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

This paper proposes a parameter-based approach for thermal monitoring of direct-torque-controlled (DTC) induction machines (IM), where the stator resistance R_s is used as a direct indicator of average stator winding temperature. The stator resistance is estimated via the DC current injection method. In a DTC drive system, the ultimate challenge is how to impose the DC current offset via the existing flux and torque control loop. In this paper, the quantitative relationship between desired current injection and the corresponding flux linkage offset is firstly derived; and the current injection is then achieved by superimposing flux linkage offset in the flux control loop. Simulation results validate the proposed methodology, and the experimental data proves that this method achieves accurate temperature estimation under various operating conditions. This simple and efficient thermal monitoring technique is particularly advantageous since it eliminates the need for embedded thermocouples, requires no hardware change to conventional DTC drive systems and has a minimal impact on the IM´s normal operation.