How to detect and control shaft current of motor products?

Shaft current is the hacker of the bearing system of the three phase motor product, and once the shaft current is generated, it will cause the motor bearing system to crash in a short time. According to the necessary conditions of current generation, voltage and circuit, the circuit is different from a simple circuit, there is no relatively clear line, it is difficult to measure the current, so the more objective and feasible detection is the detection of shaft voltage.

For different 3 phase induction motor products and bearing system structure, the resistance difference in the circuit is small, and the greater impact is the shaft voltage corresponding to different motors, when the shaft voltage reaches a certain value (generally 300-500mV), it will cause the oil film of the bearing system to be broken down, and produce a large shaft current. Shaft voltage refers to the voltage generated between the two ends of the motor shaft or between the rotating shaft and the bearing seat. It is caused by the imbalance of the magnetic circuit around the motor shaft. This unbalanced magnetic flux cuts the rotating shaft and induces the shaft voltage at both ends of the shaft.

Under normal circumstances, the lubricating oil film between the shaft and the bearing plays an insulating role. When the shaft voltage increases to a certain value, especially when the motor starts, the lubricating oil film has not been formed stably, the shaft voltage will break down the oil film to form a loop, producing a considerable shaft current, sometimes up to several hundred to thousands of amps. According to the size of the shaft voltage, it can be measured by a millivolt meter (also called a high resistance voltmeter). From the analysis of the physical distribution of the parts, the motor shaft and the housing are insulated through the grease in the bearing, when the motor shaft voltage exists, that is, there is a potential difference at both ends of the motor shaft. When the motor is energized, it can be measured directly through the measuring probe of the voltmeter, that is, one of the probes is in dynamic contact with the shaft (because the shaft is in a rotating state), but avoid the key way position to prevent hazards; The other probe of the voltmeter should be in good contact with the motor housing, that is, it is necessary to avoid the position with surface paint, and ensure that the motor housing is well grounded.
Most of the bearing systems of high-voltage three phase motors and variable frequency motors have taken necessary measures for shaft current, but this does not affect the detection of shaft voltage.