Transient Analysis of Motor Terminal Voltage, Common Mode Voltage and Bearing Voltage in 2-level and Multilevel PWM Inverter Fed Induction Motor with Long Cable

The long motor cable contributes to overvoltage and higher common-mode voltage (Vcm) at the motor terminals fed by the PWM inverter due to the voltage reflection phenomenon. Higher the Vcm, higher the induced bearing voltage (Vb) and the bearing currents. Overvoltage at the motor terminals is due to two reasons, the impedance mismatch between the cable and the motor. Another reason is the PWM inverter output pulses travel at approximately half the speed of light. If the output pulses take longer than 1/3rd of the pulse rise time, voltage reflection occurs. The rise time of the inverter output pulses can be increased by using multilevel inverters. Hence, the magnitude of motor terminal voltage, Vcm, Vb is reduced significantly. In this paper, transient analysis of motor terminal voltage, Vcm and Vb in a multilevel inverter (MLI) fed induction motor with a long cable has been presented. The simulation modeling and analysis are carried out using MATLAB Simulink. The work presented in this paper provides the solution to improve the reliability of the ASD system for long motor cable applications and also to avoid downtime costs due to bearing failure.