抄録
This paper describes an ungrounded common-mode equivalent circuit for a motor driven by a voltage-source PWM inverter. When the capacitance of the rotor was is small, the reversal of the polarities of the common-mode voltage and shaft voltage is observed. In order to model this reversal, a bridge-type equivalent circuit is proposed. On the basis of calculations and experiment, it is found the values and polarity of the shaft voltage can be are accurately determined with the proposed equivalent circuit. Furthermore, the capacitance value of the insulated rotor required to make the shaft voltage equal to or less than the dielectric breakdown voltage of the bearing grease is obtained.
