Active stator winding thermal protection for AC motors

AC motors are the main workhorses in process industries. Their malfunction may not only lead to repair or replacement of the motor, but also cause significant financial losses due to unexpected process downtime. Reliable thermal protection of ac motors is crucial for reducing the motor failure rate and prolonging a motor´s lifetime. In this paper, conventional thermal protection devices and state-of-the-art thermal protection techniques are reviewed with their advantages and limitations discussed. Since 2001, the research team at the author´s company and at the Georgia Institute of Technology has been cooperatively developing a series of active motor thermal protection methods, as a low-cost alternative approach to the traditional passive and sensor-based methods. These active thermal protection methods monitor the average stator temperature via stator resistance estimation based on the dc equivalent model of ac motors, using only motor stator voltage and current measurements. In this paper, an overview of the active thermal protection techniques for line-started, soft-starter-connected and inverter-fed ac motors is presented, with their advantages and drawbacks discussed. The active thermal protection techniques are capable of providing accurate, and non-invasive thermal protection of ac motors.