Analysis and mitigation of over voltages in long cable fed induction motor drives

Abstract

The application of inverter fed induction motors in industrial sector is growing fast due to its ability to line with smooth speed control. However, the most common problem is the appearance of the overvoltage at the motor terminal that may lead to premature failure of motor insulation. The overvoltage occurs at the motor terminal due to the surge impedance mismatch between the cable and the induction motor, use of fast switching devices and the long connecting cable between inverter and motor. As the surge impedance of the induction motor is much greater than that of the cable which connects the adjustable speed drive to the motor, as per travelling wave theory, voltage at the motor terminal will be much higher than that of the incident voltage. In addition, the inverter and motor will be placed at two distinct places in many applications due to the environmental constraints that require a long interconnecting cable. The use of long cable also leads to over voltage at the motor terminal. Further, though the implementation of fast switching devices in adjustable speed drives improve the overall performance of the system, the short rise time pulses incident on the motor terminals reflect, causing substantial over voltages at the motor terminals. Hence this necessitates a model of the inverter fed induction motor to analyse the over voltage phenomena for various parameters such as cable length, rise time etc. The power frequency equivalent model of the cable and the induction motor will not give a precise result in the analysis of overvoltage since the effect of capacitance is more at high frequencies. Hence a high frequency model is constructed for both cable and induction motor for studying the effect of overvoltage newline