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Current fault analysis of diesel generators

The reason of axial current formation is generally connected with the electromagnetic effect, because the damage caused by axial current caused by rotor electrification eventually occurs in the generator unit. With big oil, chemical equipment, especially in chemical fertilizer plant in the application of steam turbine driving centrifugal compressor unit, shaft current impact problems of long period safe operation of the unit is more and more prominent, it in the thrust bearing of diesel generating sets, radial bearing, coupling, floating ring sealing and generate spark discharge in areas such as the drive gear, damage the metal surface, the destruction of the oil film forming. If the damage is not detected in time, it will bring many problems to the operation of the machine and may cause serious damage to the main components of the machine.

The introduction of axial current fault is rare, and a brief introduction is made in combination with the practical problems encountered in production.

I. damage caused by axial current to rotor parts and its consequences

The rotating shaft must be lubricated in a certain way to keep the rotor from direct contact with the bearing, that is, the whole rotor is insulated from the stator at high speed. Because of the resistance of the rotor to the ground, once charged, the voltage to the ground will be established. When the voltage is raised to a certain value, it will break through the area with the lowest resistance and generate electric spark discharge. The main consequences of this discharge are as follows:

(1) melt metal particles in the discharge area and form extremely small electrolytic pits on the metal surface;
(2) the surface becomes rough and dull due to the accumulation of pits. If it occurs on the axle bush, pure mechanical wear will occur.
(3) the molten metal particles into the lubrication system, make the lubricant is polluted, the lubrication system of lubrication performance is bad, and contains a lot of metal particles of lubricant can reduce oil film resistor, accelerate the progress of the electric spark erosion;
(4) local high temperature is generated in bearing bearing bearing area, damaging oil film, burning metal, increasing abrasion consumption, and finally causing serious friction damage.

In rotating machinery rotor system, is the most likely to happen spark discharge part of the bearing surface of the radial and thrust bearings, type coupling working tooth surface and floating ring sealing surface and changes of these parts for running a variety of conditions, such as load, temperature, lubrication condition and the rotor vibration, etc.), all have may reduce the oil film and crack resistance, cause spark discharge phenomenon in these areas.

For radial sliding bearing, bearing the development of electric corrosion pits can make serious corrosion of babbitt, this will not only change the original clearance of bearing, and surface roughness decrease can also lead to scratch and scratch on the surface of the bearing, high temperature, and burns. High speed light load bearing surface abrasion can make part of babbitt bearing segment lose effect on rotor shaft neck of preload, the rotor rotates in easy to induce the oil film vortex, causing instability of the rotor system. And instability of oil film bearing, in turn, will cause a sharp drop in the oil film resistors, get more shaft current through the area, intensify edm, encourage each other, finally cause the damage of the bearing and rotor system.

The electric spark damage of thrust bearing is caused by the thin oil film, and the thrust tile wears away quickly under the erosion of electric spark. According to foreign reports, the thrust bearing of a turbine generator unit is corroded to the point that it cannot continue to work under the action of charging and discharging axial current. For mitchel thrust bearings, the result of electric spark erosion of thrust tiles makes the tiles not well formed into oil film, thus accelerating the pure mechanical wear. If the axial thrust is large, it will further cause the thrust bearing to burn tiles or serious damage, causing major equipment accidents.

Causes and types of axial current

In addition to the external potential applied to the rotor, the formation of axial current is mostly caused by induction of the following factors.

For motor rotors, the main causes of axial current are asymmetric effect of magnetic force line distribution and magnetization effect of rotating shaft. Asymmetrical distribution of magnetic field lines is usually caused by asymmetric gaps in laminated layers. In addition to the motor rotor, other equipment will also generate axial current due to the magnetization effect of the shaft.

The magnetization effect of shaft is rotating shaft with magnetism due to various reasons, such as rotor unbalance current winding makes a shaft magnetization, welding, friction, collision, and a magnetic eddy current device may make equipment, and set up a magnetic field. When the rotating magnetic field cuts a conductor, it induces a certain potential in these parts, and when the potential is high enough to break through the oil film, a current loop is formed. This current loop may be through the whole rotor, also may be only in the bearing or local short circuit current, formed in the floating ring seal bearing or short circuit current of floating ring and produces new magnetic field, magnetic rotation or other parts. Therefore, this kind of magneto-electric conversion will form a strong magnetic field in the unit and generate high current.