Design methodology of a cable terminator to reduce reflected voltage on AC motors

This paper investigates theory and design of a line termination network (LTN) apparatus to reduce reflected wave voltage stress on AC motor terminals fed by pulse width modulated (PWM) inverter drives. Overvoltage stress may cause dielectric system failures in motors without suitable protection. An explanation on why LTNs are successful in reducing motor failures is given. Surge impedances of drives, cables and motors are characterized since the apparatus uses resistors and capacitors to provide an AC impedance match to the effective surge impedance seen at the motor. A design methodology to determine optimum resistance and capacitance is established, predominantly constrained by maximum allowable peak motor voltage. Energy loss equations of the LTN as a function of carrier frequency and cable distance are derived. Simulation and experimental results of reflected wave stress on the motor with an LTN shows good correlation. Cost and electrical performance comparison of the LTN to other reflected wave protection techniques in the 1/2-500 hp motor range shows its beneficial aspects.