A torque-injection-based approach for thermal monitoring of induction machines with direct torque control

Abstract

This paper proposes a nonintrusive thermal monitoring method for induction machines (IM) with the direct torque control (DTC) scheme. The stator winding temperature is estimated by injecting a small DC current offset into the machine winding. Since DTC strategy does not have explicit current control loops, the primary challenge is how to inject the desired DC current via existing torque and flux loops. This paper derives a quantitative relationship between torque and DC current offset; hence the DC current can be injected by offsetting the feedback torque estimation. Simulation and experimental results demonstrate that this method achieves accurate temperature estimation under various operating conditions. This simple and efficient thermal monitoring technique is nonintrusive because it eliminates the need to use winding temperature sensors, has minimal impacts on the IM's normal operations, and requires no hardware change to existing drive systems.