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, a mathematical modeling, which describes the motor's magnetic circuit with the quadrature voltage injection, is derived and the current estimation process transfer function is identified using the least-squares method. Moreover, it is observed that the transfer function's static gain varies strongly with the value of the quadrature current reference. Practical experiments comparing the reference and the measured torques are presented.