At an academic exchange conference in City S, several leading experts in motor technology engaged in an in-depth and detailed discussion on the theory and practice of suppressing shaft current in large motors . Among the experts were several foreigners with high noses and blue eyes, speaking broken Chinese interspersed with...
"Yes" and "No" filled the discussion with a serious and candid academic atmosphere. Ms. Can was relatively more accepting of the idea that "music knows no borders," and this time she also experienced the charm of academic pursuits being borderless, which undoubtedly elevated her understanding of the axis current problem.
shaft current problem
During motor operation, a potential difference, or shaft voltage, will appear at both ends of the shaft. In small motors, the effect of shaft current is slight and generally imperceptible; however, in large motors, the shaft voltage is often high enough to break down the insulating oil film in the bearing, generating shaft current. This current flows through the circuit formed by the shaft, bearing, and base plate, causing electrolytic pitting or washboard-like burn marks on the journal and bearing shell, or causing the rolling bearing to overheat and burn out.
Causes of shaft current
The shaft current is caused by two factors: (1) the unipolar effect caused by the non-uniformity and asymmetry of the magnetic circuit; (2) the capacitive current.
●On the unipolar effect
The unipolar effect is a characteristic description of a unipolar motor. A unipolar motor, viewed from one end of the armature, has a single magnetic pole and is a commutatorless DC motor. The armature is a metal disk (considered as a combination of radial conductors) or cylinder (considered as a combination of axial conductors). When the armature conductors rotate, a radial or axial electromotive force with a constant direction is induced in the armature conductors, allowing it to function as a low-voltage, high-current power supply. To reduce contact voltage drop and brush friction losses, a liquid metal current collector is used. Figure 1 shows a model diagram of a unipolar motor.
1. Cylindrical copper rotor; 2. Disc-shaped bracket to fix the armature to the rotor;
3. Rotating shaft; 4. Hollow cylindrical stator yoke; 5. Two ring-shaped excitation coils;
Six carbon brushes are arranged in a ring at both ends of the armature.
Figure 1
● "Single-pole effect" in high-capacity high-voltage motors
Due to assembly issues and inherent limitations, large-capacity high-voltage motors are prone to magnetic flux asymmetry, leading to voltage across the shaft, a phenomenon known as the "unipolar effect," or the single-stage magnetic field effect shown in Figure 1. When the voltage across the shaft is sufficient to break down the oil film between the shaft and the bearing, a discharge occurs, forming a shaft current. This causes the lubricating and cooling oil to gradually deteriorate, and in severe cases, it can burn out the bearing, forcing the machine to shut down and causing an accident.
The asymmetry of magnetic flux can be caused by the following reasons: uneven air gap between the stator and rotor; excessive local magnetic resistance in the stator core, such as corrosion of the stator core or poor assembly of split stator cores (mostly in hydro generators); and uneven armature response in fractional slot motors, which causes asymmetry of rotor magnetic flux.
●Capacitance current
Capacitive current, also known as displacement current, differs from the current formed by the directional movement of charges. It does not actually flow from the fault point to the ground; rather, it is the equivalent current caused by the charging and discharging of a capacitor. For alternating current, because the current is constantly changing, this equivalent current always exists.
Measures to prevent shaft current
●Improve the insulation of the bearing housing to ground and cut off the shaft current loop;
● Install grounding brushes on the shaft so that the shaft current discharges to the ground along the brushes, reducing the damage of the shaft current to the bearing bush (or bearing).
Therefore, high-voltage motors with large capacity have measures to prevent shaft current: generally, an insulating pad is added to the non-load end to block shaft current; another method is to use carbon brushes to short-circuit the bearings at both ends of large motors so that the shaft current does not pass through the bearings.
