I. Introduction
Wound rotor induction motors (WRIM) are typically employed in large power industrial applications (above 1000 hp) due to their interesting advantages: on the one hand, their rotor winding, based on a set of short-circuited coils, can be accessed externally, a fact that enables the measurement of rotor currents, which can be useful for diagnostic purposes. On the other hand, this winding structure enables the insertion of external resistors which modify the speed-torque characteristic of the machine, yielding higher starting torques while maintaining a low value of inrush current [1]. These advantages make them an ideal option for applications involving ball and sag mills, cranes, pumps, fans and blowers, chippers, hoists and conveyors, and among others [1].