Suppression for discharge bearing currents in variable-frequency motors based on electromagnetic shield

The inverter outputs high frequency common mode voltage, which excites the parasitic capacitance in the motor, and the high frequency current flows through the parasitic capacitance forming high frequency discharge bearing current (HFDBC) and voltage across the stator and rotor (bearing inner race and outer race) is defined as shaft voltage[1-3]. The discharge occurred in the bearing inner ring, causing pitting and fluting in the bearing races when charges is accumulated enough in the air-gap capacitor. Actually, the high-frequency discharges bearing current are the main cause of bearing failure. Furthermore, the bearing currents cause the vibration and noise and affect the stability of the system. Therefore, the suppression of high-frequency discharge bearing currents is concerned in this paper. Bearing currents suppression techniques commonly used in an engineering project include common-mode voltage filters, ceramic bearings, and shaft-ground connection with an electric contact brush. In addition, dual-bridge inverters which can reduce bearing currents effectively have been proposed by some scholars [4]. The static charge dissipation method was utilized creatively to suppress the bearing voltages [5]. This paper proposed a novel suppression technique based on theory of electromagnetic shielding. By finite element simulation, starting performance analysis of the motor and prototype experiment, electromagnetic shielding effectiveness will be evaluated.