The shaft current is caused by two factors: (1) The unipolar effect caused by the nonuniformity and asymmetry of magnetic circuit; (2) Capacitance current.
On the unipolar effect: Unipolar effect is a characteristic description of unipolar motor. The so-called unipolar motor is a commutator-free DC motor which is viewed from one end of the armature and the magnetic pole is a single polarity. The armature is a metal disk or a cylinder. When the armature conductor rotates, the radial or axial EMF with invariable direction is induced in the armature conductor, which can be used as the power supply equipment of low voltage and high current. In order to reduce the contact voltage drop and brush friction loss, a liquid metal collector is used.
Unipolar effect of high voltage motor with large capacity: Because of assembly and self reason, large capacity high voltage motor is easy to cause the asymmetry of magnetic flux, which leads to the voltage at both ends of shaft, which is called unipolar effect. Whent he voltage at the two ends of the shaft is enough to break through the oil film between the shaft and the bearing, the discharge occurs to form the shaft current, and the lubricating and cooling oil quality gradually deteriorates, if it is serious, the bearing will be born out and forced to stop causing an accident.
Capacitance current: Capacitance current, also called displacement current, is different from the current formed by charge oriented movement, and it does not really flow from fault point to earth. It is the equivalent current caused by capacitive charge and discharge. For alternating current, this equivalent current always exists because the current is constantly changing.