength, with 500V shake table measurement, the insulation shall not be less than 0.5MΩ

 

6, to maintain the shaft and bush lubrication oil purity of the insulating medium and found that the water must be filtered into the oil treatment, or oil film dielectric strength can not meet the requirements, easy to be low voltage breakdown.