For electric motor products, bearing system problems are always a hot topic. More often, especially for motor manufacturers, bearing system failures are simply attributed to bearing quality problems. Some motors have their bearings replaced frequently, but the problem is never solved because the fundamental problem is not necessarily the bearing itself, but the shaft current.
From the analysis of the mechanism of current formation, the main shaft voltage and closed loop are necessary conditions for the formation of shaft current. In previous tweets, we systematically analyzed the reasons for the formation of shaft voltage. This article focuses on the impact of shaft current on motor performance.
Before talking about the problem, let’s talk about the magnetization of the shaft. I once experienced a faulty motor. When disassembling the motor, I found that the shaft of the motor showed obvious magnetism and had a significant adsorption effect on the screwdriver used for disassembly. The shaft voltage is the voltage induced on the shaft due to the asymmetry of the motor magnetic field, the magnetization of the shaft, electrostatic charging, etc. The shaft voltage occurs during the operation of the motor, including the voltage at both ends of the shaft, the local part of the shaft, and the shaft to the ground.
The direct target of the shaft voltage is the bearing system. When the shaft voltage reaches a certain value, the lubricating oil film (grease film) in the bearing will be broken down, thus forming a closed loop, that is, generating shaft current. Low voltage and high current are one of the characteristics of shaft current, because the shaft current will cause heating problems, causing the bearing lubricating oil (or grease) to degrade and fail due to excessive temperature, and the bearing will be in an intermittent dry friction state, resulting in the burning of the bearing itself and even sintering adhesion between the bearing and the shaft.
The shaft current accelerates the electrical corrosion of the bearing. The appearance is not very obvious at first, but it will transition to noise, and finally the bearing will fall apart and burn out due to the damage of the bearing operation-related parts. This process is related to the operating characteristics of the motor and the severity of the shaft current. Some times it will take longer, while others may only take a few hours.
From a theoretical analysis, most motors have the effect of shaft voltage, but the size varies. The shaft current problem of high-power motors, high-voltage motors and variable-frequency motors is more serious. Therefore, the design and manufacture of such motors cannot be equated with the volume and power amplification of small motors, but necessary shaft current control should be performed.
We have discussed the causes of shaft current before, such as magnetic circuit asymmetry, capacitive coupling under variable frequency state, electrostatic effect and axial residual magnetism, which will generate shaft voltage. Therefore, necessary monitoring and protection should be carried out from the design, manufacturing and use links.
In view of the above analysis, we can draw a conclusion that for motor products, the analysis of bearing system problems should not be limited to mechanical problem analysis, especially for large motors and generators, the electrical corrosion analysis of shaft current should be carried out.
Post time: Jul-23-2024
