Sensorless Torque Control of an Induction Motor through Quadrature Voltage Injection

A previously reported sensorless torque control method for induction motors uses a high-frequency voltage injection signal on the direct axis and a demodulation based on the root-mean-square filter to estimate the rotor flux angle. Further investigations evidence that the quadrature voltage injection has advantages over the direct axis voltage injection due to the high-frequency characteristics of the motor's magnetic circuit. In this paper, the previously proposed method is further developed, using a post-processing low-pass filter to eliminate the need of root-mean-square filters. It is shown that the response of the changes of high-frequency impedances to the injected signal behave like an amplitude modulation, and the design of the post-processing is briefly discussed. Practical experiments comparing the reference and the measured torques are presented.