Research On Bearing Current Problems Of Motor Driven By A Inverter

As early as in1924s, it has been recognized that internal asymmetric magnetic fieldcaused the issue of the shaft voltage and the bearing currents in the electrical machine fedby sinusoidal wave power. In order to avoid the shaft voltage and the bearing currents,the methods of design and manufacturing have been developed.40percent of theelectrical machine failures are the bearing failures, furthermore25percent of the bearingfailure is caused by the bearing currents resulted from the PWM inverter. Moreover, thisfigure increases with the widespread use of high-performance devices such as IGBT at analarming rate.With the growing concern of efficiency and energy saving, the techniquefor AC adjusting speed is widely used in high voltage high power drive systems. Firstly,the high voltage change ratio (dv/dt) of the PWM pulse increases the possibility of themotor winding insulation damage. Secondly, the bearing is easily damaged with theaction of the bearing currents caused by PWM pulse. It is researched that the relationshipbetween the life time of the bearing and the bearing currents can be expressed asS=7867204×10-2.17J，and the bearing currents of the motor fed by PWM inverterinterrelate the shaft voltage and the common mode voltage. Therefore, it is significant tostudy the shaft voltage and bearing current generation mechanism, the relationship withthe common mode voltage and influencing factors. It provides the theoretical basis forfurther studying of the bearing current suppression measures, which extends bearinglifetime and improve the reliability and durability of the whole driven system.In this paper, a lumped common mode equivalent circuit model of a variablefrequency motor fed by PWM inverter is proposed and the formulas of the keyparameters in the equivalent circuit are presented. The rationality of the common modeequivalent circuit and the accuracy of the parameter calculation are provided by thesimulation and the experiment with a3kW variable frequency motor. By analyzing of therelationship between common-mode voltage and the generation mechanism of shaft voltage bearing current, it has studied the impact of the carrier frequency and the motorrunning frequency to common mode voltage, shaft voltages and bearing currents. It hasprovided a theoretical basis for further studying the suppression measures of bearingcurrents, and increasing the lifetime of a variable frequency motor driven by PWMinverter.