Axis current causes
Axis currents are caused by two factors: (1) Unipolar effects due to non-uniform, asymmetrical magnetic circuits; (2) Capacitance currents.
●About unipolar effects
The unipolar effect is a characteristic description of a unipolar motor. The so-called unipolar motor refers to the one end of the armature, the magnetic pole is a single polarity, and it is a DC motor without a commutator. The armature is a metal disc (referred to as a radial conductor combination) or a cylinder (regarded as an axial conductor combination). When the armature conductor rotates, a direction-invariant radial or axial electromotive force is induced in the armature conductor. Can be used as low voltage, high current power supply equipment. In order to reduce the contact voltage drop and the brush friction loss, a liquid metal current collector is used. Figure 1 shows a unipolar motor model.
Large-capacity high-voltage motor "unipolar effect"
Due to assembly and its own reasons, large-capacity high-voltage motors can easily cause magnetic flux asymmetry, resulting in voltage across the shaft, called "unipolar effect", that is, the single-stage magnetic field effect shown in Figure 1.
When the voltage at both ends of the shaft is enough to break the oil film between the shaft and the bearing, discharge occurs to form the shaft current, which gradually deteriorates the lubrication and cooling of the oil. In severe cases, the bearing burns out and the machine is forced to shut down.
The magnetic flux asymmetry has the following causes: asymmetry of magnetic flux due to uneven air gaps between the stator and the rotor; large local magnetic resistance of the stator core, such as corrosion of the stator core, or split stator core (large Part of the turbine generator) The local magnetic reluctance caused by poor assembly and assembly at the site, etc.; due to the uneven armature reaction of the fractional-slot motor, asymmetry of the rotor flux is caused.
Capacitance current, also known as displacement current, is different from the current formed by the charge directed movement and does not really flow from the fault point to the ground. It is the equivalent current caused by the charge and discharge of the capacitor. For AC, because the current is constantly changing, this equivalent current is always present.
Measures to prevent shaft current
Improve the insulation of the bearing housing to ground, cut off the shaft current loop;
● Install a grounding brush on the shaft so that the shaft current discharges along the brush to the earth to reduce the damage of the shaft current to the bearing (or the bearing).
Therefore, high-capacity high-voltage motors have anti-shaft current measures: under normal circumstances is the non-loaded end plus insulating pad blocking shaft current; Another method is to take a short-term use of carbon brush short-circuit the two ends of the bearing, let Axis current does not pass through the bearing.