Characteristic Research of Bearing Currents in Inverter-Motor Drive Systems

To gain better velocity modulation function and reduce the switch loss, the PWM voltage usually possesses very high frequency and dv/dt in inverter fed induction motor system. The common mode coupling current in the common mode coupling circuit will come into being inside the motor when this style PWM voltage acts on the winding of induction motor. The common mode coupling current component which crossed the motor bearing will give rise to damage to the bearing. This paper gives a measure method for the bearing currents of induction motor, divides the two kinds of bearing currents, conduction mode bearing current and discharge mode bearing current by experiment, analyzes the characteristic of the two kinds of bearing currents and their relation to common mode voltage, common mode current and shaft voltage. The experiment result shows that the conduction mode bearing currents appear chiefly at low motor speeds and is characterized by its occurrences in synchronization with the edges of common mode voltage pulse. Its value is small and the damage to the bearings is negligible. The discharge mode bearing current appears chiefly at high motor speeds. Its occurrences are not necessarily in synchronization with the edges of the common mode voltage pulses. Contrarily, the largest peak current usually occurs sometime during the interval of the highest level of common mode voltage. Its value, contrary to the conduction mode bearing current, is large and it is the main factor caused the bearings to be damaged