LC clamp filter for voltage reflection due to long cable in induction motor drives

In many industrial applications, Pulse Width Modulated (PWM) inverter-fed induction motors needs long cables. The PWM inverter output voltage pulses behave like travelling waves on these cables. This phenomenon causes voltage reflection that results in over voltage at the motor terminals. Over voltage at the terminals of the motor is attributed by the rise time of the inverter pulses, length of the power cable that connects the inverter to the motor, or the disparity in impedance between the inverter, cable, and the motor. High dv/dt pulse at the inverter output terminal causes voltage reflection in the cable. One of the methods used to mitigate the voltage reflection at the input terminals of the motor is to increase the rise time of the voltage at these terminals. To accomplish this delay in voltage rise, a passive filter that consists of an LC clamp circuit can be used. This circuit helps to reduce the dv/dt value. Both the common mode and the differential mode issues are addressed by connecting the neutral of the LC clamp filter with diode to the mid-point of DC bus. Since the size of the filter required is small even this filter can be placed inside the inverter enclosure. Thus, the use of LC filter with diode reduces the voltage reflection issue and increases the life of the motor and the reliability of the drive systems.