The research on variable frequency system mainly focuses on the developing of system performance, while the negative effects of the system is ignored. The bearing currents issue is a typical negative effect. In this paper, a lumped common mode model of a variable frequency induction motor driven by PWM inverter is proposed and the formulas of the key parameters in the model are presented and impedance separation method is applied to verify the calculation results. By researching a 3 kW variable frequency motor, the experiment platform is built, experimental results are consistent with the simulation result, which provides the system model and the calculation of key parameters are basically rational. Finally the bearing current density is calculated. It has provided a theoretical basis for further studying of the suppression measures and increasing the life of a variable frequency motor driven by PWM inverter.