An improved DC-signal-injection method with active torque-ripple mitigation for thermal monitoring of field-oriented-controlled induction motors

This paper proposes an improved signal-injection-based stator-winding thermal-monitoring method for field-oriented-controlled (FOC) induction motors. In this method, stator-winding DC resistance is normally used as a direct indicator of winding temperature, and is obtained from injected DC currents and voltages in the motor winding. Conventionally, however, implementation of this DC-injection method causes low-frequency torque ripple, which is undesired especially for high-performance motor applications and becomes a major disadvantage of this type of thermal-monitoring scheme. This paper takes advantage of the flexibility of the FOC scheme and proposes an improved method with a form of injection that compensates the torque ripple by adding second harmonic currents. Simulation and experimental results demonstrate that the low-frequency torque ripple is successfully mitigated, and accurate temperature estimate is guaranteed under various motor operating conditions and cooling modes.