Abstract— 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 sensorbased
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-starterconnected
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.
Index Terms – AC motor, inverter, starter, stator
resistance, stator temperature, temperature estimation,
thermal protection.