Adaptive Contactor – A New Scheme to Improve Induction Motor Immunity to Voltage Sags

One of the consequences of voltage sags is unexpected motor trips or stalls. This is caused by the simple voltage-magnitude-based tripping logic used in the motor contactors. Since a motor has momentum during a voltage sag, a deep voltage sag with a short duration does not necessarily lead to motor misoperation. Based on this reasoning, a novel motor contactor operation scheme is proposed in this paper. The proposed scheme takes into account both the magnitude and duration of a voltage sag plus the momentum of the rotor, in real-time, to decide if a motor will be tripped. The core idea of the scheme is to utilize the critical clearance time (CCT) of motors as the tripping time of AC contactors. The proposed scheme combines an offline analysis and an online calculation to minimize the computation burden in real-time operation. An improved motor parameter estimation method is also proposed to ensure the accuracy of the calculated CCT. The performance of the proposed scheme is evaluated and demonstrated through comparative case studies. Moreover, some practical issues are discussed to facilitate the real implementation.