VEIM XIAMEN MOTOR MACH CO.LTD Language：中文版
Why are variable frequency motors more prone to hidden shaft currents?
Release time: 2023-03-07
Shaft voltage is a byproduct of the operation of a rotating electrical machine, that is, it accompanies the operation of the electrical machine. Whether it is a power frequency motor or a variable frequency motor, there will be shaft current hidden dangers to varying degrees. In comparison, the shaft current problems of high voltage motors and variable frequency motors are relatively serious. The shaft voltage of the power frequency motor is mainly caused by the asymmetry of the magnetic circuit, unipolar effect, electrostatic induction, capacitive current, etc., but the essential reason is the flux pulsation. However, as long as the power supply is stable and the design is symmetrical, there are no major defects in the manufacturing process. When the power frequency motor is running, especially the potential difference between the two ends of the shaft of the low-voltage motor is very small, which is not enough to generate shaft current that endangers the motor bearing system. For a variable frequency motor powered by a frequency converter, the shaft voltage of the motor is mainly generated by the vector sum of the three-phase output voltage of the power supply and the non-zero zero seat component. Compared with industrial frequency motors, variable frequency motors naturally have unavoidable factors that generate shaft currents.
Under normal circumstances, the shaft voltage of the motor is low, and the lubricating oil film in the bearing can play an insulating role and will not form a closed circuit, so no shaft current will be generated. However, when the shaft voltage reaches a certain value, especially when the bearing system of the motor has not yet reached the state of a stable oil film, a closed loop may be formed. At this time, the shaft voltage will break down the oil film to generate shaft current.
The high temperature generated by partial discharge of shaft current can cause the bearing system of the motor to be paralyzed in a short period of time, from the initial noise and vibration to the bearing heating until it burns out. In the frequency conversion speed regulation system, the electric erosion characteristic of the high-frequency shaft current to the bearing is to produce "washboard" regular stripes on the inner and outer rings of the motor bearing and the balls.
In the relevant demonstration data of motor fault analysis, it is considered that 300 millivolts is the limit shaft voltage for breakdown of the bearing oil film, and for variable frequency motors, the shaft voltage generated by variable frequency motors with a power of 100 kW and above is less likely to reach or exceed the limit voltage. Therefore, in the manufacturing process of motors, generally for motors with a center height of 315 mm and above, necessary shaft current prevention measures will be taken uniformly, such as: disconnection method or bypass method.
