How to Detect and Control Shaft Current in Motor Products
Shaft current is a hidden threat to motor bearing systems. Once shaft current is generated, it can lead to the bearing system's collapse in a short period of time. According to the necessary conditions for current generation, voltage, and circuit, this circuit is different from a simple electrical circuit, with no clearly defined paths, making current measurement difficult. Thus, a more objective and feasible detection method is to detect shaft voltage.
For different motor products and bearing system structures, the resistance differences in the circuit are relatively small, while the more significant factor is the shaft voltage of different motors. When the shaft voltage reaches a certain value (typically 300-500 mV), the oil film in the bearing system will be punctured, resulting in a significant shaft current.
Shaft voltage refers to the voltage generated between the two ends of a motor shaft or between the shaft and the bearing housing. It is caused by an imbalance in the magnetic circuit surrounding the motor shaft. This unbalanced magnetic flux cuts through the shaft, inducing shaft voltage at both ends of the shaft.
Under normal conditions, the lubrication oil film between the shaft and bearing provides insulation. When the shaft voltage increases to a certain value, especially during motor startup when the lubrication oil film has not yet formed steadily, the shaft voltage will break down the oil film, forming a circuit and generating a considerable shaft current, sometimes reaching hundreds to thousands of amperes.
Shaft voltage can be measured using a millivoltmeter (also known as a high-resistance voltmeter). From the perspective of component distribution, the motor shaft is insulated from the casing by the grease in the bearing. When shaft voltage is present, there is a potential difference between the two ends of the motor shaft. Once the motor is energized, the measuring probe of the voltmeter can be used for direct measurement—one probe making dynamic contact with the rotating shaft, avoiding the keyway for safety reasons, while the other probe should make good contact with the motor casing, avoiding painted surfaces and ensuring the motor casing is well-grounded.
Most high-voltage motors and inverter-driven motors have taken necessary measures against shaft current in their bearing systems, but this does not affect the detection of shaft voltage.