●High-capacity high-voltage motor "unipolar effect"
Due to assembly and self reasons, large-capacity high-voltage motors are prone to cause asymmetry of the magnetic flux, resulting in a voltage across the shaft, called the "unipolar effect", which is the single-stage magnetic field effect shown in Figure 1. When the voltage at both ends of the shaft is enough to break through the oil film between the shaft and the bearing, a discharge current is generated to form a shaft current, which gradually deteriorates the oil quality of the lubrication and cooling, and the bearing burns out and is forced to stop and cause an accident.
The asymmetry of the magnetic flux is roughly due to the following reasons: the asymmetry of the magnetic flux due to the uneven air gap between the stator and the rotor; the local magnetic resistance of the stator core is large, such as the corrosion of the stator core, or the split stator core (large Part of the hydro-generators) caused excessive local reluctance due to poor assembly and bonding at the site; due to the uneven armature reaction of the fractional-slot motor, the rotor flux is asymmetrical.
●Capacitor current
The capacitor current is also called the displacement current. It is different from the current formed by the charge orientation movement. It does not really flow from the fault point to the earth. It is the equivalent current caused by the charge and discharge of the capacitor. For alternating current, because the current is constantly changing, this equivalent current is always present.
Axle current measure
● Improve the insulation of the bearing housing to the ground and cut off the shaft current loop;
● Install a grounding brush on the shaft to discharge the shaft current along the brush to the ground, reducing the damage of the shaft current to the bearing (or bearing).
Therefore, the high-voltage motor with large capacity has anti-axis current measures: under normal circumstances, the insulation current is added to the non-load end to block the shaft current; the other method is to use a carbon brush to short-circuit the two-end bearing method for the large motor. The shaft current does not pass through the bearing.
